[oldlogstashjson] simplify decoding from v0 to v1

lib/logstash/codecs/oldlogstashjson.rb

@@ -5,6 +5,12 @@ class LogStash::Codecs::OldLogStashJSON < LogStash::Codecs::Base
   config_name "oldlogstashjson"
   milestone 1
 
+  # Map from v0 name to v1 name.
+  # Note: @source is gone and has no similar field.
+  V0_TO_V1 = {"@timestamp" => "@timestamp", "@message" => "message",
+              "@tags" => "tags", "@type" => "type",
+              "@source_host" => "host", "@source_path" => "path"}
+
   public
   def decode(data)
     obj = JSON.parse(data.force_encoding("UTF-8"))
@@ -12,19 +18,10 @@ class LogStash::Codecs::OldLogStashJSON < LogStash::Codecs::Base
     h  = {}
 
     # Convert the old logstash schema to the new one.
-    basics = %w(@message @tags @type)
-    basics.each do |key|
-      # Convert '@message' to 'message', etc
-      h[key[1..-1]] = obj[key] if obj.include?(key)
+    V0_TO_V1.each do |key, val|
+      h[val] = obj[key] if obj.include?(key)
     end
 
-    # fix other mappings
-    h["host"] = obj["@source_host"]
-    h["path"] = obj["@source_path"]
-    # Note: @source is gone and has no similar field.
-
-    h["@timestamp"] = obj["@timestamp"] if obj.include?("@timestamp")
-
     h.merge!(obj["@fields"]) if obj["@fields"].is_a?(Hash)
     yield LogStash::Event.new(h)
   end # def decode

spec/codecs/oldlogstashjson.rb

@@ -18,6 +18,7 @@ describe LogStash::Codecs::OldLogStashJSON do
         insist { event["message"] } == data["@message"]
         insist { event["host"] } == data["@source_host"]
         insist { event["tags"] } == data["@tags"]
+        insist { event["path"] } != nil # @source_path not in v0 test data
       end
     end
   end