# What is the best algorithm for multiplying very large numeric strings (50k or more digits)?

If we want to multiply strings with 100k digits or more, which algorithm is fast enough to achieve this ?

Algorithmist Answered question

Fast multiplication of very large numeric strings can be easily achieved in** O(n log n) **time complexity using FFT multiplication.

Below is a python code which illustrates this for multiplication of strings with **1,60,000** digits. The algorithm is fast enough for even bigger strings than this.

import math import time class Solution: def fft(self,num,opt): n = len(num) if n == 1: if opt == 1: return [complex(int(num[0]),0)] else: return [num[0]] A1 = self.fft(num[::2],opt) A2 = self.fft(num[1::2],opt) w1 = complex(math.cos(2*math.pi/n),opt*math.sin(2*math.pi/n)) w = complex(1,0) A = [0]*n for i in range(0, n//2): A[i] = A1[i] + w*A2[i] A[i+n//2] = A1[i] - w*A2[i] w = w * w1 return A def multiply(self, num1, num2): length = len(num1) + len(num2) n = 1 while n < length: n = n<<1 # n - That is, the number of digits after multiplication, satisfying the multiplication will not overflow, and is a power of 2 A1 = self.fft(num1.zfill(n)[::-1],1) A2 = self.fft(num2.zfill(n)[::-1],1) A = [0]*n for i in range(n): A[i] = A1[i]*A2[i] Ans = self.fft(A,-1) rtn = '' #up - used for carry up = 0 for num in Ans: hre = int(num.real/n+0.5+up) if hre >= 10: up = int(hre/10) rtn += str(int(hre%10)) else: up = 0 rtn += str(hre) rtn = rtn[::-1].lstrip('0') # Avoid returning an empty string when the result is 0 if rtn == '': return '0' return rtn M = Solution() start_time = time.time() number = '123238534785435' # Replace above string with the ultra large string from this link:- https://pastebin.com/J1mHcrLs print (M.multiply(number, number)) print("\n--- Multiplication of two 1,60,000 character strings took %s seconds ---" % (time.time() - start_time)) # Output of the time taken for 160k digit multiplication:- --- Multiplication of two 1,60,000 character strings took 10.12393307685852 seconds ---

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