diff --git a/other/binary_exponentiation.py b/other/binary_exponentiation.py new file mode 100644 index 000000000..57b218b55 --- /dev/null +++ b/other/binary_exponentiation.py @@ -0,0 +1,49 @@ +""" +* Binary Exponentiation +* 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(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. +"""