[GitHub] [lucene] rmuir commented on a change in pull request #710: LUCENE-10311: Make FixedBitSet#approximateCardinality faster (and actually approximate).

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rmuir commented on a change in pull request #710:
URL: https://github.com/apache/lucene/pull/710#discussion_r814137771



##########
File path: lucene/core/src/java/org/apache/lucene/util/FixedBitSet.java
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@@ -176,6 +176,30 @@ public int cardinality() {
     return (int) BitUtil.pop_array(bits, 0, numWords);
   }
 
+  @Override
+  public int approximateCardinality() {
+    // Naive sampling: compute the number of bits that are set on the first 16 longs every 1024
+    // longs and scale the result by 1024/16.
+    // This computes the pop count on ranges instead of single longs in order to take advantage of
+    // vectorization.
+
+    final int rangeLength = 16;
+    final int interval = 1024;
+
+    if (numWords < interval) {
+      return cardinality();
+    }
+
+    long popCount = 0;
+    int maxWord;
+    for (maxWord = 0; maxWord + interval < numWords; maxWord += interval) {
+      popCount += BitUtil.pop_array(bits, maxWord, rangeLength);

Review comment:
       this isn't related/review comment. just saying i would be in favor of removing these `BitUtil` methods as I think they are outdated and provide no value. I think it would be easier on our eyes to just see loops with Long.bitCount? 
   
   The other constants/methods in the `BitUtil` class actually provide value. But let's not wrap what the JDK provides efficiently for no reason?




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