Enforce VectorValues.cost() is equal to size(). (#11962)

`VectorValues` have a `cost()` method that reports an approximate number of
documents that have a vector, but also a `size()` method that reports the
accurate number of vectors in the field. Since KNN vectors only support
single-valued fields we should enforce that `cost()` returns the `size()`.

lucene/CHANGES.txt

@@ -123,6 +123,9 @@ API Changes
   necessary and also illegal as it reported a number of dimensions equal to
   zero. (Adrien Grand)
 
+* GITHUB#11962: VectorValues#cost() now delegates to VectorValues#size().
+  (Adrien Grand)
+
 New Features
 ---------------------
 * GITHUB#11795: Add ByteWritesTrackingDirectoryWrapper to expose metrics for bytes merged, flushed, and overall

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene90/Lucene90HnswVectorsReader.java

@@ -443,11 +443,6 @@ public final class Lucene90HnswVectorsReader extends KnnVectorsReader {
       return doc;
     }
 
-    @Override
-    public long cost() {
-      return ordToDoc.length;
-    }
-
     @Override
     public RandomAccessVectorValues copy() {
       return new OffHeapVectorValues(dimension, ordToDoc, dataIn.clone());

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene91/Lucene91HnswVectorsReader.java

@@ -495,11 +495,6 @@ public final class Lucene91HnswVectorsReader extends KnnVectorsReader {
       return doc;
     }
 
-    @Override
-    public long cost() {
-      return size;
-    }
-
     @Override
     public RandomAccessVectorValues copy() {
       return new OffHeapVectorValues(dimension, size, ordToDoc, dataIn.clone());

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene92/OffHeapVectorValues.java

@@ -59,11 +59,6 @@ abstract class OffHeapVectorValues extends VectorValues implements RandomAccessV
     return size;
   }
 
-  @Override
-  public long cost() {
-    return size;
-  }
-
   @Override
   public float[] vectorValue(int targetOrd) throws IOException {
     slice.seek((long) targetOrd * byteSize);
@@ -286,11 +281,6 @@ abstract class OffHeapVectorValues extends VectorValues implements RandomAccessV
       return doc = NO_MORE_DOCS;
     }
 
-    @Override
-    public long cost() {
-      return 0;
-    }
-
     @Override
     public RandomAccessVectorValues copy() throws IOException {
       throw new UnsupportedOperationException();

lucene/codecs/src/java/org/apache/lucene/codecs/simpletext/SimpleTextKnnVectorsReader.java

@@ -333,11 +333,6 @@ public class SimpleTextKnnVectorsReader extends KnnVectorsReader {
       return slowAdvance(target);
     }
 
-    @Override
-    public long cost() {
-      return size();
-    }
-
     private void readAllVectors() throws IOException {
       for (float[] value : values) {
         readVector(value);

lucene/core/src/java/org/apache/lucene/codecs/KnnVectorsWriter.java

@@ -179,11 +179,6 @@ public abstract class KnnVectorsWriter implements Accountable, Closeable {
       return size;
     }
 
-    @Override
-    public long cost() {
-      return size;
-    }
-
     @Override
     public int dimension() {
       return subs.get(0).values.dimension();

lucene/core/src/java/org/apache/lucene/codecs/lucene94/OffHeapVectorValues.java

@@ -59,11 +59,6 @@ abstract class OffHeapVectorValues extends VectorValues implements RandomAccessV
     return size;
   }
 
-  @Override
-  public long cost() {
-    return size;
-  }
-
   @Override
   public float[] vectorValue(int targetOrd) throws IOException {
     slice.seek((long) targetOrd * byteSize);
@@ -295,11 +290,6 @@ abstract class OffHeapVectorValues extends VectorValues implements RandomAccessV
       return doc = NO_MORE_DOCS;
     }
 
-    @Override
-    public long cost() {
-      return 0;
-    }
-
     @Override
     public RandomAccessVectorValues copy() throws IOException {
       throw new UnsupportedOperationException();

lucene/core/src/java/org/apache/lucene/index/BufferingKnnVectorsWriter.java

@@ -280,10 +280,5 @@ public abstract class BufferingKnnVectorsWriter extends KnnVectorsWriter {
     public int advance(int target) {
       throw new UnsupportedOperationException();
     }
-
-    @Override
-    public long cost() {
-      return docsWithFieldIter.cost();
-    }
   }
 }

lucene/core/src/java/org/apache/lucene/index/FilterVectorValues.java

@@ -48,11 +48,6 @@ public abstract class FilterVectorValues extends VectorValues {
     return in.advance(target);
   }
 
-  @Override
-  public long cost() {
-    return in.cost();
-  }
-
   @Override
   public int dimension() {
     return in.dimension();

lucene/core/src/java/org/apache/lucene/index/VectorValues.java

@@ -39,14 +39,17 @@ public abstract class VectorValues extends DocIdSetIterator {
   public abstract int dimension();
 
   /**
-   * TODO: should we use cost() for this? We rely on its always being exactly the number of
-   * documents having a value for this field, which is not guaranteed by the cost() contract, but in
-   * all the implementations so far they are the same.
+   * Return the number of vectors for this field.
    *
    * @return the number of vectors returned by this iterator
    */
   public abstract int size();
 
+  @Override
+  public final long cost() {
+    return size();
+  }
+
   /**
    * Return the vector value for the current document ID. It is illegal to call this method when the
    * iterator is not positioned: before advancing, or after failing to advance. The returned array
@@ -127,10 +130,5 @@ public abstract class VectorValues extends DocIdSetIterator {
     public int advance(int target) throws IOException {
       throw new UnsupportedOperationException();
     }
-
-    @Override
-    public long cost() {
-      return size();
-    }
   }
 }

lucene/core/src/test/org/apache/lucene/util/hnsw/MockVectorValues.java

@@ -122,9 +122,4 @@ class MockVectorValues extends VectorValues implements RandomAccessVectorValues
     }
     return NO_MORE_DOCS;
   }
-
-  @Override
-  public long cost() {
-    return size();
-  }
 }

lucene/core/src/test/org/apache/lucene/util/hnsw/TestHnswGraph.java

@@ -767,11 +767,6 @@ public class TestHnswGraph extends LuceneTestCase {
       return doc;
     }
 
-    @Override
-    public long cost() {
-      return size;
-    }
-
     @Override
     public float[] vectorValue(int ord) {
       return unitVector2d(ord / (double) size, value);