Bit mask and bit shifting example

bit_mask.py

# https://leetcode.com/problems/minimum-bit-flips-to-convert-number/

# this works by making a bit mask using left shift 1 << i, e.g. 1 << 3 == a mask of 1000
# then bitwise AND with the num, gives us a binary num that indicates whether both mask
# and num have a 1 in that digit; e.g. 10 & (1<<2) == 000, and 7 & (1<<2) == 100
# then bit shift right chops off the trailing 0s
# so in this case we'd have 0 for the start decimal num of 10, and 1 for the goal num of 7,
# meaning digit at that place has to change for the start num, and thus our count of
# operations has to increase

class Solution:
    def minBitFlips(self, start: int, goal: int) -> int:
        length = max(len(bin(start)[2:]), len(bin(goal)[2:]))  # '2:'' chops off 0b
        count = 0
        for i in range(length):
            if ((start & (1 << i)) >> i) != ((goal & (1 << i)) >> i):
                count += 1

        return count