github.com/coyove/sdss@v0.0.0-20231129015646-c2ec58cca6a2/contrib/roaring/popcnt_compat.go (about)

     1  //go:build !go1.9
     2  // +build !go1.9
     3  
     4  package roaring
     5  
     6  // bit population count, take from
     7  // https://code.google.com/p/go/issues/detail?id=4988#c11
     8  // credit: https://code.google.com/u/arnehormann/
     9  // credit: https://play.golang.org/p/U7SogJ7psJ
    10  // credit: http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
    11  func popcount(x uint64) uint64 {
    12  	x -= (x >> 1) & 0x5555555555555555
    13  	x = (x>>2)&0x3333333333333333 + x&0x3333333333333333
    14  	x += x >> 4
    15  	x &= 0x0f0f0f0f0f0f0f0f
    16  	x *= 0x0101010101010101
    17  	return x >> 56
    18  }