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 = FixedBitSet.cardinality(liveDocs, 0, liveDocs.length());
           if (numLive != numDocs) {
             throw new CheckIndexException(
                 "liveDocs count mismatch: info=" + numDocs + ", vs bits=" + numLive);

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

@@ -142,12 +142,7 @@ void onNewReader(CodecReader reader, SegmentCommitInfo info) throws IOException
 
   private boolean assertCheckLiveDocs(Bits bits, int expectedLength, int expectedDeleteCount) {
     assert bits.length() == expectedLength;
-    int deletedCount = 0;
-    for (int i = 0; i < bits.length(); i++) {
-      if (bits.get(i) == false) {
-        deletedCount++;
-      }
-    }
+    int deletedCount = bits.length() - FixedBitSet.cardinality(bits, 0, bits.length());
     assert deletedCount == expectedDeleteCount
         : "deleted: " + deletedCount + " != expected: " + expectedDeleteCount;
     return true;
@@ -255,11 +250,7 @@ boolean verifyDocCounts(CodecReader reader) {
     int count = 0;
     Bits liveDocs = getLiveDocs();
     if (liveDocs != null) {
-      for (int docID = 0; docID < info.info.maxDoc(); docID++) {
-        if (liveDocs.get(docID)) {
-          count++;
-        }
-      }
+      count = FixedBitSet.cardinality(liveDocs, 0, info.info.maxDoc());
     } else {
       count = info.info.maxDoc();
     }

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

@@ -239,6 +239,21 @@ public int cardinality(int from, int to) {
     return cardinality;
   }
 
+  /** Just like {@link #cardinality(int, int)}, return the number of set bits for {@code Bits}. */
+  public static int cardinality(Bits bits, int from, int to) {
+    assert bits != null;
+    if (bits instanceof FixedBits fixedBits) {
+      return fixedBits.bitSet.cardinality(from, to);
+    }
+    int count = 0;
+    for (int i = from; i < to; i++) {
+      if (bits.get(i)) {
+        count++;
+      }
+    }
+    return count;
+  }
+
   @Override
   public int approximateCardinality() {
     // Naive sampling: compute the number of bits that are set on the first 16 longs every 1024