Add support for similarity-based vector searches (#12679)

### Description

Background in #12579

Add support for getting "all vectors within a radius" as opposed to getting the "topK closest vectors" in the current system

### Considerations

I've tried to keep this change minimal and non-invasive by not modifying any APIs and re-using existing HNSW graphs -- changing the graph traversal and result collection criteria to:
1. Visit all nodes (reachable from the entry node in the last level) that are within an outer "traversal" radius
2. Collect all nodes that are within an inner "result" radius

### Advantages

1. Queries that have a high number of "relevant" results will get all of those (not limited by `topK`)
2. Conversely, arbitrary queries where many results are not "relevant" will not waste time in getting all `topK` (when some of them will be removed later)
3. Results of HNSW searches need not be sorted - and we can store them in a plain list as opposed to min-max heaps (saving on `heapify` calls). Merging results from segments is also cheaper, where we just concatenate results as opposed to calculating the index-level `topK`

On a higher level, finding `topK` results needed HNSW searches to happen in `#rewrite` because of an interdependence of results between segments - where we want to find the index-level `topK` from multiple segment-level results. This is kind of against Lucene's concept of segments being independently searchable sub-indexes?

Moreover, we needed explicit concurrency (#12160) to perform these in parallel, and these shortcomings would be naturally overcome with the new objective of finding "all vectors within a radius" - inherently independent of results from another segment (so we can move searches to a more fitting place?)

### Caveats

I could not find much precedent in using HNSW graphs this way (or even the radius-based search for that matter - please add links to existing work if someone is aware) and consequently marked all classes as `@lucene.experimental`

For now I have re-used lots of functionality from `AbstractKnnVectorQuery` to keep this minimal, but if the use-case is accepted more widely we can look into writing more suitable queries (as mentioned above briefly)

lucene/CHANGES.txt

@@ -171,7 +171,11 @@ API Changes
 
 New Features
 ---------------------
-(No changes)
+
+* GITHUB#12679: Add support for similarity-based vector searches using [Byte|Float]VectorSimilarityQuery. Uses a new
+  VectorSimilarityCollector to find all vectors scoring above a `resultSimilarity` while traversing the HNSW graph till
+  better-scoring nodes are available, or the best candidate is below a score of `traversalSimilarity` in the lowest
+  level. (Aditya Prakash, Kaival Parikh)
 
 Improvements
 ---------------------

lucene/core/src/java/org/apache/lucene/search/AbstractVectorSimilarityQuery.java

@@ -0,0 +1,288 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.search;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.Objects;
+import org.apache.lucene.index.LeafReader;
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.util.BitSet;
+import org.apache.lucene.util.BitSetIterator;
+import org.apache.lucene.util.Bits;
+
+/**
+ * Search for all (approximate) vectors above a similarity threshold.
+ *
+ * @lucene.experimental
+ */
+abstract class AbstractVectorSimilarityQuery extends Query {
+  protected final String field;
+  protected final float traversalSimilarity, resultSimilarity;
+  protected final Query filter;
+
+  /**
+   * Search for all (approximate) vectors above a similarity threshold using {@link
+   * VectorSimilarityCollector}. If a filter is applied, it traverses as many nodes as the cost of
+   * the filter, and then falls back to exact search if results are incomplete.
+   *
+   * @param field a field that has been indexed as a vector field.
+   * @param traversalSimilarity (lower) similarity score for graph traversal.
+   * @param resultSimilarity (higher) similarity score for result collection.
+   * @param filter a filter applied before the vector search.
+   */
+  AbstractVectorSimilarityQuery(
+      String field, float traversalSimilarity, float resultSimilarity, Query filter) {
+    if (traversalSimilarity > resultSimilarity) {
+      throw new IllegalArgumentException("traversalSimilarity should be <= resultSimilarity");
+    }
+    this.field = Objects.requireNonNull(field, "field");
+    this.traversalSimilarity = traversalSimilarity;
+    this.resultSimilarity = resultSimilarity;
+    this.filter = filter;
+  }
+
+  abstract VectorScorer createVectorScorer(LeafReaderContext context) throws IOException;
+
+  protected abstract TopDocs approximateSearch(
+      LeafReaderContext context, Bits acceptDocs, int visitLimit) throws IOException;
+
+  @Override
+  public Weight createWeight(IndexSearcher searcher, ScoreMode scoreMode, float boost)
+      throws IOException {
+    return new Weight(this) {
+      final Weight filterWeight =
+          filter == null
+              ? null
+              : searcher.createWeight(searcher.rewrite(filter), ScoreMode.COMPLETE_NO_SCORES, 1);
+
+      @Override
+      public Explanation explain(LeafReaderContext context, int doc) throws IOException {
+        if (filterWeight != null) {
+          Scorer filterScorer = filterWeight.scorer(context);
+          if (filterScorer == null || filterScorer.iterator().advance(doc) > doc) {
+            return Explanation.noMatch("Doc does not match the filter");
+          }
+        }
+
+        VectorScorer scorer = createVectorScorer(context);
+        if (scorer == null) {
+          return Explanation.noMatch("Not indexed as the correct vector field");
+        } else if (scorer.advanceExact(doc)) {
+          float score = scorer.score();
+          if (score >= resultSimilarity) {
+            return Explanation.match(boost * score, "Score above threshold");
+          } else {
+            return Explanation.noMatch("Score below threshold");
+          }
+        } else {
+          return Explanation.noMatch("No vector found for doc");
+        }
+      }
+
+      @Override
+      public Scorer scorer(LeafReaderContext context) throws IOException {
+        @SuppressWarnings("resource")
+        LeafReader leafReader = context.reader();
+        Bits liveDocs = leafReader.getLiveDocs();
+
+        // If there is no filter
+        if (filterWeight == null) {
+          // Return exhaustive results
+          TopDocs results = approximateSearch(context, liveDocs, Integer.MAX_VALUE);
+          return VectorSimilarityScorer.fromScoreDocs(this, boost, results.scoreDocs);
+        }
+
+        Scorer scorer = filterWeight.scorer(context);
+        if (scorer == null) {
+          // If the filter does not match any documents
+          return null;
+        }
+
+        BitSet acceptDocs;
+        if (liveDocs == null && scorer.iterator() instanceof BitSetIterator bitSetIterator) {
+          // If there are no deletions, and matching docs are already cached
+          acceptDocs = bitSetIterator.getBitSet();
+        } else {
+          // Else collect all matching docs
+          FilteredDocIdSetIterator filtered =
+              new FilteredDocIdSetIterator(scorer.iterator()) {
+                @Override
+                protected boolean match(int doc) {
+                  return liveDocs == null || liveDocs.get(doc);
+                }
+              };
+          acceptDocs = BitSet.of(filtered, leafReader.maxDoc());
+        }
+
+        int cardinality = acceptDocs.cardinality();
+        if (cardinality == 0) {
+          // If there are no live matching docs
+          return null;
+        }
+
+        // Perform an approximate search
+        TopDocs results = approximateSearch(context, acceptDocs, cardinality);
+
+        // If the limit was exhausted
+        if (results.totalHits.relation == TotalHits.Relation.GREATER_THAN_OR_EQUAL_TO) {
+          // Return a lazy-loading iterator
+          return VectorSimilarityScorer.fromAcceptDocs(
+              this,
+              boost,
+              createVectorScorer(context),
+              new BitSetIterator(acceptDocs, cardinality),
+              resultSimilarity);
+        } else {
+          // Return an iterator over the collected results
+          return VectorSimilarityScorer.fromScoreDocs(this, boost, results.scoreDocs);
+        }
+      }
+
+      @Override
+      public boolean isCacheable(LeafReaderContext ctx) {
+        return true;
+      }
+    };
+  }
+
+  @Override
+  public void visit(QueryVisitor visitor) {
+    if (visitor.acceptField(field)) {
+      visitor.visitLeaf(this);
+    }
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    return sameClassAs(o)
+        && Objects.equals(field, ((AbstractVectorSimilarityQuery) o).field)
+        && Float.compare(
+                ((AbstractVectorSimilarityQuery) o).traversalSimilarity, traversalSimilarity)
+            == 0
+        && Float.compare(((AbstractVectorSimilarityQuery) o).resultSimilarity, resultSimilarity)
+            == 0
+        && Objects.equals(filter, ((AbstractVectorSimilarityQuery) o).filter);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(field, traversalSimilarity, resultSimilarity, filter);
+  }
+
+  private static class VectorSimilarityScorer extends Scorer {
+    final DocIdSetIterator iterator;
+    final float[] cachedScore;
+
+    VectorSimilarityScorer(Weight weight, DocIdSetIterator iterator, float[] cachedScore) {
+      super(weight);
+      this.iterator = iterator;
+      this.cachedScore = cachedScore;
+    }
+
+    static VectorSimilarityScorer fromScoreDocs(Weight weight, float boost, ScoreDoc[] scoreDocs) {
+      // Sort in ascending order of docid
+      Arrays.sort(scoreDocs, Comparator.comparingInt(scoreDoc -> scoreDoc.doc));
+
+      float[] cachedScore = new float[1];
+      DocIdSetIterator iterator =
+          new DocIdSetIterator() {
+            int index = -1;
+
+            @Override
+            public int docID() {
+              if (index < 0) {
+                return -1;
+              } else if (index >= scoreDocs.length) {
+                return NO_MORE_DOCS;
+              } else {
+                cachedScore[0] = boost * scoreDocs[index].score;
+                return scoreDocs[index].doc;
+              }
+            }
+
+            @Override
+            public int nextDoc() {
+              index++;
+              return docID();
+            }
+
+            @Override
+            public int advance(int target) {
+              index =
+                  Arrays.binarySearch(
+                      scoreDocs,
+                      new ScoreDoc(target, 0),
+                      Comparator.comparingInt(scoreDoc -> scoreDoc.doc));
+              if (index < 0) {
+                index = -1 - index;
+              }
+              return docID();
+            }
+
+            @Override
+            public long cost() {
+              return scoreDocs.length;
+            }
+          };
+
+      return new VectorSimilarityScorer(weight, iterator, cachedScore);
+    }
+
+    static VectorSimilarityScorer fromAcceptDocs(
+        Weight weight,
+        float boost,
+        VectorScorer scorer,
+        DocIdSetIterator acceptDocs,
+        float threshold) {
+      float[] cachedScore = new float[1];
+      DocIdSetIterator iterator =
+          new FilteredDocIdSetIterator(acceptDocs) {
+            @Override
+            protected boolean match(int doc) throws IOException {
+              // Compute the dot product
+              float score = scorer.score();
+              cachedScore[0] = score * boost;
+              return score >= threshold;
+            }
+          };
+
+      return new VectorSimilarityScorer(weight, iterator, cachedScore);
+    }
+
+    @Override
+    public int docID() {
+      return iterator.docID();
+    }
+
+    @Override
+    public DocIdSetIterator iterator() {
+      return iterator;
+    }
+
+    @Override
+    public float getMaxScore(int upTo) {
+      return Float.POSITIVE_INFINITY;
+    }
+
+    @Override
+    public float score() {
+      return cachedScore[0];
+    }
+  }
+}