Finding the exponent of n = 2**x using bitwise operations [logarithm in base 2 of n]

Question

Is there a straightforward way to extracting the exponent from a power of 2 using bitwise operations only?

EDIT: Although the question was originall

Answer 1

You can do it in O(lg s) time for arbitrary length integers using a binsearch.

import sys
def floorlg(n):
    if n < 1:
        return -1
    low=0
    high=sys.getsizeof(n)*8 # not the best upper-bound guesstimate, but...
    while True:
        mid = (low+high)//2
        i = n >> mid
        if i == 1:
            return mid
        if i == 0:
            high = mid-1
        else:
            low = mid+1

For fixed size integers, a lookup table should be the fastest solution, and probably best overall.