FastFourierTransformMod

Higher precision FFT, can be used for convolutions modulo arbitrary integers as long as Nlog_2N· mod < 8.6 · 10^14 (in practice 10^16 or higher). Inputs must be in [0, mod).

Time: O(N log N), where N = |A|+|B| (twice as slow as NTT or FFT) 22 lines stress-tested

Needs: "FastFourierTransform.h"

content/numerical/FastFourierTransformMod.h — chilli, source: http://neerc.ifmo.ru/trains/toulouse/2017/fft2.pdf

typedef vector<ll> vl;
template<int M> vl convMod(const vl &a, const vl &b) {
	if (a.empty() || b.empty()) return {};
	vl res(sz(a) + sz(b) - 1);
	int B=32-__builtin_clz(sz(res)), n=1<<B, cut=int(sqrt(M));
	vector<C> L(n), R(n), outs(n), outl(n);
	rep(i,0,sz(a)) L[i] = C((int)a[i] / cut, (int)a[i] % cut);
	rep(i,0,sz(b)) R[i] = C((int)b[i] / cut, (int)b[i] % cut);
	fft(L), fft(R);
	rep(i,0,n) {
		int j = -i & (n - 1);
		outl[j] = (L[i] + conj(L[j])) * R[i] / (2.0 * n);
		outs[j] = (L[i] - conj(L[j])) * R[i] / (2.0 * n) / 1i;
	}
	fft(outl), fft(outs);
	rep(i,0,sz(res)) {
		ll av = ll(real(outl[i])+.5), cv = ll(imag(outs[i])+.5);
		ll bv = ll(imag(outl[i])+.5) + ll(real(outs[i])+.5);
		res[i] = ((av % M * cut + bv) % M * cut + cv) % M;
	}
	return res;
}