Use FixedBitSet#cardinality for counting liveDocs in CheckIndex

lucene/CHANGES.txt

@@ -239,6 +239,8 @@ Optimizations
 
 * GITHUB#14980: Add bulk off-heap scoring for float32 vectors (Chris Hegarty)
 
+* GITHUB#15045: Use FixedBitSet#cardinality for counting liveDocs in CheckIndex (Zhang Chao)
+
 Changes in Runtime Behavior
 ---------------------
 * GITHUB#14823: Decrease TieredMergePolicy's default number of segments per

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

@@ -1297,12 +1297,7 @@ public static Status.LiveDocStatus testLiveDocs(
         if (liveDocs == null) {
           throw new CheckIndexException("segment should have deletions, but liveDocs is null");
         } else {
-          int numLive = 0;
-          for (int j = 0; j < liveDocs.length(); j++) {
-            if (liveDocs.get(j)) {
-              numLive++;
-            }
-          }
+          int numLive = bitsCardinality(liveDocs);
           if (numLive != numDocs) {
             throw new CheckIndexException(
                 "liveDocs count mismatch: info=" + numDocs + ", vs bits=" + numLive);
@@ -1348,6 +1343,28 @@ public static Status.LiveDocStatus testLiveDocs(
     return status;
   }
 
+  /**
+   * Returns the cardinality of the given {@code Bits}.
+   *
+   * <p>This method processes bits in batches of 1024 using {@link Bits#applyMask} and {@link
+   * FixedBitSet#cardinality}, which is faster than checking bits one by one.
+   */
+  static int bitsCardinality(Bits bits) {
+    int cardinality = 0;
+    FixedBitSet copy = new FixedBitSet(1024);
+    for (int offset = 0; offset < bits.length(); offset += copy.length()) {
+      int numBitsToCopy = Math.min(bits.length() - offset, copy.length());
+      copy.set(0, copy.length());
+      if (numBitsToCopy < copy.length()) {
+        // Clear ghost bits
+        copy.clear(numBitsToCopy, copy.length());
+      }
+      bits.applyMask(copy, offset);
+      cardinality += copy.cardinality();
+    }
+    return cardinality;
+  }
+
   /** Test field infos. */
   public static Status.FieldInfoStatus testFieldInfos(
       CodecReader reader, PrintStream infoStream, boolean failFast) throws IOException {