log2 Word64 and count leading zeros

nlz.hs

{-# LANGUAGE BangPatterns #-}

import Data.Bits
import Data.Word

nlz :: Word64 -> Int
nlz x = go x x 0 where
	go !x !y !n
		| x < 0  = n
		| y == 0 = 64 - n
		| otherwise = go (shiftL x 1) (shiftR y 1) (n+1)

log2 :: Word64 -> Int
log2 x = 63 - nlz x