File: popcnt.go

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golang-github-dgryski-go-bits 0.0~git20151205.0.86c69b3-1
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// +build !amd64 appengine popcntgo

package bits

// Popcnt counts the number of bits set
func Popcnt(x uint64) uint64 {
	// bit population count, see
	// http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
	x -= (x >> 1) & 0x5555555555555555
	x = (x>>2)&0x3333333333333333 + x&0x3333333333333333
	x += x >> 4
	x &= 0x0f0f0f0f0f0f0f0f
	x *= 0x0101010101010101
	return x >> 56
}