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Implement number theoretic transform for large integer multiplication #282
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Previously 3 primes were used, which was suboptimal in terms of speed. Currently, the threshold for switching from 2 to 3 primes is 2^38.
Despite the simple implementation with obvious inefficiencies (e.g., not reusing the NTT of the shorter array), this leads to speed gains in multiple benchmarks, although there is a small regression in others.
The prime numbers were replaced by larger ones to allow for tighter packing. Also, we compute the maximum number of bits that can be packed into one digit more precisely.
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Breaking at this point is the right thing to do since future encounters will all `continue`.
Make ntt.rs shorter
Make ntt.rs shorter
Make ntt.rs shorter
Make ntt.rs shorter
Make ntt.rs shorter
Make ntt.rs shorter
Improve NTT planning
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This commit implements number theoretic transform (NTT) for large integer multiplication (issue #169).
On Ryzen 7 2700X, 64bit, it takes about 15ms for 2.7Mbits x 2.7Mbits and 170ms for 27Mbits x 27Mbits multiplication. This seems comparable to GMP 6.2.1.