log2.go

func log2(n uint) uint {
	const uintBits = 32 << (^uint(0) >> 63)
	var (
		seen, bit uint
		m, l, r   uint
	)
	r = uintBits
	for l < r {
		m = l + (r-l)/2

		bit = 1 << m
		if seen&bit != 0 {
			if n^bit != 0 {
				m++
			}
			break
		}
		seen |= bit

		switch n >> m {
		case 0:
			r = m // decrease
		default:
			l = m // increase
		}
	}
	return m
}

func ceilPowerOfTwo(n int) int {
	n--
	n |= n >> 1
	n |= n >> 2
	n |= n >> 4
	n |= n >> 8
	n |= n >> 16
	n++
	return n
}