Tutorial: A simple O(n log n) polynomial multiplication algorithm

Tutorial: A simple O(n log n) polynomial multiplication algorithm

Hi Codeforces!

I have something exciting to tell you guys about today! I have recently come up with a really neat and simple recursive algorithm for multiplying polynomials in time. It is so neat and simple that I think it might possibly revolutionize the way that fast polynomial multiplication is taught and coded. You don't need to know anything about FFT to understand and implement this algorithm.

Big thanks to nor, c1729 and Spheniscine for discussing the contents of the blog with me and comming up with ideas for how to improve the blog =).

I've split this blog up into two parts. The first part is intended for anyone to be able to read and understand. The second part is advanced and goes into a ton of interesting ideas and concepts related to this algorithm.

Prerequisite: Polynomial quotient and remainder, see Wiki article and this Stackexchange example.

Task:

Given two polynomials and , an integer and a non-zero complex number , where degree and degree . Your task is to calculate the polynomial in time. You may assume that is a power of two.

Solution:

We can create a divide and conquer algorithm for based on the difference of squares formula. Assuming is even, then . The idea behind the algorithm is to calculate and using 2 recursive calls, and then use that result to calculate .

So how do we actually calculate using and ?

Well, we can use the following formula:

This formula is very useful. If we substitute by , then the formula tells us how to calculate using and in linear time. With this we have the recipie for implementing a divide and conquer algorithm:

Input:

• Integer (power of 2),
• Non-zero complex number ,
• Two polynomials and .

Output:

• The polynomial .

Algorithm:

Step 1. (Base case) If , then return . Otherwise:

Step 2. Starting from and , in time calculate

Step 3. Make two recursive calls to calculate and .

Step 4. Using the formula, calculate in time. Return the result.

Here is a Python implementation following this recipie:

One final thing that I want to mention before going into the advanced section is that this algorithm can also be used to do fast unmodded polynomial multiplication, i.e. given polynomials and calculate . The trick is simply to pick large enough such that , and then use the exact same algorithm as before. can be arbitrarily picked (any non-zero complex number works).

If you want to try out implementing this algorithm yourself, then here is a very simple problem to test out your implementation on: SPOJ:POLYMUL.

(Advanced) Speeding up the algorithm

This section will be about tricks that can be used to speed up the algorithm. The first two tricks will speed up the algorithm by a factor of 2 each. The last trick is advanced, and it has the potential to both speed up the algorithm and also make it more numerically stable.

(Advanced) -is-this-fft-?

This algorithm is actually FFT in disguise. But it is also different compared to any other FFT algorithm that I've seen in the past (for example the Cooley–Tukey FFT algorithm).

(Advanced) Connection between this algorithm and NTT

Just like how there is FFT and NTT, there are two variants of this algorithm too. One using complex floating point numbers, and the other using modulo a prime (or more generally modulo an odd composite number).

(Advanced) Shorter implementations ("Codegolfed version")

It is possible to make really short but slightly less natural implementations of this algorithm. Originally I was thinking of using this shorter version in the blog, but in the end I didn't do it. So here they are. If you want a short implemention of this algorithm to use in practice, then I would recommend taking one of these implementations and porting it to C++.