Implement support for weighted rrf

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilder.java

@@ -57,7 +57,6 @@ public final class RRFRetrieverBuilder extends CompoundRetrieverBuilder<RRFRetri
     public static final ParseField RANK_CONSTANT_FIELD = new ParseField("rank_constant");
     public static final ParseField FIELDS_FIELD = new ParseField("fields");
     public static final ParseField QUERY_FIELD = new ParseField("query");
-    public static final ParseField WEIGHTS_FIELD = new ParseField("weights");
 
     public static final int DEFAULT_RANK_CONSTANT = 60;
 
@@ -92,7 +91,6 @@ public final class RRFRetrieverBuilder extends CompoundRetrieverBuilder<RRFRetri
         PARSER.declareString(ConstructingObjectParser.optionalConstructorArg(), QUERY_FIELD);
         PARSER.declareInt(ConstructingObjectParser.optionalConstructorArg(), RANK_WINDOW_SIZE_FIELD);
         PARSER.declareInt(ConstructingObjectParser.optionalConstructorArg(), RANK_CONSTANT_FIELD);
-        PARSER.declareFloatArray(ConstructingObjectParser.optionalConstructorArg(), WEIGHTS_FIELD);
         RetrieverBuilder.declareBaseParserFields(PARSER);
     }
 
@@ -327,13 +325,6 @@ public void doToXContent(XContentBuilder builder, Params params) throws IOExcept
 
         builder.field(RANK_WINDOW_SIZE_FIELD.getPreferredName(), rankWindowSize);
         builder.field(RANK_CONSTANT_FIELD.getPreferredName(), rankConstant);
-        if (weights.length > 0) {
-            builder.startArray(WEIGHTS_FIELD.getPreferredName());
-            for (float weight : weights) {
-                builder.value(weight);
-            }
-            builder.endArray();
-        }
     }
 
     // ---- FOR TESTING XCONTENT PARSING ----

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverComponent.java

@@ -94,16 +94,7 @@ public static RRFRetrieverComponent fromXContent(XContentParser parser, Retrieve
                     parser.nextToken();
                     weight = parser.floatValue();
                 } else {
-                    if (retriever != null) {
-                        throw new ParsingException(parser.getTokenLocation(), "only one retriever can be specified");
-                    }
-                    throw new ParsingException(
-                        parser.getTokenLocation(),
-                        "unknown field [{}], expected [{}] or [{}]",
-                        fieldName,
-                        RETRIEVER_FIELD.getPreferredName(),
-                        WEIGHT_FIELD.getPreferredName()
-                    );
+                    throw new ParsingException(parser.getTokenLocation(), "unknown field [{}] after retriever", fieldName);
                 }
             } while (parser.nextToken() == XContentParser.Token.FIELD_NAME);
 
@@ -115,7 +106,8 @@ public static RRFRetrieverComponent fromXContent(XContentParser parser, Retrieve
         } else {
             RetrieverBuilder retriever = parser.namedObject(RetrieverBuilder.class, firstFieldName, context);
             context.trackRetrieverUsage(retriever.getName());
-            while (parser.nextToken() != XContentParser.Token.END_OBJECT) {
+            if (parser.nextToken() != XContentParser.Token.END_OBJECT) {
+                throw new ParsingException(parser.getTokenLocation(), "unknown field [{}] after retriever", parser.currentName());
             }
             return new RRFRetrieverComponent(retriever, DEFAULT_WEIGHT);
         }

x-pack/plugin/rank-rrf/src/test/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilderParsingTests.java

@@ -273,7 +273,7 @@ public void testRRFRetrieverComponentErrorCases() throws IOException {
             }
             """;
 
-        expectParsingException(multipleRetrieversContent, "only one retriever can be specified");
+        expectParsingException(multipleRetrieversContent, "unknown field [standard] after retriever");
 
         // Test case 2: Weight without retriever
         String weightOnlyContent = """