Adds support for 7-bit quantization in BBQ index

docs/reference/elasticsearch/mapping-reference/bbq.md

@@ -244,12 +244,13 @@ A lower `visit_percentage` can further reduce memory use and speed up queries, w
 stack: ga 9.4
 ```
 By default, BBQ performs asymmetric quantization: it performs 1-bit quantization for the indexed vectors and 4-bit quantization for query vectors.
-For fields of type `bbq_disk` it is possible to change the level of quantization for indexed vectors by setting the `bits` parameter in `index_options` to `1` (default), `2` or `4`.
+For fields of type `bbq_disk` it is possible to change the level of quantization for indexed vectors by setting the `bits` parameter in `index_options` to `1` (default), `2`, `4` or `7`.
 
 If no `oversampling_factor` is specified, setting `bits` will automatically adjust that as follows:
 * `bits = 1` --> `oversampling_factor = 3.0`
 * `bits = 2` --> `oversampling_factor = 1.5`
 * `bits = 4` --> `oversampling_factor = 0` (no oversampling)
+* `bits = 7` --> `oversampling_factor = 0` (no oversampling)
 
 ```console
 PUT bbq_disk-index/_mapping

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

@@ -1836,9 +1836,9 @@ public DenseVectorIndexOptions parseIndexOptions(
                 if (indexVersion.onOrAfter(DISK_BBQ_QUANTIZE_BITS) && experimentalFeaturesEnabled) {
                     Object quantizeBitsNode = indexOptionsMap.remove("bits");
                     quantizeBits = XContentMapValues.nodeIntegerValue(quantizeBitsNode, DEFAULT_BBQ_IVF_QUANTIZE_BITS);
-                    if ((quantizeBits == 1 || quantizeBits == 2 || quantizeBits == 4) == false) {
+                    if ((quantizeBits == 1 || quantizeBits == 2 || quantizeBits == 4 || quantizeBits == 7) == false) {
                         throw new IllegalArgumentException(
-                            "'bits' must be 1, 2 or 4, got: " + quantizeBits + " for field [" + fieldName + "]"
+                            "'bits' must be 1, 2, 4 or 7, got: " + quantizeBits + " for field [" + fieldName + "]"
                         );
                     }
                 } else {
@@ -2562,7 +2562,7 @@ KnnVectorsFormat getVectorsFormat(ElementType elementType, ExecutorService mergi
             }
             if (experimentalFeaturesEnabled) {
                 return new ESNextDiskBBQVectorsFormat(
-                    ESNextDiskBBQVectorsFormat.QuantEncoding.fromId(bits >> 1),
+                    ESNextDiskBBQVectorsFormat.QuantEncoding.fromBits((byte) bits),
                     clusterSize,
                     ES920DiskBBQVectorsFormat.DEFAULT_CENTROIDS_PER_PARENT_CLUSTER,
                     elementType,
@@ -2653,6 +2653,10 @@ public boolean isOnDiskRescore() {
         public boolean doPrecondition() {
             return doPrecondition;
         }
+
+        public int getBits() {
+            return bits;
+        }
     }
 
     public record RescoreVector(float oversample) implements ToXContentObject {

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

@@ -674,6 +674,25 @@ public void testIVFParsing() throws IOException {
             assertEquals(-1, indexOptions.getFlatIndexThreshold());
             assertEquals(DYNAMIC_VISIT_RATIO, indexOptions.defaultVisitPercentage, 0.0);
         }
+        {
+            DocumentMapper mapperService = createMapperService(experimentalEnabled, fieldMapping(b -> {
+                b.field("type", "dense_vector");
+                b.field("dims", 128);
+                b.field("index", true);
+                b.field("similarity", "dot_product");
+                b.startObject("index_options");
+                b.field("type", "bbq_disk");
+                b.field("bits", 7);
+                b.endObject();
+            })).documentMapper();
+
+            DenseVectorFieldMapper denseVectorFieldMapper = (DenseVectorFieldMapper) mapperService.mappers().getMapper("field");
+            DenseVectorFieldMapper.BBQIVFIndexOptions indexOptions = (DenseVectorFieldMapper.BBQIVFIndexOptions) denseVectorFieldMapper
+                .fieldType()
+                .getIndexOptions();
+            assertEquals(7, indexOptions.bits, 0.0F);
+            assertNull(indexOptions.rescoreVector);
+        }
         {
             DocumentMapper mapperService = createMapperService(experimentalDisabled, fieldMapping(b -> {
                 b.field("type", "dense_vector");

x-pack/plugin/diskbbq/src/main/java/org/elasticsearch/xpack/diskbbq/DiskBBQPlugin.java

@@ -50,7 +50,7 @@ public VectorsFormatProvider getVectorsFormatProvider() {
                 int flatIndexThreshold = diskbbq.getFlatIndexThreshold();
                 if (Build.current().isSnapshot()) {
                     return new ESNextDiskBBQVectorsFormat(
-                        ESNextDiskBBQVectorsFormat.QuantEncoding.ONE_BIT_4BIT_QUERY,
+                        ESNextDiskBBQVectorsFormat.QuantEncoding.fromBits((byte) diskbbq.getBits()),
                         clusterSize,
                         ES920DiskBBQVectorsFormat.DEFAULT_CENTROIDS_PER_PARENT_CLUSTER,
                         elementType,