jpountz commented on code in PR #14273:
URL: https://github.com/apache/lucene/pull/14273#discussion_r2018661757
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lucene/core/src/java/org/apache/lucene/util/FixedBitSet.java:
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@@ -204,6 +205,40 @@ public int cardinality() {
return Math.toIntExact(tot);
}
+ /**
+ * Return the number of set bits between indexes {@code from} inclusive and
{@code to} exclusive.
+ */
+ public int cardinality(int from, int to) {
+ Objects.checkFromToIndex(from, to, length());
+
+ int cardinality = 0;
+
+ // First, align `from` with a word start, ie. a multiple of Long.SIZE (64)
+ if ((from & 0x3F) != 0) {
+ long bits = this.bits[from >> 6] >>> from;
+ int numBitsTilNextWord = -from & 0x3F;
+ if (to - from < numBitsTilNextWord) {
+ bits &= (1L << (to - from)) - 1L;
+ return Long.bitCount(bits);
+ }
+ cardinality += Long.bitCount(bits);
+ from += numBitsTilNextWord;
+ assert (from & 0x3F) == 0;
+ }
+
+ for (int i = from >> 6, end = to >> 6; i < end; ++i) {
+ cardinality += Long.bitCount(bits[i]);
+ }
+
+ // Now handle bits between the last complete word and to
+ if ((to & 0x3F) != 0) {
+ long bits = this.bits[to >> 6] << -to;
Review Comment:
You are right. We need the low "to % 64" bits of `bits[to >> 6]`. `x << -to`
is nice because it requires fewer instructions but it also moves bits to a
different index. This makes iterating over set bits slightly more cumbersome,
but doesn't matter for counting bits. Hence why `forEach` applies a mask
instead. I haven't checked actual performance, I suspect that it doesn't matter
in practice.
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