Adds new bit element_type for dense_vectors (#110059)

This commit adds `bit` vector support by adding `element_type: bit` for
vectors. This new element type works for indexed and non-indexed
vectors. Additionally, it works with `hnsw` and `flat` index types. No
quantization based codec works with this element type, this is
consistent with `byte` vectors.

`bit` vectors accept up to `32768` dimensions in size and expect vectors
that are being indexed to be encoded either as a hexidecimal string or a
`byte[]` array where each element of the `byte` array represents `8`
bits of the vector.

`bit` vectors support script usage and regular query usage. When
indexed, all comparisons done are `xor` and `popcount` summations (aka,
hamming distance), and the scores are transformed and normalized given
the vector dimensions. Note, indexed bit vectors require `l2_norm` to be
the similarity.

For scripts, `l1norm` is the same as `hamming` distance and `l2norm` is
`sqrt(l1norm)`. `dotProduct` and `cosineSimilarity` are not supported.

Note, the dimensions expected by this element_type are always to be
divisible by `8`, and the `byte[]` vectors provided for index must be
have size `dim/8` size, where each byte element represents `8` bits of
the vectors.

closes: https://github.com/elastic/elasticsearch/issues/48322

docs/changelog/110059.yaml

@@ -0,0 +1,32 @@
+pr: 110059
+summary: Adds new `bit` `element_type` for `dense_vectors`
+area: Vector Search
+type: feature
+issues: []
+highlight:
+  title: Adds new `bit` `element_type` for `dense_vectors`
+  body: |-
+    This adds `bit` vector support by adding `element_type: bit` for
+    vectors. This new element type works for indexed and non-indexed
+    vectors. Additionally, it works with `hnsw` and `flat` index types. No
+    quantization based codec works with this element type, this is
+    consistent with `byte` vectors.
+
+    `bit` vectors accept up to `32768` dimensions in size and expect vectors
+    that are being indexed to be encoded either as a hexidecimal string or a
+    `byte[]` array where each element of the `byte` array represents `8`
+    bits of the vector.
+
+    `bit` vectors support script usage and regular query usage. When
+    indexed, all comparisons done are `xor` and `popcount` summations (aka,
+    hamming distance), and the scores are transformed and normalized given
+    the vector dimensions.
+
+    For scripts, `l1norm` is the same as `hamming` distance and `l2norm` is
+    `sqrt(l1norm)`. `dotProduct` and `cosineSimilarity` are not supported. 
+
+    Note, the dimensions expected by this element_type are always to be
+    divisible by `8`, and the `byte[]` vectors provided for index must be
+    have size `dim/8` size, where each byte element represents `8` bits of
+    the vectors.
+  notable: true

docs/reference/mapping/types/dense-vector.asciidoc

@@ -183,11 +183,23 @@ The following mapping parameters are accepted:
 `element_type`::
 (Optional, string)
 The data type used to encode vectors. The supported data types are
-`float` (default) and `byte`. `float` indexes a 4-byte floating-point
+`float` (default), `byte`, and bit.
-value per dimension. `byte` indexes a 1-byte integer value per dimension.
+
-Using `byte` can result in a substantially smaller index size with the
+.Valid values for `element_type`
-trade off of lower precision. Vectors using `byte` require dimensions with
+[%collapsible%open]
-integer values between -128 to 127, inclusive for both indexing and searching.
+====
+`float`:::
+indexes a 4-byte floating-point
+value per dimension. This is the default value.
+
+`byte`:::
+indexes a 1-byte integer value per dimension.
+
+`bit`:::
+indexes a single bit per dimension. Useful for very high-dimensional vectors or models that specifically support bit vectors.
+NOTE: when using `bit`, the number of dimensions must be a multiple of 8 and must represent the number of bits.
+
+====
 
 `dims`::
 (Optional, integer)
@@ -205,7 +217,11 @@ API>>. Defaults to `true`.
 The vector similarity metric to use in kNN search. Documents are ranked by
 their vector field's similarity to the query vector. The `_score` of each
 document will be derived from the similarity, in a way that ensures scores are
-positive and that a larger score corresponds to a higher ranking. Defaults to `cosine`.
+positive and that a larger score corresponds to a higher ranking.
+Defaults to `l2_norm` when `element_type: bit` otherwise defaults to `cosine`.
+
+NOTE: `bit` vectors only support `l2_norm` as their similarity metric.
+
 +
 ^*^ This parameter can only be specified when `index` is `true`.
 +
@@ -217,6 +233,9 @@ Computes similarity based on the L^2^ distance (also known as Euclidean
 distance) between the vectors. The document `_score` is computed as
 `1 / (1 + l2_norm(query, vector)^2)`.
 
+For `bit` vectors, instead of using `l2_norm`, the `hamming` distance between the vectors is used. The `_score`
+transformation is `(numBits - hamming(a, b)) / numBits`
+
 `dot_product`:::
 Computes the dot product of two unit vectors. This option provides an optimized way
 to perform cosine similarity. The constraints and computed score are defined
@@ -320,3 +339,112 @@ any issues, but features in technical preview are not subject to the support SLA
 of official GA features.
 
 `dense_vector` fields support <<synthetic-source,synthetic `_source`>> .
+
+[[dense-vector-index-bit]]
+==== Indexing & Searching bit vectors
+
+When using `element_type: bit`, this will treat all vectors as bit vectors. Bit vectors utilize only a single
+bit per dimension and are internally encoded as bytes. This can be useful for very high-dimensional vectors or models.
+
+When using `bit`, the number of dimensions must be a multiple of 8 and must represent the number of bits. Additionally,
+with `bit` vectors, the typical vector similarity values are effectively all scored the same, e.g. with `hamming` distance.
+
+Let's compare two `byte[]` arrays, each representing 40 individual bits.
+
+`[-127, 0, 1, 42, 127]` in bits `1000000100000000000000010010101001111111`
+`[127, -127, 0, 1, 42]` in bits `0111111110000001000000000000000100101010`
+
+When comparing these two bit, vectors, we first take the {wikipedia}/Hamming_distance[`hamming` distance].
+
+`xor` result:
+```
+1000000100000000000000010010101001111111
+^
+0111111110000001000000000000000100101010
+=
+1111111010000001000000010010101101010101
+```
+
+Then, we gather the count of `1` bits in the `xor` result: `18`. To scale for scoring, we subtract from the total number
+of bits and divide by the total number of bits: `(40 - 18) / 40 = 0.55`. This would be the `_score` betwee these two
+vectors.
+
+Here is an example of indexing and searching bit vectors:
+
+[source,console]
+--------------------------------------------------
+PUT my-bit-vectors
+{
+  "mappings": {
+    "properties": {
+      "my_vector": {
+        "type": "dense_vector",
+        "dims": 40, <1>
+        "element_type": "bit"
+      }
+    }
+  }
+}
+--------------------------------------------------
+<1> The number of dimensions that represents the number of bits
+
+[source,console]
+--------------------------------------------------
+POST /my-bit-vectors/_bulk?refresh
+{"index": {"_id" : "1"}}
+{"my_vector": [127, -127, 0, 1, 42]} <1>
+{"index": {"_id" : "2"}}
+{"my_vector": "8100012a7f"} <2>
+--------------------------------------------------
+// TEST[continued]
+<1> 5 bytes representing the 40 bit dimensioned vector
+<2> A hexidecimal string representing the 40 bit dimensioned vector
+
+Then, when searching, you can use the `knn` query to search for similar bit vectors:
+
+[source,console]
+--------------------------------------------------
+POST /my-bit-vectors/_search?filter_path=hits.hits
+{
+  "query": {
+    "knn": {
+      "query_vector": [127, -127, 0, 1, 42],
+      "field": "my_vector"
+    }
+  }
+}
+--------------------------------------------------
+// TEST[continued]
+
+[source,console-result]
+----
+{
+    "hits": {
+        "hits": [
+            {
+                "_index": "my-bit-vectors",
+                "_id": "1",
+                "_score": 1.0,
+                "_source": {
+                    "my_vector": [
+                        127,
+                        -127,
+                        0,
+                        1,
+                        42
+                    ]
+                }
+            },
+            {
+                "_index": "my-bit-vectors",
+                "_id": "2",
+                "_score": 0.55,
+                "_source": {
+                    "my_vector": "8100012a7f"
+                }
+            }
+        ]
+    }
+}
+----
+

docs/reference/vectors/vector-functions.asciidoc

@@ -1,4 +1,3 @@
-[role="xpack"]
 [[vector-functions]]
 ===== Functions for vector fields
 
@@ -17,6 +16,8 @@ This is the list of available vector functions and vector access methods:
 6. <<vector-functions-accessing-vectors,`doc[<field>].vectorValue`>> – returns a vector's value as an array of floats
 7. <<vector-functions-accessing-vectors,`doc[<field>].magnitude`>> – returns a vector's magnitude
 
+NOTE: The `cosineSimilarity` and `dotProduct` functions are not supported for `bit` vectors.
+
 NOTE: The recommended way to access dense vectors is through the
 `cosineSimilarity`, `dotProduct`, `l1norm` or `l2norm` functions. Please note
 however, that you should call these functions only once per script. For example,
@@ -193,7 +194,7 @@ we added `1` in the denominator.
 ====== Hamming distance
 
 The `hamming` function calculates {wikipedia}/Hamming_distance[Hamming distance] between a given query vector and
-document vectors. It is only available for byte vectors.
+document vectors. It is only available for byte and bit vectors.
 
 [source,console]
 --------------------------------------------------
@@ -278,10 +279,14 @@ You can access vector values directly through the following functions:
 
 - `doc[<field>].vectorValue` – returns a vector's value as an array of floats
 
+NOTE: For `bit` vectors, it does return a `float[]`, where each element represents 8 bits.
+
 - `doc[<field>].magnitude` – returns a vector's magnitude as a float
 (for vectors created prior to version 7.5 the magnitude is not stored.
 So this function calculates it anew every time it is called).
 
+NOTE: For `bit` vectors, this is just the square root of the sum of `1` bits.
+
 For example, the script below implements a cosine similarity using these
 two functions:
 
@@ -319,3 +324,14 @@ GET my-index-000001/_search
   }
 }
 --------------------------------------------------
+[[vector-functions-bit-vectors]]
+====== Bit vectors and vector functions
+
+When using `bit` vectors, not all the vector functions are available. The supported functions are:
+
+* <<vector-functions-hamming,`hamming`>> – calculates Hamming distance, the sum of the bitwise XOR of the two vectors
+* <<vector-functions-l1,`l1norm`>> – calculates L^1^ distance, this is simply the `hamming` distance
+* <<vector-functions-l2,`l2norm`>> - calculates L^2^ distance, this is the square root of the `hamming` distance
+
+Currently, the `cosineSimilarity` and `dotProduct` functions are not supported for `bit` vectors.
+

modules/lang-painless/src/yamlRestTest/resources/rest-api-spec/test/painless/146_dense_vector_bit_basic.yml

@@ -0,0 +1,392 @@
+setup:
+  - requires:
+      cluster_features: ["mapper.vectors.bit_vectors"]
+      reason: "support for bit vectors added in 8.15"
+      test_runner_features: headers
+
+  - do:
+      indices.create:
+        index: test-index
+        body:
+          mappings:
+            properties:
+              vector:
+                type: dense_vector
+                index: false
+                element_type: bit
+                dims: 40
+              indexed_vector:
+                type: dense_vector
+                element_type: bit
+                dims: 40
+                index: true
+                similarity: l2_norm
+
+  - do:
+      index:
+        index: test-index
+        id: "1"
+        body:
+          vector: [8, 5, -15, 1, -7]
+          indexed_vector: [8, 5, -15, 1, -7]
+
+  - do:
+      index:
+        index: test-index
+        id: "2"
+        body:
+          vector: [-1, 115, -3, 4, -128]
+          indexed_vector: [-1, 115, -3, 4, -128]
+
+  - do:
+      index:
+        index: test-index
+        id: "3"
+        body:
+          vector: [2, 18, -5, 0, -124]
+          indexed_vector: [2, 18, -5, 0, -124]
+
+  - do:
+      indices.refresh: {}
+
+---
+"Test vector magnitude equality":
+  - skip:
+      features: close_to
+
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "doc['vector'].magnitude"
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - close_to: {hits.hits.0._score: {value: 4.690416, error: 0.01}}
+
+  - match: {hits.hits.1._id: "1"}
+  - close_to: {hits.hits.1._score: {value: 3.8729835, error: 0.01}}
+
+  - match: {hits.hits.2._id: "3"}
+  - close_to: {hits.hits.2._score: {value: 3.4641016, error: 0.01}}
+
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "doc['indexed_vector'].magnitude"
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - close_to: {hits.hits.0._score: {value: 4.690416, error: 0.01}}
+
+  - match: {hits.hits.1._id: "1"}
+  - close_to: {hits.hits.1._score: {value: 3.8729835, error: 0.01}}
+
+  - match: {hits.hits.2._id: "3"}
+  - close_to: {hits.hits.2._score: {value: 3.4641016, error: 0.01}}
+
+---
+"Dot Product is not supported":
+  - do:
+      catch: bad_request
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "dotProduct(params.query_vector, 'vector')"
+                params:
+                  query_vector: [0, 111, -13, 14, -124]
+  - do:
+      catch: bad_request
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "dotProduct(params.query_vector, 'vector')"
+                params:
+                  query_vector: "006ff30e84"
+
+---
+"Cosine Similarity is not supported":
+  - do:
+      catch: bad_request
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "cosineSimilarity(params.query_vector, 'vector')"
+                params:
+                  query_vector: [0, 111, -13, 14, -124]
+  - do:
+      catch: bad_request
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "cosineSimilarity(params.query_vector, 'vector')"
+                params:
+                  query_vector: "006ff30e84"
+
+  - do:
+      catch: bad_request
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "cosineSimilarity(params.query_vector, 'indexed_vector')"
+                params:
+                  query_vector: [0, 111, -13, 14, -124]
+---
+"L1 norm":
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "l1norm(params.query_vector, 'vector')"
+                params:
+                  query_vector: [0, 111, -13, 14, -124]
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0._score: 17.0}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1._score: 16.0}
+
+  - match: {hits.hits.2._id: "3"}
+  - match: {hits.hits.2._score: 11.0}
+
+---
+"L1 norm hexidecimal":
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "l1norm(params.query_vector, 'vector')"
+                params:
+                  query_vector: "006ff30e84"
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0._score: 17.0}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1._score: 16.0}
+
+  - match: {hits.hits.2._id: "3"}
+  - match: {hits.hits.2._score: 11.0}
+---
+"L2 norm":
+  - requires:
+      test_runner_features: close_to
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "l2norm(params.query_vector, 'vector')"
+                params:
+                  query_vector: [0, 111, -13, 14, -124]
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - close_to: {hits.hits.0._score: {value: 4.123, error: 0.001}}
+
+  - match: {hits.hits.1._id: "1"}
+  - close_to: {hits.hits.1._score: {value: 4, error: 0.001}}
+
+  - match: {hits.hits.2._id: "3"}
+  - close_to: {hits.hits.2._score: {value: 3.316, error: 0.001}}
+---
+"L2 norm hexidecimal":
+  - requires:
+      test_runner_features: close_to
+
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "l2norm(params.query_vector, 'vector')"
+                params:
+                  query_vector: "006ff30e84"
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - close_to: {hits.hits.0._score: {value: 4.123, error: 0.001}}
+
+  - match: {hits.hits.1._id: "1"}
+  - close_to: {hits.hits.1._score: {value: 4, error: 0.001}}
+
+  - match: {hits.hits.2._id: "3"}
+  - close_to: {hits.hits.2._score: {value: 3.316, error: 0.001}}
+---
+"Hamming distance":
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "hamming(params.query_vector, 'vector')"
+                params:
+                  query_vector: [0, 111, -13, 14, -124]
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0._score: 17.0}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1._score: 16.0}
+
+  - match: {hits.hits.2._id: "3"}
+  - match: {hits.hits.2._score: 11.0}
+
+
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "hamming(params.query_vector, 'indexed_vector')"
+                params:
+                  query_vector: [0, 111, -13, 14, -124]
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0._score: 17.0}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1._score: 16.0}
+
+  - match: {hits.hits.2._id: "3"}
+  - match: {hits.hits.2._score: 11.0}
+---
+"Hamming distance hexidecimal":
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "hamming(params.query_vector, 'vector')"
+                params:
+                  query_vector: "006ff30e84"
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0._score: 17.0}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1._score: 16.0}
+
+  - match: {hits.hits.2._id: "3"}
+  - match: {hits.hits.2._score: 11.0}
+
+
+  - do:
+      headers:
+        Content-Type: application/json
+      search:
+        rest_total_hits_as_int: true
+        body:
+          query:
+            script_score:
+              query: {match_all: {} }
+              script:
+                source: "hamming(params.query_vector, 'indexed_vector')"
+                params:
+                  query_vector: "006ff30e84"
+
+  - match: {hits.total: 3}
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0._score: 17.0}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1._score: 16.0}
+
+  - match: {hits.hits.2._id: "3"}
+  - match: {hits.hits.2._score: 11.0}

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/101_knn_nested_search_bits.yml

@@ -0,0 +1,301 @@
+setup:
+  - requires:
+      cluster_features: "mapper.vectors.bit_vectors"
+      test_runner_features: close_to
+      reason: 'bit vectors added in 8.15'
+  - do:
+      indices.create:
+        index: test
+        body:
+          settings:
+            index:
+              number_of_shards: 2
+          mappings:
+            properties:
+              name:
+                type: keyword
+              nested:
+                type: nested
+                properties:
+                  paragraph_id:
+                    type: keyword
+                  vector:
+                    type: dense_vector
+                    dims: 40
+                    index: true
+                    element_type: bit
+                    similarity: l2_norm
+
+  - do:
+      index:
+        index: test
+        id: "1"
+        body:
+          name: cow.jpg
+          nested:
+          - paragraph_id: 0
+            vector: [100, 20, -34, 15, -100]
+          - paragraph_id: 1
+            vector: [40, 30, -3, 1, -20]
+
+  - do:
+      index:
+        index: test
+        id: "2"
+        body:
+          name: moose.jpg
+          nested:
+          - paragraph_id: 0
+            vector: [-1, 100, -13, 14, -127]
+          - paragraph_id: 2
+            vector: [0, 100, 0, 15, -127]
+          - paragraph_id: 3
+            vector: [0, 1, 0, 2, -15]
+
+  - do:
+      index:
+        index: test
+        id: "3"
+        body:
+          name: rabbit.jpg
+          nested:
+            - paragraph_id: 0
+              vector: [1, 111, -13, 14, -1]
+
+  - do:
+      indices.refresh: {}
+
+---
+"nested kNN search only":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [-1, 90, -10, 14, -127]
+            k: 2
+            num_candidates: 3
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0.fields.name.0: "moose.jpg"}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1.fields.name.0: "cow.jpg"}
+
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [-1, 90, -10, 14, -127]
+            k: 2
+            num_candidates: 3
+            inner_hits: {size: 1, "fields": ["nested.paragraph_id"], _source: false}
+
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0.fields.name.0: "moose.jpg"}
+  - match: {hits.hits.0.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0"}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1.fields.name.0: "cow.jpg"}
+  - match: {hits.hits.1.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0"}
+
+---
+"nested kNN search filtered":
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [-1, 90, -10, 14, -127]
+            k: 2
+            num_candidates: 3
+            filter: {term: {name: "rabbit.jpg"}}
+
+  - match: {hits.total.value: 1}
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [-1, 90, -10, 14, -127]
+            k: 3
+            num_candidates: 3
+            filter: {term: {name: "rabbit.jpg"}}
+            inner_hits: {size: 1, fields: ["nested.paragraph_id"], _source: false}
+
+  - match: {hits.total.value: 1}
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+  - match: {hits.hits.0.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0"}
+---
+"nested kNN search inner_hits size > 1":
+  - do:
+      index:
+        index: test
+        id: "4"
+        body:
+          name: moose.jpg
+          nested:
+            - paragraph_id: 0
+              vector: [-1, 90, -10, 14, -127]
+            - paragraph_id: 2
+              vector: [ 0, 100.0, 0, 14, -127 ]
+            - paragraph_id: 3
+              vector: [ 0, 1.0, 0, 2, -15 ]
+
+  - do:
+      index:
+        index: test
+        id: "5"
+        body:
+          name: moose.jpg
+          nested:
+            - paragraph_id: 0
+              vector: [ -1, 100, -13, 14, -127 ]
+            - paragraph_id: 2
+              vector: [ 0, 100, 0, 15, -127 ]
+            - paragraph_id: 3
+              vector: [ 0, 1, 0, 2, -15 ]
+
+  - do:
+      index:
+        index: test
+        id: "6"
+        body:
+          name: moose.jpg
+          nested:
+            - paragraph_id: 0
+              vector: [ -1, 100, -13, 15, -127 ]
+            - paragraph_id: 2
+              vector: [ 0, 100, 0, 15, -127 ]
+            - paragraph_id: 3
+              vector: [ 0, 1, 0, 2, -15 ]
+  - do:
+      indices.refresh: { }
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [-1, 90, -10, 15, -127]
+            k: 3
+            num_candidates: 5
+            inner_hits: {size: 2, fields: ["nested.paragraph_id"], _source: false}
+
+  - match: {hits.total.value: 3}
+  - length: { hits.hits.0.inner_hits.nested.hits.hits: 2 }
+  - length: { hits.hits.1.inner_hits.nested.hits.hits: 2 }
+  - length: { hits.hits.2.inner_hits.nested.hits.hits: 2 }
+
+  - match: { hits.hits.0.fields.name.0: "moose.jpg" }
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0" }
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [-1, 90, -10, 15, -127]
+            k: 5
+            num_candidates: 5
+            inner_hits: {size: 2, fields: ["nested.paragraph_id"], _source: false}
+
+  - match: {hits.total.value: 5}
+  # All these initial matches are "moose.jpg", which has 3 nested vectors, but two are closest
+  - match: {hits.hits.0.fields.name.0: "moose.jpg"}
+  - length: { hits.hits.0.inner_hits.nested.hits.hits: 2 }
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0" }
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.1.fields.nested.0.paragraph_id.0: "2" }
+  - match: {hits.hits.1.fields.name.0: "moose.jpg"}
+  - length: { hits.hits.1.inner_hits.nested.hits.hits: 2 }
+  - match: { hits.hits.1.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0" }
+  - match: { hits.hits.1.inner_hits.nested.hits.hits.1.fields.nested.0.paragraph_id.0: "2" }
+  - match: {hits.hits.2.fields.name.0: "moose.jpg"}
+  - length: { hits.hits.2.inner_hits.nested.hits.hits: 2 }
+  - match: { hits.hits.2.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0" }
+  - match: { hits.hits.2.inner_hits.nested.hits.hits.1.fields.nested.0.paragraph_id.0: "2" }
+  - match: {hits.hits.3.fields.name.0: "moose.jpg"}
+  - length: { hits.hits.3.inner_hits.nested.hits.hits: 2 }
+  - match: { hits.hits.3.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0" }
+  - match: { hits.hits.3.inner_hits.nested.hits.hits.1.fields.nested.0.paragraph_id.0: "2" }
+  # Rabbit only has one passage vector
+  - match: {hits.hits.4.fields.name.0: "cow.jpg"}
+  - length: { hits.hits.4.inner_hits.nested.hits.hits: 2 }
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [ -1, 90, -10, 15, -127 ]
+            k: 3
+            num_candidates: 3
+            filter: {term: {name: "cow.jpg"}}
+            inner_hits: {size: 3, fields: ["nested.paragraph_id"], _source: false}
+
+  - match: {hits.total.value: 1}
+  - match: { hits.hits.0._id: "1" }
+  - length: { hits.hits.0.inner_hits.nested.hits.hits: 2 }
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.fields.nested.0.paragraph_id.0: "0" }
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.1.fields.nested.0.paragraph_id.0: "1" }
+---
+"nested kNN search inner_hits & boosting":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [-1, 90, -10, 15, -127]
+            k: 3
+            num_candidates: 5
+            inner_hits: {size: 2, fields: ["nested.paragraph_id"], _source: false}
+
+  - close_to: { hits.hits.0._score: {value: 0.8, error: 0.00001} }
+  - close_to: { hits.hits.0.inner_hits.nested.hits.hits.0._score: {value: 0.8, error: 0.00001} }
+  - close_to: { hits.hits.1._score: {value: 0.625, error: 0.00001} }
+  - close_to: { hits.hits.1.inner_hits.nested.hits.hits.0._score: {value: 0.625, error: 0.00001} }
+  - close_to: { hits.hits.2._score: {value: 0.5, error: 0.00001} }
+  - close_to: { hits.hits.2.inner_hits.nested.hits.hits.0._score: {value: 0.5, error: 0.00001} }
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nested.vector
+            query_vector: [-1, 90, -10, 15, -127]
+            k: 3
+            num_candidates: 5
+            boost: 2
+            inner_hits: {size: 2, fields: ["nested.paragraph_id"], _source: false}
+  - close_to: { hits.hits.0._score:  {value: 1.6, error: 0.00001} }
+  - close_to: { hits.hits.0.inner_hits.nested.hits.hits.0._score:  {value: 1.6, error: 0.00001} }
+  - close_to: { hits.hits.1._score:  {value: 1.25, error: 0.00001} }
+  - close_to: { hits.hits.1.inner_hits.nested.hits.hits.0._score:  {value: 1.25, error: 0.00001} }
+  - close_to: { hits.hits.2._score:  {value: 1, error: 0.00001} }
+  - close_to: { hits.hits.2.inner_hits.nested.hits.hits.0._score:  {value: 1.0, error: 0.00001} }

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/170_knn_search_hex_encoded_byte_vectors.yml

@@ -116,8 +116,9 @@ setup:
 ---
 "Knn search with hex string for byte field - dimensions mismatch" :
   # [64, 10, -30, 10] - is encoded as '400ae20a'
+  # the error message has been adjusted in later versions
   - do:
-      catch: /the query vector has a different dimension \[4\] than the index vectors \[3\]/
+      catch: /dimension|dimensions \[4\] than the document|index vectors \[3\]/
       search:
         index: knn_hex_vector_index
         body:

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/175_knn_query_hex_encoded_byte_vectors.yml

@@ -116,8 +116,9 @@ setup:
 ---
 "Knn query with hex string for byte field - dimensions mismatch" :
   # [64, 10, -30, 10] - is encoded as '400ae20a'
+  # the error message has been adjusted in later versions
   - do:
-      catch: /the query vector has a different dimension \[4\] than the index vectors \[3\]/
+      catch: /dimension|dimensions \[4\] than the document|index vectors \[3\]/
       search:
         index: knn_hex_vector_index
         body:

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/45_knn_search_bit.yml

@@ -0,0 +1,356 @@
+setup:
+  - requires:
+      cluster_features: "mapper.vectors.bit_vectors"
+      reason: 'mapper.vectors.bit_vectors'
+
+  - do:
+      indices.create:
+        index: test
+        body:
+          mappings:
+            properties:
+              name:
+                type: keyword
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 40
+                index: true
+                similarity: l2_norm
+
+  - do:
+      index:
+        index: test
+        id: "1"
+        body:
+          name: cow.jpg
+          vector: [2, -1, 1, 4, -3]
+
+  - do:
+      index:
+        index: test
+        id: "2"
+        body:
+          name: moose.jpg
+          vector: [127.0, -128.0, 0.0, 1.0, -1.0]
+
+  - do:
+      index:
+        index: test
+        id: "3"
+        body:
+          name: rabbit.jpg
+          vector: [5, 4.0, 3, 2.0, 127]
+
+  - do:
+      indices.refresh: {}
+
+---
+"kNN search only":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [127, 127, -128, -128, 127]
+            k: 2
+            num_candidates: 3
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0.fields.name.0: "moose.jpg"}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1.fields.name.0: "cow.jpg"}
+
+---
+"kNN search plus query":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [127.0, -128.0, 0.0, 1.0, -1.0]
+            k: 2
+            num_candidates: 3
+          query:
+            term:
+              name: rabbit.jpg
+
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+
+  - match: {hits.hits.1._id: "2"}
+  - match: {hits.hits.1.fields.name.0: "moose.jpg"}
+
+---
+"kNN search with filter":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [5.0, 4, 3.0, 2, 127.0]
+            k: 2
+            num_candidates: 3
+            filter:
+              term:
+                name: "rabbit.jpg"
+
+  - match: {hits.total.value: 1}
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [2, -1, 1, 4, -3]
+            k: 2
+            num_candidates: 3
+            filter:
+              - term:
+                  name: "rabbit.jpg"
+              - term:
+                  _id: 2
+
+  - match: {hits.total.value: 0}
+
+---
+"Vector similarity search only":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            num_candidates: 3
+            k: 3
+            field: vector
+            similarity: 0.98
+            query_vector: [5, 4.0, 3, 2.0, 127]
+
+  - length: {hits.hits: 1}
+
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+---
+"Vector similarity with filter only":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            num_candidates: 3
+            k: 3
+            field: vector
+            similarity: 0.98
+            query_vector: [5, 4.0, 3, 2.0, 127]
+            filter: {"term": {"name": "rabbit.jpg"}}
+
+  - length: {hits.hits: 1}
+
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            num_candidates: 3
+            k: 3
+            field: vector
+            similarity: 0.98
+            query_vector: [5, 4.0, 3, 2.0, 127]
+            filter: {"term": {"name": "cow.jpg"}}
+
+  - length: {hits.hits: 0}
+---
+"dim mismatch":
+  - do:
+      catch: bad_request
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [1, 2, 3, 4, 5, 6]
+            k: 2
+            num_candidates: 3
+---
+"disallow quantized vector types":
+  - do:
+      catch: bad_request
+      indices.create:
+        index: test
+        body:
+          mappings:
+            properties:
+              name:
+                type: keyword
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 32
+                index: true
+                similarity: l2_norm
+                index_options:
+                  type: int8_flat
+
+  - do:
+      catch: bad_request
+      indices.create:
+        index: test
+        body:
+          mappings:
+            properties:
+              name:
+                type: keyword
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 32
+                index: true
+                similarity: l2_norm
+                index_options:
+                  type: int4_flat
+
+  - do:
+      catch: bad_request
+      indices.create:
+        index: test
+        body:
+          mappings:
+            properties:
+              name:
+                type: keyword
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 32
+                index: true
+                similarity: l2_norm
+                index_options:
+                  type: int8_hnsw
+
+  - do:
+      catch: bad_request
+      indices.create:
+        index: test
+        body:
+          mappings:
+            properties:
+              name:
+                type: keyword
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 32
+                index: true
+                similarity: l2_norm
+                index_options:
+                  type: int4_hnsw
+---
+"disallow vector index type change to quantized type":
+  - do:
+      catch: bad_request
+      indices.put_mapping:
+        index: test
+        body:
+          properties:
+            vector:
+              type: dense_vector
+              element_type: bit
+              dims: 32
+              index: true
+              similarity: l2_norm
+              index_options:
+                type: int4_hnsw
+  - do:
+      catch: bad_request
+      indices.put_mapping:
+        index: test
+        body:
+          properties:
+            vector:
+              type: dense_vector
+              element_type: bit
+              dims: 32
+              index: true
+              similarity: l2_norm
+              index_options:
+                type: int8_hnsw
+---
+"Defaults to l2_norm with bit vectors":
+  - do:
+      indices.create:
+        index: default_to_l2_norm_bit
+        body:
+          mappings:
+            properties:
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 40
+                index: true
+
+  - do:
+      indices.get_mapping:
+        index: default_to_l2_norm_bit
+
+  - match: { default_to_l2_norm_bit.mappings.properties.vector.similarity: l2_norm }
+
+---
+"Only allow l2_norm with bit vectors":
+  - do:
+      catch: bad_request
+      indices.create:
+        index: dot_product_fails_for_bits
+        body:
+          mappings:
+            properties:
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 40
+                index: true
+                similarity: dot_product
+
+  - do:
+      catch: bad_request
+      indices.create:
+        index: cosine_product_fails_for_bits
+        body:
+          mappings:
+            properties:
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 40
+                index: true
+                similarity: cosine
+
+  - do:
+      catch: bad_request
+      indices.create:
+        index: cosine_product_fails_for_bits
+        body:
+          mappings:
+            properties:
+              type: dense_vector
+              element_type: bit
+              dims: 40
+              index: true
+              similarity: max_inner_product

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/45_knn_search_bit_flat.yml

@@ -0,0 +1,223 @@
+setup:
+  - requires:
+      cluster_features: "mapper.vectors.bit_vectors"
+      reason: 'mapper.vectors.bit_vectors'
+
+  - do:
+      indices.create:
+        index: test
+        body:
+          mappings:
+            properties:
+              name:
+                type: keyword
+              vector:
+                type: dense_vector
+                element_type: bit
+                dims: 40
+                index: true
+                similarity: l2_norm
+                index_options:
+                  type: flat
+
+  - do:
+      index:
+        index: test
+        id: "1"
+        body:
+          name: cow.jpg
+          vector: [2, -1, 1, 4, -3]
+
+  - do:
+      index:
+        index: test
+        id: "2"
+        body:
+          name: moose.jpg
+          vector: [127.0, -128.0, 0.0, 1.0, -1.0]
+
+  - do:
+      index:
+        index: test
+        id: "3"
+        body:
+          name: rabbit.jpg
+          vector: [5, 4.0, 3, 2.0, 127]
+
+  - do:
+      indices.refresh: {}
+
+---
+"kNN search only":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [127, 127, -128, -128, 127]
+            k: 2
+            num_candidates: 3
+
+  - match: {hits.hits.0._id: "2"}
+  - match: {hits.hits.0.fields.name.0: "moose.jpg"}
+
+  - match: {hits.hits.1._id: "1"}
+  - match: {hits.hits.1.fields.name.0: "cow.jpg"}
+
+---
+"kNN search plus query":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [127.0, -128.0, 0.0, 1.0, -1.0]
+            k: 2
+            num_candidates: 3
+          query:
+            term:
+              name: rabbit.jpg
+
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+
+  - match: {hits.hits.1._id: "2"}
+  - match: {hits.hits.1.fields.name.0: "moose.jpg"}
+
+---
+"kNN search with filter":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [5.0, 4, 3.0, 2, 127.0]
+            k: 2
+            num_candidates: 3
+            filter:
+              term:
+                name: "rabbit.jpg"
+
+  - match: {hits.total.value: 1}
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [2, -1, 1, 4, -3]
+            k: 2
+            num_candidates: 3
+            filter:
+              - term:
+                  name: "rabbit.jpg"
+              - term:
+                  _id: 2
+
+  - match: {hits.total.value: 0}
+
+---
+"Vector similarity search only":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            num_candidates: 3
+            k: 3
+            field: vector
+            similarity: 0.98
+            query_vector: [5, 4.0, 3, 2.0, 127]
+
+  - length: {hits.hits: 1}
+
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+---
+"Vector similarity with filter only":
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            num_candidates: 3
+            k: 3
+            field: vector
+            similarity: 0.98
+            query_vector: [5, 4.0, 3, 2.0, 127]
+            filter: {"term": {"name": "rabbit.jpg"}}
+
+  - length: {hits.hits: 1}
+
+  - match: {hits.hits.0._id: "3"}
+  - match: {hits.hits.0.fields.name.0: "rabbit.jpg"}
+
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            num_candidates: 3
+            k: 3
+            field: vector
+            similarity: 0.98
+            query_vector: [5, 4.0, 3, 2.0, 127]
+            filter: {"term": {"name": "cow.jpg"}}
+
+  - length: {hits.hits: 0}
+---
+"dim mismatch":
+  - do:
+      catch: bad_request
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: vector
+            query_vector: [1, 2, 3, 4, 5, 6]
+            k: 2
+            num_candidates: 3
+---
+"disallow vector index type change to quantized type":
+  - do:
+      catch: bad_request
+      indices.put_mapping:
+        index: test
+        body:
+         properties:
+           vector:
+             type: dense_vector
+             element_type: bit
+             dims: 32
+             index: true
+             similarity: l2_norm
+             index_options:
+               type: int4_hnsw
+  - do:
+      catch: bad_request
+      indices.put_mapping:
+        index: test
+        body:
+          properties:
+            vector:
+              type: dense_vector
+              element_type: bit
+              dims: 32
+              index: true
+              similarity: l2_norm
+              index_options:
+                type: int8_hnsw

server/src/main/java/module-info.java

@@ -449,7 +449,10 @@ module org.elasticsearch.server {
         with
             org.elasticsearch.index.codec.vectors.ES813FlatVectorFormat,
             org.elasticsearch.index.codec.vectors.ES813Int8FlatVectorFormat,
-            org.elasticsearch.index.codec.vectors.ES814HnswScalarQuantizedVectorsFormat;
+            org.elasticsearch.index.codec.vectors.ES814HnswScalarQuantizedVectorsFormat,
+            org.elasticsearch.index.codec.vectors.ES815HnswBitVectorsFormat,
+            org.elasticsearch.index.codec.vectors.ES815BitFlatVectorFormat;
+
     provides org.apache.lucene.codecs.Codec with Elasticsearch814Codec;
 
     provides org.apache.logging.log4j.core.util.ContextDataProvider with org.elasticsearch.common.logging.DynamicContextDataProvider;

server/src/main/java/org/elasticsearch/index/codec/vectors/ES813FlatVectorFormat.java

@@ -54,11 +54,11 @@ public class ES813FlatVectorFormat extends KnnVectorsFormat {
         return new ES813FlatVectorReader(format.fieldsReader(state));
     }
 
-    public static class ES813FlatVectorWriter extends KnnVectorsWriter {
+    static class ES813FlatVectorWriter extends KnnVectorsWriter {
 
         private final FlatVectorsWriter writer;
 
-        public ES813FlatVectorWriter(FlatVectorsWriter writer) {
+        ES813FlatVectorWriter(FlatVectorsWriter writer) {
             super();
             this.writer = writer;
         }
@@ -94,11 +94,11 @@ public class ES813FlatVectorFormat extends KnnVectorsFormat {
         }
     }
 
-    public static class ES813FlatVectorReader extends KnnVectorsReader {
+    static class ES813FlatVectorReader extends KnnVectorsReader {
 
         private final FlatVectorsReader reader;
 
-        public ES813FlatVectorReader(FlatVectorsReader reader) {
+        ES813FlatVectorReader(FlatVectorsReader reader) {
             super();
             this.reader = reader;
         }

server/src/main/java/org/elasticsearch/index/codec/vectors/ES815BitFlatVectorFormat.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.index.codec.vectors;
+
+import org.apache.lucene.codecs.KnnVectorsFormat;
+import org.apache.lucene.codecs.KnnVectorsReader;
+import org.apache.lucene.codecs.KnnVectorsWriter;
+import org.apache.lucene.codecs.hnsw.FlatVectorsFormat;
+import org.apache.lucene.index.SegmentReadState;
+import org.apache.lucene.index.SegmentWriteState;
+
+import java.io.IOException;
+
+public class ES815BitFlatVectorFormat extends KnnVectorsFormat {
+
+    static final String NAME = "ES815BitFlatVectorFormat";
+
+    private final FlatVectorsFormat format = new ES815BitFlatVectorsFormat();
+
+    /**
+     * Sole constructor
+     */
+    public ES815BitFlatVectorFormat() {
+        super(NAME);
+    }
+
+    @Override
+    public KnnVectorsWriter fieldsWriter(SegmentWriteState state) throws IOException {
+        return new ES813FlatVectorFormat.ES813FlatVectorWriter(format.fieldsWriter(state));
+    }
+
+    @Override
+    public KnnVectorsReader fieldsReader(SegmentReadState state) throws IOException {
+        return new ES813FlatVectorFormat.ES813FlatVectorReader(format.fieldsReader(state));
+    }
+
+    @Override
+    public String toString() {
+        return NAME;
+    }
+}

server/src/main/java/org/elasticsearch/index/codec/vectors/ES815BitFlatVectorsFormat.java

@@ -0,0 +1,143 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.index.codec.vectors;
+
+import org.apache.lucene.codecs.hnsw.FlatVectorsFormat;
+import org.apache.lucene.codecs.hnsw.FlatVectorsReader;
+import org.apache.lucene.codecs.hnsw.FlatVectorsScorer;
+import org.apache.lucene.codecs.hnsw.FlatVectorsWriter;
+import org.apache.lucene.codecs.lucene99.Lucene99FlatVectorsFormat;
+import org.apache.lucene.index.SegmentReadState;
+import org.apache.lucene.index.SegmentWriteState;
+import org.apache.lucene.index.VectorSimilarityFunction;
+import org.apache.lucene.util.VectorUtil;
+import org.apache.lucene.util.hnsw.RandomAccessVectorValues;
+import org.apache.lucene.util.hnsw.RandomVectorScorer;
+import org.apache.lucene.util.hnsw.RandomVectorScorerSupplier;
+import org.apache.lucene.util.quantization.RandomAccessQuantizedByteVectorValues;
+
+import java.io.IOException;
+
+class ES815BitFlatVectorsFormat extends FlatVectorsFormat {
+
+    private final FlatVectorsFormat delegate = new Lucene99FlatVectorsFormat(FlatBitVectorScorer.INSTANCE);
+
+    @Override
+    public FlatVectorsWriter fieldsWriter(SegmentWriteState segmentWriteState) throws IOException {
+        return delegate.fieldsWriter(segmentWriteState);
+    }
+
+    @Override
+    public FlatVectorsReader fieldsReader(SegmentReadState segmentReadState) throws IOException {
+        return delegate.fieldsReader(segmentReadState);
+    }
+
+    static class FlatBitVectorScorer implements FlatVectorsScorer {
+
+        static final FlatBitVectorScorer INSTANCE = new FlatBitVectorScorer();
+
+        static void checkDimensions(int queryLen, int fieldLen) {
+            if (queryLen != fieldLen) {
+                throw new IllegalArgumentException("vector query dimension: " + queryLen + " differs from field dimension: " + fieldLen);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return super.toString();
+        }
+
+        @Override
+        public RandomVectorScorerSupplier getRandomVectorScorerSupplier(
+            VectorSimilarityFunction vectorSimilarityFunction,
+            RandomAccessVectorValues randomAccessVectorValues
+        ) throws IOException {
+            assert randomAccessVectorValues instanceof RandomAccessVectorValues.Bytes;
+            assert vectorSimilarityFunction == VectorSimilarityFunction.EUCLIDEAN;
+            if (randomAccessVectorValues instanceof RandomAccessVectorValues.Bytes randomAccessVectorValuesBytes) {
+                assert randomAccessVectorValues instanceof RandomAccessQuantizedByteVectorValues == false;
+                return switch (vectorSimilarityFunction) {
+                    case DOT_PRODUCT, MAXIMUM_INNER_PRODUCT, COSINE, EUCLIDEAN -> new HammingScorerSupplier(randomAccessVectorValuesBytes);
+                };
+            }
+            throw new IllegalArgumentException("Unsupported vector type or similarity function");
+        }
+
+        @Override
+        public RandomVectorScorer getRandomVectorScorer(
+            VectorSimilarityFunction vectorSimilarityFunction,
+            RandomAccessVectorValues randomAccessVectorValues,
+            byte[] bytes
+        ) {
+            assert randomAccessVectorValues instanceof RandomAccessVectorValues.Bytes;
+            assert vectorSimilarityFunction == VectorSimilarityFunction.EUCLIDEAN;
+            if (randomAccessVectorValues instanceof RandomAccessVectorValues.Bytes randomAccessVectorValuesBytes) {
+                checkDimensions(bytes.length, randomAccessVectorValuesBytes.dimension());
+                return switch (vectorSimilarityFunction) {
+                    case DOT_PRODUCT, MAXIMUM_INNER_PRODUCT, COSINE, EUCLIDEAN -> new HammingVectorScorer(
+                        randomAccessVectorValuesBytes,
+                        bytes
+                    );
+                };
+            }
+            throw new IllegalArgumentException("Unsupported vector type or similarity function");
+        }
+
+        @Override
+        public RandomVectorScorer getRandomVectorScorer(
+            VectorSimilarityFunction vectorSimilarityFunction,
+            RandomAccessVectorValues randomAccessVectorValues,
+            float[] floats
+        ) {
+            throw new IllegalArgumentException("Unsupported vector type");
+        }
+    }
+
+    static float hammingScore(byte[] a, byte[] b) {
+        return ((a.length * Byte.SIZE) - VectorUtil.xorBitCount(a, b)) / (float) (a.length * Byte.SIZE);
+    }
+
+    static class HammingVectorScorer extends RandomVectorScorer.AbstractRandomVectorScorer {
+        private final byte[] query;
+        private final RandomAccessVectorValues.Bytes byteValues;
+
+        HammingVectorScorer(RandomAccessVectorValues.Bytes byteValues, byte[] query) {
+            super(byteValues);
+            this.query = query;
+            this.byteValues = byteValues;
+        }
+
+        @Override
+        public float score(int i) throws IOException {
+            return hammingScore(byteValues.vectorValue(i), query);
+        }
+    }
+
+    static class HammingScorerSupplier implements RandomVectorScorerSupplier {
+        private final RandomAccessVectorValues.Bytes byteValues, byteValues1, byteValues2;
+
+        HammingScorerSupplier(RandomAccessVectorValues.Bytes byteValues) throws IOException {
+            this.byteValues = byteValues;
+            this.byteValues1 = byteValues.copy();
+            this.byteValues2 = byteValues.copy();
+        }
+
+        @Override
+        public RandomVectorScorer scorer(int i) throws IOException {
+            byte[] query = byteValues1.vectorValue(i);
+            return new HammingVectorScorer(byteValues2, query);
+        }
+
+        @Override
+        public RandomVectorScorerSupplier copy() throws IOException {
+            return new HammingScorerSupplier(byteValues);
+        }
+    }
+
+}

server/src/main/java/org/elasticsearch/index/codec/vectors/ES815HnswBitVectorsFormat.java

@@ -0,0 +1,74 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.index.codec.vectors;
+
+import org.apache.lucene.codecs.KnnVectorsFormat;
+import org.apache.lucene.codecs.KnnVectorsReader;
+import org.apache.lucene.codecs.KnnVectorsWriter;
+import org.apache.lucene.codecs.hnsw.FlatVectorsFormat;
+import org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsReader;
+import org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsWriter;
+import org.apache.lucene.index.SegmentReadState;
+import org.apache.lucene.index.SegmentWriteState;
+
+import java.io.IOException;
+
+public class ES815HnswBitVectorsFormat extends KnnVectorsFormat {
+
+    static final String NAME = "ES815HnswBitVectorsFormat";
+
+    static final int MAXIMUM_MAX_CONN = 512;
+    static final int MAXIMUM_BEAM_WIDTH = 3200;
+
+    private final int maxConn;
+    private final int beamWidth;
+
+    private final FlatVectorsFormat flatVectorsFormat = new ES815BitFlatVectorsFormat();
+
+    public ES815HnswBitVectorsFormat() {
+        this(16, 100);
+    }
+
+    public ES815HnswBitVectorsFormat(int maxConn, int beamWidth) {
+        super(NAME);
+        if (maxConn <= 0 || maxConn > MAXIMUM_MAX_CONN) {
+            throw new IllegalArgumentException(
+                "maxConn must be positive and less than or equal to " + MAXIMUM_MAX_CONN + "; maxConn=" + maxConn
+            );
+        }
+        if (beamWidth <= 0 || beamWidth > MAXIMUM_BEAM_WIDTH) {
+            throw new IllegalArgumentException(
+                "beamWidth must be positive and less than or equal to " + MAXIMUM_BEAM_WIDTH + "; beamWidth=" + beamWidth
+            );
+        }
+        this.maxConn = maxConn;
+        this.beamWidth = beamWidth;
+    }
+
+    @Override
+    public KnnVectorsWriter fieldsWriter(SegmentWriteState state) throws IOException {
+        return new Lucene99HnswVectorsWriter(state, maxConn, beamWidth, flatVectorsFormat.fieldsWriter(state), 1, null);
+    }
+
+    @Override
+    public KnnVectorsReader fieldsReader(SegmentReadState state) throws IOException {
+        return new Lucene99HnswVectorsReader(state, flatVectorsFormat.fieldsReader(state));
+    }
+
+    @Override
+    public String toString() {
+        return "ES815HnswBitVectorsFormat(name=ES815HnswBitVectorsFormat, maxConn="
+            + maxConn
+            + ", beamWidth="
+            + beamWidth
+            + ", flatVectorFormat="
+            + flatVectorsFormat
+            + ")";
+    }
+}

server/src/main/java/org/elasticsearch/index/mapper/MapperFeatures.java

@@ -25,7 +25,8 @@ public class MapperFeatures implements FeatureSpecification {
             PassThroughObjectMapper.PASS_THROUGH_PRIORITY,
             RangeFieldMapper.NULL_VALUES_OFF_BY_ONE_FIX,
             SourceFieldMapper.SYNTHETIC_SOURCE_FALLBACK,
-            DenseVectorFieldMapper.INT4_QUANTIZATION
+            DenseVectorFieldMapper.INT4_QUANTIZATION,
+            DenseVectorFieldMapper.BIT_VECTORS
         );
     }
 }

server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java

@@ -31,6 +31,7 @@ import org.apache.lucene.index.VectorSimilarityFunction;
 import org.apache.lucene.search.FieldExistsQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.join.BitSetProducer;
+import org.apache.lucene.util.BitUtil;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.VectorUtil;
 import org.elasticsearch.common.ParsingException;
@@ -41,6 +42,8 @@ import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.codec.vectors.ES813FlatVectorFormat;
 import org.elasticsearch.index.codec.vectors.ES813Int8FlatVectorFormat;
 import org.elasticsearch.index.codec.vectors.ES814HnswScalarQuantizedVectorsFormat;
+import org.elasticsearch.index.codec.vectors.ES815BitFlatVectorFormat;
+import org.elasticsearch.index.codec.vectors.ES815HnswBitVectorsFormat;
 import org.elasticsearch.index.fielddata.FieldDataContext;
 import org.elasticsearch.index.fielddata.IndexFieldData;
 import org.elasticsearch.index.mapper.ArraySourceValueFetcher;
@@ -100,6 +103,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
     }
 
     public static final NodeFeature INT4_QUANTIZATION = new NodeFeature("mapper.vectors.int4_quantization");
+    public static final NodeFeature BIT_VECTORS = new NodeFeature("mapper.vectors.bit_vectors");
 
     public static final IndexVersion MAGNITUDE_STORED_INDEX_VERSION = IndexVersions.V_7_5_0;
     public static final IndexVersion INDEXED_BY_DEFAULT_INDEX_VERSION = IndexVersions.FIRST_DETACHED_INDEX_VERSION;
@@ -109,6 +113,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
 
     public static final String CONTENT_TYPE = "dense_vector";
     public static short MAX_DIMS_COUNT = 4096; // maximum allowed number of dimensions
+    public static int MAX_DIMS_COUNT_BIT = 4096 * Byte.SIZE; // maximum allowed number of dimensions
 
     public static short MIN_DIMS_FOR_DYNAMIC_FLOAT_MAPPING = 128; // minimum number of dims for floats to be dynamically mapped to vector
     public static final int MAGNITUDE_BYTES = 4;
@@ -134,17 +139,28 @@ public class DenseVectorFieldMapper extends FieldMapper {
                 throw new MapperParsingException("Property [dims] on field [" + n + "] must be an integer but got [" + o + "]");
             }
             int dims = XContentMapValues.nodeIntegerValue(o);
-            if (dims < 1 || dims > MAX_DIMS_COUNT) {
+            int maxDims = elementType.getValue() == ElementType.BIT ? MAX_DIMS_COUNT_BIT : MAX_DIMS_COUNT;
+            int minDims = elementType.getValue() == ElementType.BIT ? Byte.SIZE : 1;
+            if (dims < minDims || dims > maxDims) {
                 throw new MapperParsingException(
                     "The number of dimensions for field ["
                         + n
-                        + "] should be in the range [1, "
+                        + "] should be in the range ["
-                        + MAX_DIMS_COUNT
+                        + minDims
+                        + ", "
+                        + maxDims
                         + "] but was ["
                         + dims
                         + "]"
                 );
             }
+            if (elementType.getValue() == ElementType.BIT) {
+                if (dims % Byte.SIZE != 0) {
+                    throw new MapperParsingException(
+                        "The number of dimensions for field [" + n + "] should be a multiple of 8 but was [" + dims + "]"
+                    );
+                }
+            }
             return dims;
         }, m -> toType(m).fieldType().dims, XContentBuilder::field, Object::toString).setSerializerCheck((id, ic, v) -> v != null)
             .setMergeValidator((previous, current, c) -> previous == null || Objects.equals(previous, current));
@@ -171,13 +187,27 @@ public class DenseVectorFieldMapper extends FieldMapper {
                 "similarity",
                 false,
                 m -> toType(m).fieldType().similarity,
-                (Supplier<VectorSimilarity>) () -> indexedByDefault && indexed.getValue() ? VectorSimilarity.COSINE : null,
+                (Supplier<VectorSimilarity>) () -> {
+                    if (indexedByDefault && indexed.getValue()) {
+                        return elementType.getValue() == ElementType.BIT ? VectorSimilarity.L2_NORM : VectorSimilarity.COSINE;
+                    }
+                    return null;
+                },
                 VectorSimilarity.class
-            ).acceptsNull().setSerializerCheck((id, ic, v) -> v != null);
+            ).acceptsNull().setSerializerCheck((id, ic, v) -> v != null).addValidator(vectorSim -> {
+                if (vectorSim == null) {
+                    return;
+                }
+                if (elementType.getValue() == ElementType.BIT && vectorSim != VectorSimilarity.L2_NORM) {
+                    throw new IllegalArgumentException(
+                        "The [" + VectorSimilarity.L2_NORM + "] similarity is the only supported similarity for bit vectors"
+                    );
+                }
+            });
             this.indexOptions = new Parameter<>(
                 "index_options",
                 true,
-                () -> defaultInt8Hnsw && elementType.getValue() != ElementType.BYTE && this.indexed.getValue()
+                () -> defaultInt8Hnsw && elementType.getValue() == ElementType.FLOAT && this.indexed.getValue()
                     ? new Int8HnswIndexOptions(
                         Lucene99HnswVectorsFormat.DEFAULT_MAX_CONN,
                         Lucene99HnswVectorsFormat.DEFAULT_BEAM_WIDTH,
@@ -266,7 +296,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
 
     public enum ElementType {
 
-        BYTE(1) {
+        BYTE {
 
             @Override
             public String toString() {
@@ -371,7 +401,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             }
 
             @Override
-            public double computeDotProduct(VectorData vectorData) {
+            public double computeSquaredMagnitude(VectorData vectorData) {
                 return VectorUtil.dotProduct(vectorData.asByteVector(), vectorData.asByteVector());
             }
 
@@ -428,7 +458,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
                 byte[] decodedVector = HexFormat.of().parseHex(context.parser().text());
                 fieldMapper.checkDimensionMatches(decodedVector.length, context);
                 VectorData vectorData = VectorData.fromBytes(decodedVector);
-                double squaredMagnitude = computeDotProduct(vectorData);
+                double squaredMagnitude = computeSquaredMagnitude(vectorData);
                 checkVectorMagnitude(
                     fieldMapper.fieldType().similarity,
                     errorByteElementsAppender(decodedVector),
@@ -463,7 +493,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
 
             @Override
             int getNumBytes(int dimensions) {
-                return dimensions * elementBytes;
+                return dimensions;
             }
 
             @Override
@@ -494,7 +524,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             }
         },
 
-        FLOAT(4) {
+        FLOAT {
 
             @Override
             public String toString() {
@@ -596,7 +626,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             }
 
             @Override
-            public double computeDotProduct(VectorData vectorData) {
+            public double computeSquaredMagnitude(VectorData vectorData) {
                 return VectorUtil.dotProduct(vectorData.asFloatVector(), vectorData.asFloatVector());
             }
 
@@ -656,7 +686,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
 
             @Override
             int getNumBytes(int dimensions) {
-                return dimensions * elementBytes;
+                return dimensions * Float.BYTES;
             }
 
             @Override
@@ -665,14 +695,250 @@ public class DenseVectorFieldMapper extends FieldMapper {
                     ? ByteBuffer.wrap(new byte[numBytes]).order(ByteOrder.LITTLE_ENDIAN)
                     : ByteBuffer.wrap(new byte[numBytes]);
             }
+        },
+
+        BIT {
+
+            @Override
+            public String toString() {
+                return "bit";
+            }
+
+            @Override
+            public void writeValue(ByteBuffer byteBuffer, float value) {
+                byteBuffer.put((byte) value);
+            }
+
+            @Override
+            public void readAndWriteValue(ByteBuffer byteBuffer, XContentBuilder b) throws IOException {
+                b.value(byteBuffer.get());
+            }
+
+            private KnnByteVectorField createKnnVectorField(String name, byte[] vector, VectorSimilarityFunction function) {
+                if (vector == null) {
+                    throw new IllegalArgumentException("vector value must not be null");
+                }
+                FieldType denseVectorFieldType = new FieldType();
+                denseVectorFieldType.setVectorAttributes(vector.length, VectorEncoding.BYTE, function);
+                denseVectorFieldType.freeze();
+                return new KnnByteVectorField(name, vector, denseVectorFieldType);
+            }
+
+            @Override
+            IndexFieldData.Builder fielddataBuilder(DenseVectorFieldType denseVectorFieldType, FieldDataContext fieldDataContext) {
+                return new VectorIndexFieldData.Builder(
+                    denseVectorFieldType.name(),
+                    CoreValuesSourceType.KEYWORD,
+                    denseVectorFieldType.indexVersionCreated,
+                    this,
+                    denseVectorFieldType.dims,
+                    denseVectorFieldType.indexed,
+                    r -> r
+                );
+            }
+
+            @Override
+            public void checkVectorBounds(float[] vector) {
+                checkNanAndInfinite(vector);
+
+                StringBuilder errorBuilder = null;
+
+                for (int index = 0; index < vector.length; ++index) {
+                    float value = vector[index];
+
+                    if (value % 1.0f != 0.0f) {
+                        errorBuilder = new StringBuilder(
+                            "element_type ["
+                                + this
+                                + "] vectors only support non-decimal values but found decimal value ["
+                                + value
+                                + "] at dim ["
+                                + index
+                                + "];"
+                        );
+                        break;
+                    }
+
+                    if (value < Byte.MIN_VALUE || value > Byte.MAX_VALUE) {
+                        errorBuilder = new StringBuilder(
+                            "element_type ["
+                                + this
+                                + "] vectors only support integers between ["
+                                + Byte.MIN_VALUE
+                                + ", "
+                                + Byte.MAX_VALUE
+                                + "] but found ["
+                                + value
+                                + "] at dim ["
+                                + index
+                                + "];"
+                        );
+                        break;
+                    }
+                }
+
+                if (errorBuilder != null) {
+                    throw new IllegalArgumentException(appendErrorElements(errorBuilder, vector).toString());
+                }
+            }
+
+            @Override
+            void checkVectorMagnitude(
+                VectorSimilarity similarity,
+                Function<StringBuilder, StringBuilder> appender,
+                float squaredMagnitude
+            ) {}
+
+            @Override
+            public double computeSquaredMagnitude(VectorData vectorData) {
+                int count = 0;
+                int i = 0;
+                byte[] byteBits = vectorData.asByteVector();
+                for (int upperBound = byteBits.length & -8; i < upperBound; i += 8) {
+                    count += Long.bitCount((long) BitUtil.VH_NATIVE_LONG.get(byteBits, i));
+                }
+
+                while (i < byteBits.length) {
+                    count += Integer.bitCount(byteBits[i] & 255);
+                    ++i;
+                }
+                return count;
+            }
+
+            private VectorData parseVectorArray(DocumentParserContext context, DenseVectorFieldMapper fieldMapper) throws IOException {
+                int index = 0;
+                byte[] vector = new byte[fieldMapper.fieldType().dims / Byte.SIZE];
+                for (XContentParser.Token token = context.parser().nextToken(); token != Token.END_ARRAY; token = context.parser()
+                    .nextToken()) {
+                    fieldMapper.checkDimensionExceeded(index, context);
+                    ensureExpectedToken(Token.VALUE_NUMBER, token, context.parser());
+                    final int value;
+                    if (context.parser().numberType() != XContentParser.NumberType.INT) {
+                        float floatValue = context.parser().floatValue(true);
+                        if (floatValue % 1.0f != 0.0f) {
+                            throw new IllegalArgumentException(
+                                "element_type ["
+                                    + this
+                                    + "] vectors only support non-decimal values but found decimal value ["
+                                    + floatValue
+                                    + "] at dim ["
+                                    + index
+                                    + "];"
+                            );
+                        }
+                        value = (int) floatValue;
+                    } else {
+                        value = context.parser().intValue(true);
+                    }
+                    if (value < Byte.MIN_VALUE || value > Byte.MAX_VALUE) {
+                        throw new IllegalArgumentException(
+                            "element_type ["
+                                + this
+                                + "] vectors only support integers between ["
+                                + Byte.MIN_VALUE
+                                + ", "
+                                + Byte.MAX_VALUE
+                                + "] but found ["
+                                + value
+                                + "] at dim ["
+                                + index
+                                + "];"
+                        );
+                    }
+                    if (index >= vector.length) {
+                        throw new IllegalArgumentException(
+                            "The number of dimensions for field ["
+                                + fieldMapper.fieldType().name()
+                                + "] should be ["
+                                + fieldMapper.fieldType().dims
+                                + "] but found ["
+                                + (index + 1) * Byte.SIZE
+                                + "]"
+                        );
+                    }
+                    vector[index++] = (byte) value;
+                }
+                fieldMapper.checkDimensionMatches(index * Byte.SIZE, context);
+                return VectorData.fromBytes(vector);
+            }
+
+            private VectorData parseHexEncodedVector(DocumentParserContext context, DenseVectorFieldMapper fieldMapper) throws IOException {
+                byte[] decodedVector = HexFormat.of().parseHex(context.parser().text());
+                fieldMapper.checkDimensionMatches(decodedVector.length * Byte.SIZE, context);
+                return VectorData.fromBytes(decodedVector);
+            }
+
+            @Override
+            VectorData parseKnnVector(DocumentParserContext context, DenseVectorFieldMapper fieldMapper) throws IOException {
+                XContentParser.Token token = context.parser().currentToken();
+                return switch (token) {
+                    case START_ARRAY -> parseVectorArray(context, fieldMapper);
+                    case VALUE_STRING -> parseHexEncodedVector(context, fieldMapper);
+                    default -> throw new ParsingException(
+                        context.parser().getTokenLocation(),
+                        format("Unsupported type [%s] for provided value [%s]", token, context.parser().text())
+                    );
+                };
+            }
+
+            @Override
+            public void parseKnnVectorAndIndex(DocumentParserContext context, DenseVectorFieldMapper fieldMapper) throws IOException {
+                VectorData vectorData = parseKnnVector(context, fieldMapper);
+                Field field = createKnnVectorField(
+                    fieldMapper.fieldType().name(),
+                    vectorData.asByteVector(),
+                    fieldMapper.fieldType().similarity.vectorSimilarityFunction(fieldMapper.indexCreatedVersion, this)
+                );
+                context.doc().addWithKey(fieldMapper.fieldType().name(), field);
+            }
+
+            @Override
+            int getNumBytes(int dimensions) {
+                assert dimensions % Byte.SIZE == 0;
+                return dimensions / Byte.SIZE;
+            }
+
+            @Override
+            ByteBuffer createByteBuffer(IndexVersion indexVersion, int numBytes) {
+                return ByteBuffer.wrap(new byte[numBytes]);
+            }
+
+            @Override
+            int parseDimensionCount(DocumentParserContext context) throws IOException {
+                XContentParser.Token currentToken = context.parser().currentToken();
+                return switch (currentToken) {
+                    case START_ARRAY -> {
+                        int index = 0;
+                        for (Token token = context.parser().nextToken(); token != Token.END_ARRAY; token = context.parser().nextToken()) {
+                            index++;
+                        }
+                        yield index * Byte.SIZE;
+                    }
+                    case VALUE_STRING -> {
+                        byte[] decodedVector = HexFormat.of().parseHex(context.parser().text());
+                        yield decodedVector.length * Byte.SIZE;
+                    }
+                    default -> throw new ParsingException(
+                        context.parser().getTokenLocation(),
+                        format("Unsupported type [%s] for provided value [%s]", currentToken, context.parser().text())
+                    );
+                };
+            }
+
+            @Override
+            public void checkDimensions(int dvDims, int qvDims) {
+                if (dvDims != qvDims * Byte.SIZE) {
+                    throw new IllegalArgumentException(
+                        "The query vector has a different number of dimensions ["
+                            + qvDims * Byte.SIZE
+                            + "] than the document vectors ["
+                            + dvDims
+                            + "]."
+                    );
+                }
+            }
         };
 
-        final int elementBytes;
-
-        ElementType(int elementBytes) {
-            this.elementBytes = elementBytes;
-        }
-
         public abstract void writeValue(ByteBuffer byteBuffer, float value);
 
         public abstract void readAndWriteValue(ByteBuffer byteBuffer, XContentBuilder b) throws IOException;
@@ -695,6 +961,14 @@ public class DenseVectorFieldMapper extends FieldMapper {
             float squaredMagnitude
         );
 
+        public void checkDimensions(int dvDims, int qvDims) {
+            if (dvDims != qvDims) {
+                throw new IllegalArgumentException(
+                    "The query vector has a different number of dimensions [" + qvDims + "] than the document vectors [" + dvDims + "]."
+                );
+            }
+        }
+
         int parseDimensionCount(DocumentParserContext context) throws IOException {
             int index = 0;
             for (Token token = context.parser().nextToken(); token != Token.END_ARRAY; token = context.parser().nextToken()) {
@@ -775,7 +1049,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             return sb -> appendErrorElements(sb, vector);
         }
 
-        public abstract double computeDotProduct(VectorData vectorData);
+        public abstract double computeSquaredMagnitude(VectorData vectorData);
 
         public static ElementType fromString(String name) {
             return valueOf(name.trim().toUpperCase(Locale.ROOT));
@@ -786,7 +1060,9 @@ public class DenseVectorFieldMapper extends FieldMapper {
         ElementType.BYTE.toString(),
         ElementType.BYTE,
         ElementType.FLOAT.toString(),
-        ElementType.FLOAT
+        ElementType.FLOAT,
+        ElementType.BIT.toString(),
+        ElementType.BIT
     );
 
     public enum VectorSimilarity {
@@ -795,6 +1071,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             float score(float similarity, ElementType elementType, int dim) {
                 return switch (elementType) {
                     case BYTE, FLOAT -> 1f / (1f + similarity * similarity);
+                    case BIT -> (dim - similarity) / dim;
                 };
             }
 
@@ -806,8 +1083,10 @@ public class DenseVectorFieldMapper extends FieldMapper {
         COSINE {
             @Override
             float score(float similarity, ElementType elementType, int dim) {
+                assert elementType != ElementType.BIT;
                 return switch (elementType) {
                     case BYTE, FLOAT -> (1 + similarity) / 2f;
+                    default -> throw new IllegalArgumentException("Unsupported element type [" + elementType + "]");
                 };
             }
 
@@ -824,6 +1103,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
                 return switch (elementType) {
                     case BYTE -> 0.5f + similarity / (float) (dim * (1 << 15));
                     case FLOAT -> (1 + similarity) / 2f;
+                    default -> throw new IllegalArgumentException("Unsupported element type [" + elementType + "]");
                 };
             }
 
@@ -837,6 +1117,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             float score(float similarity, ElementType elementType, int dim) {
                 return switch (elementType) {
                     case BYTE, FLOAT -> similarity < 0 ? 1 / (1 + -1 * similarity) : similarity + 1;
+                    default -> throw new IllegalArgumentException("Unsupported element type [" + elementType + "]");
                 };
             }
 
@@ -863,7 +1144,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             this.type = type;
         }
 
-        abstract KnnVectorsFormat getVectorsFormat();
+        abstract KnnVectorsFormat getVectorsFormat(ElementType elementType);
 
         boolean supportsElementType(ElementType elementType) {
             return true;
@@ -1002,7 +1283,8 @@ public class DenseVectorFieldMapper extends FieldMapper {
         }
 
         @Override
-        KnnVectorsFormat getVectorsFormat() {
+        KnnVectorsFormat getVectorsFormat(ElementType elementType) {
+            assert elementType == ElementType.FLOAT;
             return new ES813Int8FlatVectorFormat(confidenceInterval, 7, false);
         }
 
@@ -1021,7 +1303,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
 
         @Override
         boolean supportsElementType(ElementType elementType) {
-            return elementType != ElementType.BYTE;
+            return elementType == ElementType.FLOAT;
         }
 
         @Override
@@ -1047,7 +1329,10 @@ public class DenseVectorFieldMapper extends FieldMapper {
         }
 
         @Override
-        KnnVectorsFormat getVectorsFormat() {
+        KnnVectorsFormat getVectorsFormat(ElementType elementType) {
+            if (elementType.equals(ElementType.BIT)) {
+                return new ES815BitFlatVectorFormat();
+            }
             return new ES813FlatVectorFormat();
         }
 
@@ -1083,7 +1368,8 @@ public class DenseVectorFieldMapper extends FieldMapper {
         }
 
         @Override
-        public KnnVectorsFormat getVectorsFormat() {
+        public KnnVectorsFormat getVectorsFormat(ElementType elementType) {
+            assert elementType == ElementType.FLOAT;
             return new ES814HnswScalarQuantizedVectorsFormat(m, efConstruction, confidenceInterval, 4, true);
         }
 
@@ -1126,7 +1412,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
 
         @Override
         boolean supportsElementType(ElementType elementType) {
-            return elementType != ElementType.BYTE;
+            return elementType == ElementType.FLOAT;
         }
 
         @Override
@@ -1153,7 +1439,8 @@ public class DenseVectorFieldMapper extends FieldMapper {
         }
 
         @Override
-        public KnnVectorsFormat getVectorsFormat() {
+        public KnnVectorsFormat getVectorsFormat(ElementType elementType) {
+            assert elementType == ElementType.FLOAT;
             return new ES813Int8FlatVectorFormat(confidenceInterval, 4, true);
         }
 
@@ -1186,7 +1473,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
 
         @Override
         boolean supportsElementType(ElementType elementType) {
-            return elementType != ElementType.BYTE;
+            return elementType == ElementType.FLOAT;
         }
 
         @Override
@@ -1216,7 +1503,8 @@ public class DenseVectorFieldMapper extends FieldMapper {
         }
 
         @Override
-        public KnnVectorsFormat getVectorsFormat() {
+        public KnnVectorsFormat getVectorsFormat(ElementType elementType) {
+            assert elementType == ElementType.FLOAT;
             return new ES814HnswScalarQuantizedVectorsFormat(m, efConstruction, confidenceInterval, 7, false);
         }
 
@@ -1261,7 +1549,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
 
         @Override
         boolean supportsElementType(ElementType elementType) {
-            return elementType != ElementType.BYTE;
+            return elementType == ElementType.FLOAT;
         }
 
         @Override
@@ -1291,7 +1579,10 @@ public class DenseVectorFieldMapper extends FieldMapper {
         }
 
         @Override
-        public KnnVectorsFormat getVectorsFormat() {
+        public KnnVectorsFormat getVectorsFormat(ElementType elementType) {
+            if (elementType == ElementType.BIT) {
+                return new ES815HnswBitVectorsFormat(m, efConstruction);
+            }
             return new Lucene99HnswVectorsFormat(m, efConstruction, 1, null);
         }
 
@@ -1412,48 +1703,6 @@ public class DenseVectorFieldMapper extends FieldMapper {
             throw new IllegalArgumentException("Field [" + name() + "] of type [" + typeName() + "] doesn't support term queries");
         }
 
-        public Query createKnnQuery(
-            byte[] queryVector,
-            int numCands,
-            Query filter,
-            Float similarityThreshold,
-            BitSetProducer parentFilter
-        ) {
-            if (isIndexed() == false) {
-                throw new IllegalArgumentException(
-                    "to perform knn search on field [" + name() + "], its mapping must have [index] set to [true]"
-                );
-            }
-
-            if (queryVector.length != dims) {
-                throw new IllegalArgumentException(
-                    "the query vector has a different dimension [" + queryVector.length + "] than the index vectors [" + dims + "]"
-                );
-            }
-
-            if (elementType != ElementType.BYTE) {
-                throw new IllegalArgumentException(
-                    "only [" + ElementType.BYTE + "] elements are supported when querying field [" + name() + "]"
-                );
-            }
-
-            if (similarity == VectorSimilarity.DOT_PRODUCT || similarity == VectorSimilarity.COSINE) {
-                float squaredMagnitude = VectorUtil.dotProduct(queryVector, queryVector);
-                elementType.checkVectorMagnitude(similarity, ElementType.errorByteElementsAppender(queryVector), squaredMagnitude);
-            }
-            Query knnQuery = parentFilter != null
-                ? new ESDiversifyingChildrenByteKnnVectorQuery(name(), queryVector, filter, numCands, parentFilter)
-                : new ESKnnByteVectorQuery(name(), queryVector, numCands, filter);
-            if (similarityThreshold != null) {
-                knnQuery = new VectorSimilarityQuery(
-                    knnQuery,
-                    similarityThreshold,
-                    similarity.score(similarityThreshold, elementType, dims)
-                );
-            }
-            return knnQuery;
-        }
-
         public Query createExactKnnQuery(VectorData queryVector) {
             if (isIndexed() == false) {
                 throw new IllegalArgumentException(
@@ -1463,15 +1712,17 @@ public class DenseVectorFieldMapper extends FieldMapper {
             return switch (elementType) {
                 case BYTE -> createExactKnnByteQuery(queryVector.asByteVector());
                 case FLOAT -> createExactKnnFloatQuery(queryVector.asFloatVector());
+                case BIT -> createExactKnnBitQuery(queryVector.asByteVector());
             };
         }
 
+        private Query createExactKnnBitQuery(byte[] queryVector) {
+            elementType.checkDimensions(dims, queryVector.length);
+            return new DenseVectorQuery.Bytes(queryVector, name());
+        }
+
         private Query createExactKnnByteQuery(byte[] queryVector) {
-            if (queryVector.length != dims) {
+            elementType.checkDimensions(dims, queryVector.length);
-                throw new IllegalArgumentException(
-                    "the query vector has a different dimension [" + queryVector.length + "] than the index vectors [" + dims + "]"
-                );
-            }
             if (similarity == VectorSimilarity.DOT_PRODUCT || similarity == VectorSimilarity.COSINE) {
                 float squaredMagnitude = VectorUtil.dotProduct(queryVector, queryVector);
                 elementType.checkVectorMagnitude(similarity, ElementType.errorByteElementsAppender(queryVector), squaredMagnitude);
@@ -1480,11 +1731,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
         }
 
         private Query createExactKnnFloatQuery(float[] queryVector) {
-            if (queryVector.length != dims) {
+            elementType.checkDimensions(dims, queryVector.length);
-                throw new IllegalArgumentException(
-                    "the query vector has a different dimension [" + queryVector.length + "] than the index vectors [" + dims + "]"
-                );
-            }
             elementType.checkVectorBounds(queryVector);
             if (similarity == VectorSimilarity.DOT_PRODUCT || similarity == VectorSimilarity.COSINE) {
                 float squaredMagnitude = VectorUtil.dotProduct(queryVector, queryVector);
@@ -1521,9 +1768,31 @@ public class DenseVectorFieldMapper extends FieldMapper {
             return switch (getElementType()) {
                 case BYTE -> createKnnByteQuery(queryVector.asByteVector(), numCands, filter, similarityThreshold, parentFilter);
                 case FLOAT -> createKnnFloatQuery(queryVector.asFloatVector(), numCands, filter, similarityThreshold, parentFilter);
+                case BIT -> createKnnBitQuery(queryVector.asByteVector(), numCands, filter, similarityThreshold, parentFilter);
             };
         }
 
+        private Query createKnnBitQuery(
+            byte[] queryVector,
+            int numCands,
+            Query filter,
+            Float similarityThreshold,
+            BitSetProducer parentFilter
+        ) {
+            elementType.checkDimensions(dims, queryVector.length);
+            Query knnQuery = parentFilter != null
+                ? new ESDiversifyingChildrenByteKnnVectorQuery(name(), queryVector, filter, numCands, parentFilter)
+                : new ESKnnByteVectorQuery(name(), queryVector, numCands, filter);
+            if (similarityThreshold != null) {
+                knnQuery = new VectorSimilarityQuery(
+                    knnQuery,
+                    similarityThreshold,
+                    similarity.score(similarityThreshold, elementType, dims)
+                );
+            }
+            return knnQuery;
+        }
+
         private Query createKnnByteQuery(
             byte[] queryVector,
             int numCands,
@@ -1531,11 +1800,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             Float similarityThreshold,
             BitSetProducer parentFilter
         ) {
-            if (queryVector.length != dims) {
+            elementType.checkDimensions(dims, queryVector.length);
-                throw new IllegalArgumentException(
-                    "the query vector has a different dimension [" + queryVector.length + "] than the index vectors [" + dims + "]"
-                );
-            }
 
             if (similarity == VectorSimilarity.DOT_PRODUCT || similarity == VectorSimilarity.COSINE) {
                 float squaredMagnitude = VectorUtil.dotProduct(queryVector, queryVector);
@@ -1561,11 +1826,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
             Float similarityThreshold,
             BitSetProducer parentFilter
         ) {
-            if (queryVector.length != dims) {
+            elementType.checkDimensions(dims, queryVector.length);
-                throw new IllegalArgumentException(
-                    "the query vector has a different dimension [" + queryVector.length + "] than the index vectors [" + dims + "]"
-                );
-            }
             elementType.checkVectorBounds(queryVector);
             if (similarity == VectorSimilarity.DOT_PRODUCT || similarity == VectorSimilarity.COSINE) {
                 float squaredMagnitude = VectorUtil.dotProduct(queryVector, queryVector);
@@ -1701,7 +1962,7 @@ public class DenseVectorFieldMapper extends FieldMapper {
         vectorData.addToBuffer(byteBuffer);
         if (indexCreatedVersion.onOrAfter(MAGNITUDE_STORED_INDEX_VERSION)) {
             // encode vector magnitude at the end
-            double dotProduct = elementType.computeDotProduct(vectorData);
+            double dotProduct = elementType.computeSquaredMagnitude(vectorData);
             float vectorMagnitude = (float) Math.sqrt(dotProduct);
             byteBuffer.putFloat(vectorMagnitude);
         }
@@ -1780,9 +2041,9 @@ public class DenseVectorFieldMapper extends FieldMapper {
     public KnnVectorsFormat getKnnVectorsFormatForField(KnnVectorsFormat defaultFormat) {
         final KnnVectorsFormat format;
         if (indexOptions == null) {
-            format = defaultFormat;
+            format = fieldType().elementType == ElementType.BIT ? new ES815HnswBitVectorsFormat() : defaultFormat;
         } else {
-            format = indexOptions.getVectorsFormat();
+            format = indexOptions.getVectorsFormat(fieldType().elementType);
         }
         // It's legal to reuse the same format name as this is the same on-disk format.
         return new KnnVectorsFormat(format.getName()) {

server/src/main/java/org/elasticsearch/index/mapper/vectors/VectorDVLeafFieldData.java

@@ -17,6 +17,8 @@ import org.elasticsearch.index.fielddata.SortedBinaryDocValues;
 import org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper.ElementType;
 import org.elasticsearch.script.field.DocValuesScriptFieldFactory;
 import org.elasticsearch.script.field.vectors.BinaryDenseVectorDocValuesField;
+import org.elasticsearch.script.field.vectors.BitBinaryDenseVectorDocValuesField;
+import org.elasticsearch.script.field.vectors.BitKnnDenseVectorDocValuesField;
 import org.elasticsearch.script.field.vectors.ByteBinaryDenseVectorDocValuesField;
 import org.elasticsearch.script.field.vectors.ByteKnnDenseVectorDocValuesField;
 import org.elasticsearch.script.field.vectors.KnnDenseVectorDocValuesField;
@@ -58,12 +60,14 @@ final class VectorDVLeafFieldData implements LeafFieldData {
                 return switch (elementType) {
                     case BYTE -> new ByteKnnDenseVectorDocValuesField(reader.getByteVectorValues(field), name, dims);
                     case FLOAT -> new KnnDenseVectorDocValuesField(reader.getFloatVectorValues(field), name, dims);
+                    case BIT -> new BitKnnDenseVectorDocValuesField(reader.getByteVectorValues(field), name, dims);
                 };
             } else {
                 BinaryDocValues values = DocValues.getBinary(reader, field);
                 return switch (elementType) {
                     case BYTE -> new ByteBinaryDenseVectorDocValuesField(values, name, elementType, dims);
                     case FLOAT -> new BinaryDenseVectorDocValuesField(values, name, elementType, dims, indexVersion);
+                    case BIT -> new BitBinaryDenseVectorDocValuesField(values, name, elementType, dims);
                 };
             }
         } catch (IOException e) {

server/src/main/java/org/elasticsearch/script/VectorScoreScriptUtils.java

@@ -56,7 +56,7 @@ public class VectorScoreScriptUtils {
          */
         public ByteDenseVectorFunction(ScoreScript scoreScript, DenseVectorDocValuesField field, List<Number> queryVector) {
             super(scoreScript, field);
-            DenseVector.checkDimensions(field.get().getDims(), queryVector.size());
+            field.getElementType().checkDimensions(field.get().getDims(), queryVector.size());
             this.queryVector = new byte[queryVector.size()];
             float[] validateValues = new float[queryVector.size()];
             int queryMagnitude = 0;
@@ -168,7 +168,7 @@ public class VectorScoreScriptUtils {
         public L1Norm(ScoreScript scoreScript, Object queryVector, String fieldName) {
             DenseVectorDocValuesField field = (DenseVectorDocValuesField) scoreScript.field(fieldName);
             function = switch (field.getElementType()) {
-                case BYTE -> {
+                case BYTE, BIT -> {
                     if (queryVector instanceof List) {
                         yield new ByteL1Norm(scoreScript, field, (List<Number>) queryVector);
                     } else if (queryVector instanceof String s) {
@@ -219,8 +219,8 @@ public class VectorScoreScriptUtils {
         @SuppressWarnings("unchecked")
         public Hamming(ScoreScript scoreScript, Object queryVector, String fieldName) {
             DenseVectorDocValuesField field = (DenseVectorDocValuesField) scoreScript.field(fieldName);
-            if (field.getElementType() != DenseVectorFieldMapper.ElementType.BYTE) {
+            if (field.getElementType() == DenseVectorFieldMapper.ElementType.FLOAT) {
-                throw new IllegalArgumentException("hamming distance is only supported for byte vectors");
+                throw new IllegalArgumentException("hamming distance is only supported for byte or bit vectors");
             }
             if (queryVector instanceof List) {
                 function = new ByteHammingDistance(scoreScript, field, (List<Number>) queryVector);
@@ -278,7 +278,7 @@ public class VectorScoreScriptUtils {
         public L2Norm(ScoreScript scoreScript, Object queryVector, String fieldName) {
             DenseVectorDocValuesField field = (DenseVectorDocValuesField) scoreScript.field(fieldName);
             function = switch (field.getElementType()) {
-                case BYTE -> {
+                case BYTE, BIT -> {
                     if (queryVector instanceof List) {
                         yield new ByteL2Norm(scoreScript, field, (List<Number>) queryVector);
                     } else if (queryVector instanceof String s) {
@@ -342,7 +342,7 @@ public class VectorScoreScriptUtils {
         public DotProduct(ScoreScript scoreScript, Object queryVector, String fieldName) {
             DenseVectorDocValuesField field = (DenseVectorDocValuesField) scoreScript.field(fieldName);
             function = switch (field.getElementType()) {
-                case BYTE -> {
+                case BYTE, BIT -> {
                     if (queryVector instanceof List) {
                         yield new ByteDotProduct(scoreScript, field, (List<Number>) queryVector);
                     } else if (queryVector instanceof String s) {
@@ -406,7 +406,7 @@ public class VectorScoreScriptUtils {
         public CosineSimilarity(ScoreScript scoreScript, Object queryVector, String fieldName) {
             DenseVectorDocValuesField field = (DenseVectorDocValuesField) scoreScript.field(fieldName);
             function = switch (field.getElementType()) {
-                case BYTE -> {
+                case BYTE, BIT -> {
                     if (queryVector instanceof List) {
                         yield new ByteCosineSimilarity(scoreScript, field, (List<Number>) queryVector);
                     } else if (queryVector instanceof String s) {

server/src/main/java/org/elasticsearch/script/field/vectors/BitBinaryDenseVector.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.script.field.vectors;
+
+import org.apache.lucene.util.BytesRef;
+
+import java.util.List;
+
+public class BitBinaryDenseVector extends ByteBinaryDenseVector {
+
+    public BitBinaryDenseVector(byte[] vectorValue, BytesRef docVector, int dims) {
+        super(vectorValue, docVector, dims);
+    }
+
+    @Override
+    public void checkDimensions(int qvDims) {
+        if (qvDims != dims) {
+            throw new IllegalArgumentException(
+                "The query vector has a different number of dimensions ["
+                    + qvDims * Byte.SIZE
+                    + "] than the document vectors ["
+                    + dims * Byte.SIZE
+                    + "]."
+            );
+        }
+    }
+
+    @Override
+    public int l1Norm(byte[] queryVector) {
+        return hamming(queryVector);
+    }
+
+    @Override
+    public double l1Norm(List<Number> queryVector) {
+        return hamming(queryVector);
+    }
+
+    @Override
+    public double l2Norm(byte[] queryVector) {
+        return Math.sqrt(hamming(queryVector));
+    }
+
+    @Override
+    public double l2Norm(List<Number> queryVector) {
+        return Math.sqrt(hamming(queryVector));
+    }
+
+    @Override
+    public int dotProduct(byte[] queryVector) {
+        throw new UnsupportedOperationException("dotProduct is not supported for bit vectors.");
+    }
+
+    @Override
+    public double cosineSimilarity(float[] queryVector, boolean normalizeQueryVector) {
+        throw new UnsupportedOperationException("cosineSimilarity is not supported for bit vectors.");
+    }
+
+    @Override
+    public double dotProduct(List<Number> queryVector) {
+        throw new UnsupportedOperationException("dotProduct is not supported for bit vectors.");
+    }
+
+    @Override
+    public double cosineSimilarity(byte[] queryVector, float qvMagnitude) {
+        throw new UnsupportedOperationException("cosineSimilarity is not supported for bit vectors.");
+    }
+
+    @Override
+    public double cosineSimilarity(List<Number> queryVector) {
+        throw new UnsupportedOperationException("cosineSimilarity is not supported for bit vectors.");
+    }
+
+    @Override
+    public double dotProduct(float[] queryVector) {
+        throw new UnsupportedOperationException("dotProduct is not supported for bit vectors.");
+    }
+
+    @Override
+    public int getDims() {
+        return dims * Byte.SIZE;
+    }
+}

server/src/main/java/org/elasticsearch/script/field/vectors/BitBinaryDenseVectorDocValuesField.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.script.field.vectors;
+
+import org.apache.lucene.index.BinaryDocValues;
+import org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper.ElementType;
+
+public class BitBinaryDenseVectorDocValuesField extends ByteBinaryDenseVectorDocValuesField {
+
+    public BitBinaryDenseVectorDocValuesField(BinaryDocValues input, String name, ElementType elementType, int dims) {
+        super(input, name, elementType, dims / 8);
+    }
+
+    @Override
+    protected DenseVector getVector() {
+        return new BitBinaryDenseVector(vectorValue, value, dims);
+    }
+}

server/src/main/java/org/elasticsearch/script/field/vectors/BitKnnDenseVector.java

@@ -0,0 +1,95 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.script.field.vectors;
+
+import java.util.List;
+
+public class BitKnnDenseVector extends ByteKnnDenseVector {
+
+    public BitKnnDenseVector(byte[] vector) {
+        super(vector);
+    }
+
+    @Override
+    public void checkDimensions(int qvDims) {
+        if (qvDims != docVector.length) {
+            throw new IllegalArgumentException(
+                "The query vector has a different number of dimensions ["
+                    + qvDims * Byte.SIZE
+                    + "] than the document vectors ["
+                    + docVector.length * Byte.SIZE
+                    + "]."
+            );
+        }
+    }
+
+    @Override
+    public float getMagnitude() {
+        if (magnitudeCalculated == false) {
+            magnitude = DenseVector.getBitMagnitude(docVector, docVector.length);
+            magnitudeCalculated = true;
+        }
+        return magnitude;
+    }
+
+    @Override
+    public int l1Norm(byte[] queryVector) {
+        return hamming(queryVector);
+    }
+
+    @Override
+    public double l1Norm(List<Number> queryVector) {
+        return hamming(queryVector);
+    }
+
+    @Override
+    public double l2Norm(byte[] queryVector) {
+        return Math.sqrt(hamming(queryVector));
+    }
+
+    @Override
+    public double l2Norm(List<Number> queryVector) {
+        return Math.sqrt(hamming(queryVector));
+    }
+
+    @Override
+    public int dotProduct(byte[] queryVector) {
+        throw new UnsupportedOperationException("dotProduct is not supported for bit vectors.");
+    }
+
+    @Override
+    public double cosineSimilarity(float[] queryVector, boolean normalizeQueryVector) {
+        throw new UnsupportedOperationException("cosineSimilarity is not supported for bit vectors.");
+    }
+
+    @Override
+    public double dotProduct(List<Number> queryVector) {
+        throw new UnsupportedOperationException("dotProduct is not supported for bit vectors.");
+    }
+
+    @Override
+    public double cosineSimilarity(byte[] queryVector, float qvMagnitude) {
+        throw new UnsupportedOperationException("cosineSimilarity is not supported for bit vectors.");
+    }
+
+    @Override
+    public double cosineSimilarity(List<Number> queryVector) {
+        throw new UnsupportedOperationException("cosineSimilarity is not supported for bit vectors.");
+    }
+
+    @Override
+    public double dotProduct(float[] queryVector) {
+        throw new UnsupportedOperationException("dotProduct is not supported for bit vectors.");
+    }
+
+    @Override
+    public int getDims() {
+        return docVector.length * Byte.SIZE;
+    }
+}

server/src/main/java/org/elasticsearch/script/field/vectors/BitKnnDenseVectorDocValuesField.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.script.field.vectors;
+
+import org.apache.lucene.index.ByteVectorValues;
+import org.elasticsearch.core.Nullable;
+import org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper;
+
+public class BitKnnDenseVectorDocValuesField extends ByteKnnDenseVectorDocValuesField {
+
+    public BitKnnDenseVectorDocValuesField(@Nullable ByteVectorValues input, String name, int dims) {
+        super(input, name, dims / 8, DenseVectorFieldMapper.ElementType.BIT);
+    }
+
+    @Override
+    protected DenseVector getVector() {
+        return new BitKnnDenseVector(vector);
+    }
+
+}

server/src/main/java/org/elasticsearch/script/field/vectors/ByteBinaryDenseVector.java

@@ -21,7 +21,7 @@ public class ByteBinaryDenseVector implements DenseVector {
 
     private final BytesRef docVector;
     private final byte[] vectorValue;
-    private final int dims;
+    protected final int dims;
 
     private float[] floatDocVector;
     private boolean magnitudeDecoded;

server/src/main/java/org/elasticsearch/script/field/vectors/ByteBinaryDenseVectorDocValuesField.java

@@ -17,11 +17,11 @@ import java.io.IOException;
 
 public class ByteBinaryDenseVectorDocValuesField extends DenseVectorDocValuesField {
 
-    private final BinaryDocValues input;
+    protected final BinaryDocValues input;
-    private final int dims;
+    protected final int dims;
-    private final byte[] vectorValue;
+    protected final byte[] vectorValue;
-    private boolean decoded;
+    protected boolean decoded;
-    private BytesRef value;
+    protected BytesRef value;
 
     public ByteBinaryDenseVectorDocValuesField(BinaryDocValues input, String name, ElementType elementType, int dims) {
         super(name, elementType);
@@ -50,13 +50,17 @@ public class ByteBinaryDenseVectorDocValuesField extends DenseVectorDocValuesFie
         return value == null;
     }
 
+    protected DenseVector getVector() {
+        return new ByteBinaryDenseVector(vectorValue, value, dims);
+    }
+
     @Override
     public DenseVector get() {
         if (isEmpty()) {
             return DenseVector.EMPTY;
         }
         decodeVectorIfNecessary();
-        return new ByteBinaryDenseVector(vectorValue, value, dims);
+        return getVector();
     }
 
     @Override
@@ -65,7 +69,7 @@ public class ByteBinaryDenseVectorDocValuesField extends DenseVectorDocValuesFie
             return defaultValue;
         }
         decodeVectorIfNecessary();
-        return new ByteBinaryDenseVector(vectorValue, value, dims);
+        return getVector();
     }
 
     @Override

server/src/main/java/org/elasticsearch/script/field/vectors/ByteKnnDenseVectorDocValuesField.java

@@ -23,7 +23,11 @@ public class ByteKnnDenseVectorDocValuesField extends DenseVectorDocValuesField
     protected final int dims;
 
     public ByteKnnDenseVectorDocValuesField(@Nullable ByteVectorValues input, String name, int dims) {
-        super(name, ElementType.BYTE);
+        this(input, name, dims, ElementType.BYTE);
+    }
+
+    protected ByteKnnDenseVectorDocValuesField(@Nullable ByteVectorValues input, String name, int dims, ElementType elementType) {
+        super(name, elementType);
         this.dims = dims;
         this.input = input;
     }
@@ -57,13 +61,17 @@ public class ByteKnnDenseVectorDocValuesField extends DenseVectorDocValuesField
         return vector == null;
     }
 
+    protected DenseVector getVector() {
+        return new ByteKnnDenseVector(vector);
+    }
+
     @Override
     public DenseVector get() {
         if (isEmpty()) {
             return DenseVector.EMPTY;
         }
 
-        return new ByteKnnDenseVector(vector);
+        return getVector();
     }
 
     @Override
@@ -72,7 +80,7 @@ public class ByteKnnDenseVectorDocValuesField extends DenseVectorDocValuesField
             return defaultValue;
         }
 
-        return new ByteKnnDenseVector(vector);
+        return getVector();
     }
 
     @Override

server/src/main/java/org/elasticsearch/script/field/vectors/DenseVector.java

@@ -8,6 +8,7 @@
 
 package org.elasticsearch.script.field.vectors;
 
+import org.apache.lucene.util.BitUtil;
 import org.apache.lucene.util.VectorUtil;
 
 import java.util.List;
@@ -25,6 +26,10 @@ import java.util.List;
  */
 public interface DenseVector {
 
+    default void checkDimensions(int qvDims) {
+        checkDimensions(getDims(), qvDims);
+    }
+
     float[] getVector();
 
     float getMagnitude();
@@ -38,13 +43,13 @@ public interface DenseVector {
     @SuppressWarnings("unchecked")
     default double dotProduct(Object queryVector) {
         if (queryVector instanceof float[] floats) {
-            checkDimensions(getDims(), floats.length);
+            checkDimensions(floats.length);
             return dotProduct(floats);
         } else if (queryVector instanceof List<?> list) {
-            checkDimensions(getDims(), list.size());
+            checkDimensions(list.size());
             return dotProduct((List<Number>) list);
         } else if (queryVector instanceof byte[] bytes) {
-            checkDimensions(getDims(), bytes.length);
+            checkDimensions(bytes.length);
             return dotProduct(bytes);
         }
 
@@ -60,13 +65,13 @@ public interface DenseVector {
     @SuppressWarnings("unchecked")
     default double l1Norm(Object queryVector) {
         if (queryVector instanceof float[] floats) {
-            checkDimensions(getDims(), floats.length);
+            checkDimensions(floats.length);
             return l1Norm(floats);
         } else if (queryVector instanceof List<?> list) {
-            checkDimensions(getDims(), list.size());
+            checkDimensions(list.size());
             return l1Norm((List<Number>) list);
         } else if (queryVector instanceof byte[] bytes) {
-            checkDimensions(getDims(), bytes.length);
+            checkDimensions(bytes.length);
             return l1Norm(bytes);
         }
 
@@ -80,11 +85,11 @@ public interface DenseVector {
     @SuppressWarnings("unchecked")
     default int hamming(Object queryVector) {
         if (queryVector instanceof List<?> list) {
-            checkDimensions(getDims(), list.size());
+            checkDimensions(list.size());
             return hamming((List<Number>) list);
         }
         if (queryVector instanceof byte[] bytes) {
-            checkDimensions(getDims(), bytes.length);
+            checkDimensions(bytes.length);
             return hamming(bytes);
         }
 
@@ -100,13 +105,13 @@ public interface DenseVector {
     @SuppressWarnings("unchecked")
     default double l2Norm(Object queryVector) {
         if (queryVector instanceof float[] floats) {
-            checkDimensions(getDims(), floats.length);
+            checkDimensions(floats.length);
             return l2Norm(floats);
         } else if (queryVector instanceof List<?> list) {
-            checkDimensions(getDims(), list.size());
+            checkDimensions(list.size());
             return l2Norm((List<Number>) list);
         } else if (queryVector instanceof byte[] bytes) {
-            checkDimensions(getDims(), bytes.length);
+            checkDimensions(bytes.length);
             return l2Norm(bytes);
         }
 
@@ -150,13 +155,13 @@ public interface DenseVector {
     @SuppressWarnings("unchecked")
     default double cosineSimilarity(Object queryVector) {
         if (queryVector instanceof float[] floats) {
-            checkDimensions(getDims(), floats.length);
+            checkDimensions(floats.length);
             return cosineSimilarity(floats);
         } else if (queryVector instanceof List<?> list) {
-            checkDimensions(getDims(), list.size());
+            checkDimensions(list.size());
             return cosineSimilarity((List<Number>) list);
         } else if (queryVector instanceof byte[] bytes) {
-            checkDimensions(getDims(), bytes.length);
+            checkDimensions(bytes.length);
             return cosineSimilarity(bytes);
         }
 
@@ -184,6 +189,20 @@ public interface DenseVector {
         return (float) Math.sqrt(mag);
     }
 
+    static float getBitMagnitude(byte[] vector, int dims) {
+        int count = 0;
+        int i = 0;
+        for (int upperBound = dims & -8; i < upperBound; i += 8) {
+            count += Long.bitCount((long) BitUtil.VH_NATIVE_LONG.get(vector, i));
+        }
+
+        while (i < dims) {
+            count += Integer.bitCount(vector[i] & 255);
+            ++i;
+        }
+        return (float) Math.sqrt(count);
+    }
+
     static float getMagnitude(float[] vector) {
         return (float) Math.sqrt(VectorUtil.dotProduct(vector, vector));
     }

server/src/main/resources/META-INF/services/org.apache.lucene.codecs.KnnVectorsFormat

@@ -1,3 +1,5 @@
 org.elasticsearch.index.codec.vectors.ES813FlatVectorFormat
 org.elasticsearch.index.codec.vectors.ES813Int8FlatVectorFormat
 org.elasticsearch.index.codec.vectors.ES814HnswScalarQuantizedVectorsFormat
+org.elasticsearch.index.codec.vectors.ES815HnswBitVectorsFormat
+org.elasticsearch.index.codec.vectors.ES815BitFlatVectorFormat

server/src/test/java/org/elasticsearch/index/codec/vectors/BaseKnnBitVectorsFormatTestCase.java

@@ -0,0 +1,149 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.index.codec.vectors;
+
+import org.apache.lucene.document.Document;
+import org.apache.lucene.document.Field;
+import org.apache.lucene.document.KnnByteVectorField;
+import org.apache.lucene.document.NumericDocValuesField;
+import org.apache.lucene.document.StringField;
+import org.apache.lucene.index.ByteVectorValues;
+import org.apache.lucene.index.DirectoryReader;
+import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.IndexWriter;
+import org.apache.lucene.index.IndexWriterConfig;
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.index.StoredFields;
+import org.apache.lucene.index.Term;
+import org.apache.lucene.index.VectorSimilarityFunction;
+import org.apache.lucene.search.Sort;
+import org.apache.lucene.search.SortField;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.tests.index.BaseIndexFileFormatTestCase;
+import org.elasticsearch.common.logging.LogConfigurator;
+
+import java.io.IOException;
+
+import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
+
+abstract class BaseKnnBitVectorsFormatTestCase extends BaseIndexFileFormatTestCase {
+
+    static {
+        LogConfigurator.loadLog4jPlugins();
+        LogConfigurator.configureESLogging(); // native access requires logging to be initialized
+    }
+
+    @Override
+    protected void addRandomFields(Document doc) {
+        doc.add(new KnnByteVectorField("v2", randomVector(30), similarityFunction));
+    }
+
+    protected VectorSimilarityFunction similarityFunction;
+
+    protected VectorSimilarityFunction randomSimilarity() {
+        return VectorSimilarityFunction.values()[random().nextInt(VectorSimilarityFunction.values().length)];
+    }
+
+    byte[] randomVector(int dims) {
+        byte[] vector = new byte[dims];
+        random().nextBytes(vector);
+        return vector;
+    }
+
+    public void testRandom() throws Exception {
+        IndexWriterConfig iwc = newIndexWriterConfig();
+        if (random().nextBoolean()) {
+            iwc.setIndexSort(new Sort(new SortField("sortkey", SortField.Type.INT)));
+        }
+        String fieldName = "field";
+        try (Directory dir = newDirectory(); IndexWriter iw = new IndexWriter(dir, iwc)) {
+            int numDoc = atLeast(100);
+            int dimension = atLeast(10);
+            if (dimension % 2 != 0) {
+                dimension++;
+            }
+            byte[] scratch = new byte[dimension];
+            int numValues = 0;
+            byte[][] values = new byte[numDoc][];
+            for (int i = 0; i < numDoc; i++) {
+                if (random().nextInt(7) != 3) {
+                    // usually index a vector value for a doc
+                    values[i] = randomVector(dimension);
+                    ++numValues;
+                }
+                if (random().nextBoolean() && values[i] != null) {
+                    // sometimes use a shared scratch array
+                    System.arraycopy(values[i], 0, scratch, 0, scratch.length);
+                    add(iw, fieldName, i, scratch, similarityFunction);
+                } else {
+                    add(iw, fieldName, i, values[i], similarityFunction);
+                }
+                if (random().nextInt(10) == 2) {
+                    // sometimes delete a random document
+                    int idToDelete = random().nextInt(i + 1);
+                    iw.deleteDocuments(new Term("id", Integer.toString(idToDelete)));
+                    // and remember that it was deleted
+                    if (values[idToDelete] != null) {
+                        values[idToDelete] = null;
+                        --numValues;
+                    }
+                }
+                if (random().nextInt(10) == 3) {
+                    iw.commit();
+                }
+            }
+            int numDeletes = 0;
+            try (IndexReader reader = DirectoryReader.open(iw)) {
+                int valueCount = 0, totalSize = 0;
+                for (LeafReaderContext ctx : reader.leaves()) {
+                    ByteVectorValues vectorValues = ctx.reader().getByteVectorValues(fieldName);
+                    if (vectorValues == null) {
+                        continue;
+                    }
+                    totalSize += vectorValues.size();
+                    StoredFields storedFields = ctx.reader().storedFields();
+                    int docId;
+                    while ((docId = vectorValues.nextDoc()) != NO_MORE_DOCS) {
+                        byte[] v = vectorValues.vectorValue();
+                        assertEquals(dimension, v.length);
+                        String idString = storedFields.document(docId).getField("id").stringValue();
+                        int id = Integer.parseInt(idString);
+                        if (ctx.reader().getLiveDocs() == null || ctx.reader().getLiveDocs().get(docId)) {
+                            assertArrayEquals(idString, values[id], v);
+                            ++valueCount;
+                        } else {
+                            ++numDeletes;
+                            assertNull(values[id]);
+                        }
+                    }
+                }
+                assertEquals(numValues, valueCount);
+                assertEquals(numValues, totalSize - numDeletes);
+            }
+        }
+    }
+
+    private void add(IndexWriter iw, String field, int id, byte[] vector, VectorSimilarityFunction similarity) throws IOException {
+        add(iw, field, id, random().nextInt(100), vector, similarity);
+    }
+
+    private void add(IndexWriter iw, String field, int id, int sortKey, byte[] vector, VectorSimilarityFunction similarityFunction)
+        throws IOException {
+        Document doc = new Document();
+        if (vector != null) {
+            doc.add(new KnnByteVectorField(field, vector, similarityFunction));
+        }
+        doc.add(new NumericDocValuesField("sortkey", sortKey));
+        String idString = Integer.toString(id);
+        doc.add(new StringField("id", idString, Field.Store.YES));
+        Term idTerm = new Term("id", idString);
+        iw.updateDocument(idTerm, doc);
+    }
+
+}

server/src/test/java/org/elasticsearch/index/codec/vectors/ES813FlatVectorFormatTests.java

@@ -12,8 +12,15 @@ import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.KnnVectorsFormat;
 import org.apache.lucene.codecs.lucene99.Lucene99Codec;
 import org.apache.lucene.tests.index.BaseKnnVectorsFormatTestCase;
+import org.elasticsearch.common.logging.LogConfigurator;
 
 public class ES813FlatVectorFormatTests extends BaseKnnVectorsFormatTestCase {
+
+    static {
+        LogConfigurator.loadLog4jPlugins();
+        LogConfigurator.configureESLogging(); // native access requires logging to be initialized
+    }
+
     @Override
     protected Codec getCodec() {
         return new Lucene99Codec() {

server/src/test/java/org/elasticsearch/index/codec/vectors/ES813Int8FlatVectorFormatTests.java

@@ -12,8 +12,15 @@ import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.KnnVectorsFormat;
 import org.apache.lucene.codecs.lucene99.Lucene99Codec;
 import org.apache.lucene.tests.index.BaseKnnVectorsFormatTestCase;
+import org.elasticsearch.common.logging.LogConfigurator;
 
 public class ES813Int8FlatVectorFormatTests extends BaseKnnVectorsFormatTestCase {
+
+    static {
+        LogConfigurator.loadLog4jPlugins();
+        LogConfigurator.configureESLogging(); // native access requires logging to be initialized
+    }
+
     @Override
     protected Codec getCodec() {
         return new Lucene99Codec() {

server/src/test/java/org/elasticsearch/index/codec/vectors/ES815BitFlatVectorFormatTests.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.index.codec.vectors;
+
+import org.apache.lucene.codecs.Codec;
+import org.apache.lucene.codecs.KnnVectorsFormat;
+import org.apache.lucene.codecs.lucene99.Lucene99Codec;
+import org.apache.lucene.index.VectorSimilarityFunction;
+import org.junit.Before;
+
+public class ES815BitFlatVectorFormatTests extends BaseKnnBitVectorsFormatTestCase {
+
+    @Override
+    protected Codec getCodec() {
+        return new Lucene99Codec() {
+            @Override
+            public KnnVectorsFormat getKnnVectorsFormatForField(String field) {
+                return new ES815BitFlatVectorFormat();
+            }
+        };
+    }
+
+    @Before
+    public void init() {
+        similarityFunction = VectorSimilarityFunction.EUCLIDEAN;
+    }
+
+}

server/src/test/java/org/elasticsearch/index/codec/vectors/ES815HnswBitVectorsFormatTests.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.index.codec.vectors;
+
+import org.apache.lucene.codecs.Codec;
+import org.apache.lucene.codecs.KnnVectorsFormat;
+import org.apache.lucene.codecs.lucene99.Lucene99Codec;
+import org.apache.lucene.index.VectorSimilarityFunction;
+import org.junit.Before;
+
+public class ES815HnswBitVectorsFormatTests extends BaseKnnBitVectorsFormatTestCase {
+
+    @Override
+    protected Codec getCodec() {
+        return new Lucene99Codec() {
+            @Override
+            public KnnVectorsFormat getKnnVectorsFormatForField(String field) {
+                return new ES815HnswBitVectorsFormat();
+            }
+        };
+    }
+
+    @Before
+    public void init() {
+        similarityFunction = VectorSimilarityFunction.EUCLIDEAN;
+    }
+}

server/src/test/java/org/elasticsearch/index/mapper/vectors/BinaryDenseVectorScriptDocValuesTests.java

@@ -236,8 +236,8 @@ public class BinaryDenseVectorScriptDocValuesTests extends ESTestCase {
 
     public static BytesRef mockEncodeDenseVector(float[] values, ElementType elementType, IndexVersion indexVersion) {
         int numBytes = indexVersion.onOrAfter(DenseVectorFieldMapper.MAGNITUDE_STORED_INDEX_VERSION)
-            ? elementType.elementBytes * values.length + DenseVectorFieldMapper.MAGNITUDE_BYTES
+            ? elementType.getNumBytes(values.length) + DenseVectorFieldMapper.MAGNITUDE_BYTES
-            : elementType.elementBytes * values.length;
+            : elementType.getNumBytes(values.length);
         double dotProduct = 0f;
         ByteBuffer byteBuffer = elementType.createByteBuffer(indexVersion, numBytes);
         for (float value : values) {

server/src/test/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapperTests.java

@@ -71,11 +71,13 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
     private final ElementType elementType;
     private final boolean indexed;
     private final boolean indexOptionsSet;
+    private final int dims;
 
     public DenseVectorFieldMapperTests() {
-        this.elementType = randomFrom(ElementType.BYTE, ElementType.FLOAT);
+        this.elementType = randomFrom(ElementType.BYTE, ElementType.FLOAT, ElementType.BIT);
         this.indexed = randomBoolean();
         this.indexOptionsSet = this.indexed && randomBoolean();
+        this.dims = ElementType.BIT == elementType ? 4 * Byte.SIZE : 4;
     }
 
     @Override
@@ -89,7 +91,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
     }
 
     private void indexMapping(XContentBuilder b, IndexVersion indexVersion) throws IOException {
-        b.field("type", "dense_vector").field("dims", 4);
+        b.field("type", "dense_vector").field("dims", dims);
         if (elementType != ElementType.FLOAT) {
             b.field("element_type", elementType.toString());
         }
@@ -108,7 +110,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
             b.endObject();
         }
         if (indexed) {
-            b.field("similarity", "dot_product");
+            b.field("similarity", elementType == ElementType.BIT ? "l2_norm" : "dot_product");
             if (indexOptionsSet) {
                 b.startObject("index_options");
                 b.field("type", "hnsw");
@@ -121,52 +123,86 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
 
     @Override
     protected Object getSampleValueForDocument() {
-        return elementType == ElementType.BYTE ? List.of((byte) 1, (byte) 1, (byte) 1, (byte) 1) : List.of(0.5, 0.5, 0.5, 0.5);
+        return elementType == ElementType.FLOAT ? List.of(0.5, 0.5, 0.5, 0.5) : List.of((byte) 1, (byte) 1, (byte) 1, (byte) 1);
     }
 
     @Override
     protected void registerParameters(ParameterChecker checker) throws IOException {
         checker.registerConflictCheck(
             "dims",
-            fieldMapping(b -> b.field("type", "dense_vector").field("dims", 4)),
+            fieldMapping(b -> b.field("type", "dense_vector").field("dims", dims)),
-            fieldMapping(b -> b.field("type", "dense_vector").field("dims", 5))
+            fieldMapping(b -> b.field("type", "dense_vector").field("dims", dims + 8))
         );
         checker.registerConflictCheck(
             "similarity",
-            fieldMapping(b -> b.field("type", "dense_vector").field("dims", 4).field("index", true).field("similarity", "dot_product")),
+            fieldMapping(b -> b.field("type", "dense_vector").field("dims", dims).field("index", true).field("similarity", "dot_product")),
-            fieldMapping(b -> b.field("type", "dense_vector").field("dims", 4).field("index", true).field("similarity", "l2_norm"))
+            fieldMapping(b -> b.field("type", "dense_vector").field("dims", dims).field("index", true).field("similarity", "l2_norm"))
         );
         checker.registerConflictCheck(
             "index",
-            fieldMapping(b -> b.field("type", "dense_vector").field("dims", 4).field("index", true).field("similarity", "dot_product")),
+            fieldMapping(b -> b.field("type", "dense_vector").field("dims", dims).field("index", true).field("similarity", "dot_product")),
-            fieldMapping(b -> b.field("type", "dense_vector").field("dims", 4).field("index", false))
+            fieldMapping(b -> b.field("type", "dense_vector").field("dims", dims).field("index", false))
         );
         checker.registerConflictCheck(
             "element_type",
             fieldMapping(
                 b -> b.field("type", "dense_vector")
-                    .field("dims", 4)
+                    .field("dims", dims)
                     .field("index", true)
                     .field("similarity", "dot_product")
                     .field("element_type", "byte")
             ),
             fieldMapping(
                 b -> b.field("type", "dense_vector")
-                    .field("dims", 4)
+                    .field("dims", dims)
                     .field("index", true)
                     .field("similarity", "dot_product")
                     .field("element_type", "float")
             )
         );
+        checker.registerConflictCheck(
+            "element_type",
+            fieldMapping(
+                b -> b.field("type", "dense_vector")
+                    .field("dims", dims)
+                    .field("index", true)
+                    .field("similarity", "l2_norm")
+                    .field("element_type", "float")
+            ),
+            fieldMapping(
+                b -> b.field("type", "dense_vector")
+                    .field("dims", dims)
+                    .field("index", true)
+                    .field("similarity", "l2_norm")
+                    .field("element_type", "bit")
+            )
+        );
+        checker.registerConflictCheck(
+            "element_type",
+            fieldMapping(
+                b -> b.field("type", "dense_vector")
+                    .field("dims", dims)
+                    .field("index", true)
+                    .field("similarity", "l2_norm")
+                    .field("element_type", "byte")
+            ),
+            fieldMapping(
+                b -> b.field("type", "dense_vector")
+                    .field("dims", dims)
+                    .field("index", true)
+                    .field("similarity", "l2_norm")
+                    .field("element_type", "bit")
+            )
+        );
         checker.registerUpdateCheck(
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "flat")
                 .endObject(),
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "int8_flat")
@@ -175,13 +211,13 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         );
         checker.registerUpdateCheck(
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "flat")
                 .endObject(),
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "hnsw")
@@ -190,13 +226,13 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         );
         checker.registerUpdateCheck(
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "flat")
                 .endObject(),
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "int8_hnsw")
@@ -205,13 +241,13 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         );
         checker.registerUpdateCheck(
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "int8_flat")
                 .endObject(),
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "hnsw")
@@ -220,13 +256,13 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         );
         checker.registerUpdateCheck(
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "int8_flat")
                 .endObject(),
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "int8_hnsw")
@@ -235,13 +271,13 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         );
         checker.registerUpdateCheck(
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "hnsw")
                 .endObject(),
             b -> b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("index", true)
                 .startObject("index_options")
                 .field("type", "int8_hnsw")
@@ -252,7 +288,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
             "index_options",
             fieldMapping(
                 b -> b.field("type", "dense_vector")
-                    .field("dims", 4)
+                    .field("dims", dims)
                     .field("index", true)
                     .startObject("index_options")
                     .field("type", "hnsw")
@@ -260,7 +296,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
             ),
             fieldMapping(
                 b -> b.field("type", "dense_vector")
-                    .field("dims", 4)
+                    .field("dims", dims)
                     .field("index", true)
                     .startObject("index_options")
                     .field("type", "flat")
@@ -353,7 +389,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         mapping = mapping(b -> {
             b.startObject("field");
             b.field("type", "dense_vector")
-                .field("dims", 4)
+                .field("dims", dims)
                 .field("similarity", "cosine")
                 .field("index", true)
                 .startObject("index_options")
@@ -648,7 +684,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
                 () -> createDocumentMapper(
                     fieldMapping(
                         b -> b.field("type", "dense_vector")
-                            .field("dims", 4)
+                            .field("dims", dims)
                             .field("element_type", "byte")
                             .field("similarity", "l2_norm")
                             .field("index", true)
@@ -1020,6 +1056,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
                 }
                 yield floats;
             }
+            case BIT -> randomByteArrayOfLength(vectorFieldType.getVectorDimensions() / 8);
         };
     }
 
@@ -1196,7 +1233,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         boolean setEfConstruction = randomBoolean();
         MapperService mapperService = createMapperService(fieldMapping(b -> {
             b.field("type", "dense_vector");
-            b.field("dims", 4);
+            b.field("dims", dims);
             b.field("index", true);
             b.field("similarity", "dot_product");
             b.startObject("index_options");
@@ -1234,7 +1271,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         for (String quantizedFlatFormat : new String[] { "int8_flat", "int4_flat" }) {
             MapperService mapperService = createMapperService(fieldMapping(b -> {
                 b.field("type", "dense_vector");
-                b.field("dims", 4);
+                b.field("dims", dims);
                 b.field("index", true);
                 b.field("similarity", "dot_product");
                 b.startObject("index_options");
@@ -1275,7 +1312,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         float confidenceInterval = (float) randomDoubleBetween(0.90f, 1.0f, true);
         MapperService mapperService = createMapperService(fieldMapping(b -> {
             b.field("type", "dense_vector");
-            b.field("dims", 4);
+            b.field("dims", dims);
             b.field("index", true);
             b.field("similarity", "dot_product");
             b.startObject("index_options");
@@ -1316,7 +1353,7 @@ public class DenseVectorFieldMapperTests extends MapperTestCase {
         float confidenceInterval = (float) randomDoubleBetween(0.90f, 1.0f, true);
         MapperService mapperService = createMapperService(fieldMapping(b -> {
             b.field("type", "dense_vector");
-            b.field("dims", 4);
+            b.field("dims", dims);
             b.field("index", true);
             b.field("similarity", "dot_product");
             b.startObject("index_options");

server/src/test/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldTypeTests.java

@@ -185,10 +185,12 @@ public class DenseVectorFieldTypeTests extends FieldTypeTestCase {
                 queryVector[i] = randomByte();
                 floatQueryVector[i] = queryVector[i];
             }
-            Query query = field.createKnnQuery(queryVector, 10, null, null, producer);
+            VectorData vectorData = new VectorData(null, queryVector);
+            Query query = field.createKnnQuery(vectorData, 10, null, null, producer);
             assertThat(query, instanceOf(DiversifyingChildrenByteKnnVectorQuery.class));
 
-            query = field.createKnnQuery(floatQueryVector, 10, null, null, producer);
+            vectorData = new VectorData(floatQueryVector, null);
+            query = field.createKnnQuery(vectorData, 10, null, null, producer);
             assertThat(query, instanceOf(DiversifyingChildrenByteKnnVectorQuery.class));
         }
     }
@@ -321,7 +323,8 @@ public class DenseVectorFieldTypeTests extends FieldTypeTestCase {
             for (int i = 0; i < 4096; i++) {
                 queryVector[i] = randomByte();
             }
-            Query query = fieldWith4096dims.createKnnQuery(queryVector, 10, null, null, null);
+            VectorData vectorData = new VectorData(null, queryVector);
+            Query query = fieldWith4096dims.createKnnQuery(vectorData, 10, null, null, null);
             assertThat(query, instanceOf(KnnByteVectorQuery.class));
         }
     }
@@ -359,7 +362,10 @@ public class DenseVectorFieldTypeTests extends FieldTypeTestCase {
         );
         assertThat(e.getMessage(), containsString("The [cosine] similarity does not support vectors with zero magnitude."));
 
-        e = expectThrows(IllegalArgumentException.class, () -> cosineField.createKnnQuery(new byte[] { 0, 0, 0 }, 10, null, null, null));
+        e = expectThrows(
+            IllegalArgumentException.class,
+            () -> cosineField.createKnnQuery(new VectorData(null, new byte[] { 0, 0, 0 }), 10, null, null, null)
+        );
         assertThat(e.getMessage(), containsString("The [cosine] similarity does not support vectors with zero magnitude."));
     }
 }

server/src/test/java/org/elasticsearch/script/VectorScoreScriptUtilsTests.java

@@ -114,10 +114,10 @@ public class VectorScoreScriptUtilsTests extends ESTestCase {
             );
 
             e = expectThrows(IllegalArgumentException.class, () -> new Hamming(scoreScript, queryVector, fieldName));
-            assertThat(e.getMessage(), containsString("hamming distance is only supported for byte vectors"));
+            assertThat(e.getMessage(), containsString("hamming distance is only supported for byte or bit vectors"));
 
             e = expectThrows(IllegalArgumentException.class, () -> new Hamming(scoreScript, invalidQueryVector, fieldName));
-            assertThat(e.getMessage(), containsString("hamming distance is only supported for byte vectors"));
+            assertThat(e.getMessage(), containsString("hamming distance is only supported for byte or bit vectors"));
 
             // Check scripting infrastructure integration
             DotProduct dotProduct = new DotProduct(scoreScript, queryVector, fieldName);

server/src/test/java/org/elasticsearch/search/vectors/AbstractKnnVectorQueryBuilderTestCase.java

@@ -122,7 +122,7 @@ abstract class AbstractKnnVectorQueryBuilderTestCase extends AbstractQueryTestCa
         Query filterQuery = booleanQuery.clauses().isEmpty() ? null : booleanQuery;
         // The field should always be resolved to the concrete field
         Query knnVectorQueryBuilt = switch (elementType()) {
-            case BYTE -> new ESKnnByteVectorQuery(
+            case BYTE, BIT -> new ESKnnByteVectorQuery(
                 VECTOR_FIELD,
                 queryBuilder.queryVector().asByteVector(),
                 queryBuilder.numCands(),
@@ -145,7 +145,10 @@ abstract class AbstractKnnVectorQueryBuilderTestCase extends AbstractQueryTestCa
         SearchExecutionContext context = createSearchExecutionContext();
         KnnVectorQueryBuilder query = new KnnVectorQueryBuilder(VECTOR_FIELD, new float[] { 1.0f, 2.0f }, 10, null);
         IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> query.doToQuery(context));
-        assertThat(e.getMessage(), containsString("the query vector has a different dimension [2] than the index vectors [3]"));
+        assertThat(
+            e.getMessage(),
+            containsString("The query vector has a different number of dimensions [2] than the document vectors [3]")
+        );
     }
 
     public void testNonexistentField() {

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/common/EmbeddingRequestChunker.java

@@ -46,6 +46,7 @@ public class EmbeddingRequestChunker {
             return switch (elementType) {
                 case BYTE -> EmbeddingType.BYTE;
                 case FLOAT -> EmbeddingType.FLOAT;
+                case BIT -> throw new IllegalArgumentException("Bit vectors are not supported");
             };
         }
     };