Use cached cardinality for AcceptDocs cost (#15343)

* Use cached cardinality for AcceptDocs cost

* adding changes

Author: benwtrent

lucene/CHANGES.txt

@@ -200,6 +200,9 @@ Optimizations
 
 * GITHUB#15261: Implement longValues for MultiFieldNormValues to speedup CombinedQuery (Ge Song)
 
+* GITHUB#15343: Ensure that `AcceptDocs#cost()` only ever calls `BitSets#cardinality()`
+  once per instance to avoid redundant computation. (Ben Trent)
+
 Bug Fixes
 ---------------------
 * GITHUB#14161: PointInSetQuery's constructor now throws IllegalArgumentException

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

@@ -190,7 +190,7 @@ public Bits bits() throws IOException {
     @Override
     public int cost() throws IOException {
       createBitSetAcceptDocsIfNecessary();
-      return acceptBitSet.cardinality();
+      return cardinality;
     }
 
     @Override