Return float[] instead of List<Double> in valueFetcher for rank_vectors

docs/changelog/126702.yaml

@@ -0,0 +1,5 @@
+pr: 126702
+summary: "Return float[] instead of List<Double> in `valueFetcher`"
+area: Search
+type: enhancement
+issues: []

x-pack/plugin/rank-vectors/src/main/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapper.java

@@ -180,7 +180,20 @@ public ValueFetcher valueFetcher(SearchExecutionContext context, String format)
             return new ArraySourceValueFetcher(name(), context) {
                 @Override
                 protected Object parseSourceValue(Object value) {
-                    return value;
+                    List<?> outerList = (List<?>) value;
+                    List<Object> vectors = new ArrayList<>(outerList.size());
+                    for (Object o : outerList) {
+                        if (o instanceof List<?> innerList) {
+                            float[] vector = new float[innerList.size()];
+                            for (int i = 0; i < vector.length; i++) {
+                                vector[i] = ((Number) innerList.get(i)).floatValue();
+                            }
+                            vectors.add(vector);
+                        } else {
+                            vectors.add(o);
+                        }
+                    }
+                    return vectors;
                 }
             };
         }

x-pack/plugin/rank-vectors/src/test/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapperTests.java

@@ -383,19 +383,10 @@ protected void assertFetch(MapperService mapperService, String field, Object val
             switch (denseVectorFieldType.getElementType()) {
                 case BYTE -> assumeFalse("byte element type testing not currently added", false);
                 case FLOAT -> {
-                    List<float[]> fetchedFloatsList = new ArrayList<>();
-                    for (var f : fromNative) {
-                        float[] fetchedFloats = new float[denseVectorFieldType.getVectorDimensions()];
-                        assert f instanceof List;
-                        List<?> vector = (List<?>) f;
-                        int i = 0;
-                        for (Object v : vector) {
-                            assert v instanceof Number;
-                            fetchedFloats[i++] = ((Number) v).floatValue();
-                        }
-                        fetchedFloatsList.add(fetchedFloats);
+                    float[][] fetchedFloats = new float[fromNative.size()][];
+                    for (int i = 0; i < fromNative.size(); i++) {
+                        fetchedFloats[i] = (float[]) fromNative.get(i);
                     }
-                    float[][] fetchedFloats = fetchedFloatsList.toArray(new float[0][]);
                     assertThat("fetching " + value, fetchedFloats, equalTo(value));
                 }
             }

x-pack/plugin/rank-vectors/src/test/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldTypeTests.java

@@ -19,10 +19,13 @@
 
 import java.io.IOException;
 import java.util.Collections;
+import java.util.HexFormat;
 import java.util.List;
 import java.util.Set;
 
 import static org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper.BBQ_MIN_DIMS;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.iterableWithSize;
 
 public class RankVectorsFieldTypeTests extends FieldTypeTestCase {
 
@@ -86,9 +89,16 @@ public void testDocValueFormat() {
 
     public void testFetchSourceValue() throws IOException {
         RankVectorsFieldType fft = createFloatFieldType();
-        List<List<Double>> vector = List.of(List.of(0.0, 1.0, 2.0, 3.0, 4.0, 6.0));
-        assertEquals(vector, fetchSourceValue(fft, vector));
+        List<List<Double>> vectorFromXContent = List.of(List.of(0.0, 1.0, 2.0, 3.0, 4.0, 6.0));
+        List<float[]> vector = List.of(new float[] { 0.0f, 1.0f, 2.0f, 3.0f, 4.0f, 6.0f });
+        assertThat(fetchSourceValue(fft, vectorFromXContent), iterableWithSize(1));
+        assertThat(fetchSourceValue(fft, vectorFromXContent).get(0), equalTo(vector.get(0)));
         RankVectorsFieldType bft = createByteFieldType();
-        assertEquals(vector, fetchSourceValue(bft, vector));
+        assertThat(fetchSourceValue(bft, vectorFromXContent), iterableWithSize(1));
+        assertThat(fetchSourceValue(bft, vectorFromXContent).get(0), equalTo(vector.get(0)));
+        String hexStr = HexFormat.of().formatHex(new byte[] { 0, 1, 2, 3, 4, 6 });
+        List<String> hexVecs = List.of(hexStr);
+        assertThat(fetchSourceValue(bft, hexVecs), iterableWithSize(1));
+        assertThat(fetchSourceValue(bft, hexVecs).get(0), equalTo(hexStr));
     }
 }