easyice commented on PR #15045:
URL: https://github.com/apache/lucene/pull/15045#issuecomment-3220493289
It s a nice idea! although it requires allocating an
`FixedBitSet(bits.length())`, it is still much faster than checking bits one by
one.
Here are some JMH numbers:
```
Benchmark (density) (size) Mode Cnt
Score Error Units
FixedBitSetBenchmark.countWithCardinality 0.5 1024 thrpt 5
53.244 ± 3.754 ops/us
FixedBitSetBenchmark.countWithFallbackGet 0.5 1024 thrpt 5
1.912 ± 0.090 ops/us
FixedBitSetBenchmark.countWithFixedBitSetGet 0.5 1024 thrpt 5
1.758 ± 0.047 ops/us
```
`countWithCardinality` this approach, use `applyMask` and `cardinality`
`countWithFallbackGet` uses a subclass of `Bits` to mock a `Bits` object
that is not an instance of `FixedBitSet`
`countWithFixedBitSetGet` use `FixedBitSet#get`
<details>
<summary >Code</summary>
```java
@BenchmarkMode(Mode.Throughput)
@OutputTimeUnit(TimeUnit.MICROSECONDS)
@State(Scope.Benchmark)
@Warmup(iterations = 3, time = 3)
@Measurement(iterations = 5, time = 5)
@Fork(1)
public class FixedBitSetBenchmark {
@Param({"1024"})
private int size;
@Param({"0.5"})
private float density; // the percentage of 1 in the bitset
private Bits bits; // FixedBitSet#asReadOnlyBits
Bits fallbackBits; // will not use FixedBitSet
@Setup(Level.Trial)
public void setup() {
FixedBitSet bitSet = new FixedBitSet(size);
int numSet = (int) (size * density);
if (numSet == size) {
bitSet.set(0, size);
} else if (numSet > 0) {
Random random = new Random(0);
for (int i = 0; i < numSet; i++) {
bitSet.set(random.nextInt(size - 1));
}
}
bits = bitSet.asReadOnlyBits();
fallbackBits =
new Bits() {
@Override
public boolean get(int index) {
return index % 2 == 0;
}
@Override
public int length() {
return size;
}
};
}
@Benchmark
public void countWithCardinality(Blackhole bh) {
int count = 0;
FixedBitSet bitSet = new FixedBitSet(size);
bitSet.set(0, size);
bits.applyMask(bitSet, 0);
count = bitSet.cardinality();
bh.consume(count);
}
@Benchmark
public void countWithFixedBitSetGet(Blackhole bh) {
int count = 0;
for (int i = 0; i < bits.length(); i++) {
if (bits.get(i)) {
count++;
}
}
bh.consume(count);
}
@Benchmark
public void countWithFallbackGet(Blackhole bh) {
int count = 0;
for (int i = 0; i < fallbackBits.length(); i++) {
if (fallbackBits.get(i)) {
count++;
}
}
bh.consume(count);
}
}
```
</details>
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