bit tricks

1. check if number is power of 2
The trick is that, if number is a power of two it contains only single bit with value 1.
x & (x-1) unsets right most bit with value 1. And in this case it unsets the only bit with value 1, if number is a power of 2.

idea was taken from this article:
http://articles.leetcode.com/2010/09/number-of-1-bits.html

check_base_2 = lambda x: x and not(x & (x-1))

#bin(16)   #10000#bin(16-1) #01111
#  10000# &#  01111#  00000
assert check_base_2(16)
assert check_base_2(32)
assert check_base_2(128)
assert check_base_2(8)
assert check_base_2(256)
assert check_base_2(3) != True