""" * Binary Exponentiation for Powers * This is a method to find a^b in a time complexity of O(log b) * This is one of the most commonly used methods of finding powers. * Also useful in cases where solution to (a^b)%c is required, * where a,b,c can be numbers over the computers calculation limits. * Done using iteration, can also be done using recursion * @author chinmoy159 * @version 1.0 dated 10/08/2017 """ def b_expo(a, b): res = 1 while b > 0: if b & 1: res *= a a *= a b >>= 1 return res def b_expo_mod(a, b, c): res = 1 while b > 0: if b & 1: res = ((res % c) * (a % c)) % c a *= a b >>= 1 return res """ * Wondering how this method works ! * It's pretty simple. * Let's say you need to calculate a ^ b * RULE 1 : a ^ b = (a*a) ^ (b/2) ---- example : 4 ^ 4 = (4*4) ^ (4/2) = 16 ^ 2 * RULE 2 : IF b is ODD, then ---- a ^ b = a * (a ^ (b - 1)) :: where (b - 1) is even. * Once b is even, repeat the process to get a ^ b * Repeat the process till b = 1 OR b = 0, because a^1 = a AND a^0 = 1 * * As far as the modulo is concerned, * the fact : (a*b) % c = ((a%c) * (b%c)) % c * Now apply RULE 1 OR 2 whichever is required. """