Deprecate COSINE VectorSimilarity function

lucene/CHANGES.txt

@@ -232,6 +232,8 @@ API Changes
 
 * GITHUB#13145: Deprecate ByteBufferIndexInput as it will be removed in Lucene 10.0. (Uwe Schindler)
 
+* GITHUB#13281: Mark COSINE VectorSimilarityFunction as deprecated. (Pulkit Gupta)
+
 New Features
 ---------------------
 

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

@@ -66,7 +66,10 @@ public float compare(byte[] v1, byte[] v2) {
  * vectors to unit length, and instead use {@link VectorSimilarityFunction#DOT_PRODUCT}. You
  * should only use this function if you need to preserve the original vectors and cannot normalize
  * them in advance. The similarity score is normalised to assure it is positive.
+ *
+ * @deprecated Use MAXIMUM_INNER_PRODUCT or DOT_PRODUCT instead
  */
+ @Deprecated
  COSINE {
  @Override
  public float compare(float[] v1, float[] v2) {

lucene/core/src/java/org/apache/lucene/util/VectorUtil.java

@@ -70,7 +70,9 @@ public static float dotProduct(float[] a, float[] b) {
  * Returns the cosine similarity between the two vectors.
  *
  * @throws IllegalArgumentException if the vectors' dimensions differ.
+ * @deprecated use dot-product instead using normalized vectors
  */
+ @Deprecated
  public static float cosine(float[] a, float[] b) {
  if (a.length != b.length) {
  throw new IllegalArgumentException("vector dimensions differ: " + a.length + "!=" + b.length);
@@ -80,7 +82,12 @@ public static float cosine(float[] a, float[] b) {
  return r;
  }
 
- /** Returns the cosine similarity between the two vectors. */
+ /**
+ * Returns the cosine similarity between the two vectors.
+ *
+ * @deprecated use dot-product instead using normalized vectors
+ */
+ @Deprecated
  public static float cosine(byte[] a, byte[] b) {
  if (a.length != b.length) {
  throw new IllegalArgumentException("vector dimensions differ: " + a.length + "!=" + b.length);