package util

// log2FloorMasks defines the masks to use when quickly calculating
// floor(log2(x)) in a constant log2(32) = 5 steps, where x is a uint32, using
// shifts.  They are derived from (2^(2^x) - 1) * (2^(2^x)), for x in 4..0.
var log2FloorMasks = []uint32{0xffff0000, 0xff00, 0xf0, 0xc, 0x2}

// FastLog2Floor calculates and returns floor(log2(x)) in a constant 5 steps.
func FastLog2Floor(n uint32) uint8 {
	rv := uint8(0)
	exponent := uint8(16)
	for i := 0; i < 5; i++ {
		if n&log2FloorMasks[i] != 0 {
			rv += exponent
			n >>= exponent
		}
		exponent >>= 1
	}
	return rv
}