public void Ntt_Fft_Many()
{
var n = 1 << 16;
var a = Enumerable.Range(3, n).Select(x => (long)x).ToArray();
var ntt = new Ntt(n);
var t = ntt.Fft(a);
var r = ntt.Fft(t, true);
CollectionAssert.AreEqual(a, r);
}
// 長さは n 以下で OK。
public static long[] Convolution(long[] a, long[] b)
{
var ntt = new Ntt(a.Length + b.Length - 1);
var fa = ntt.Fft(a);
var fb = ntt.Fft(b);
for (int i = 0; i < ntt.n; ++i)
{
fa[i] = fa[i] * fb[i] % p;
}
return(ntt.Fft(fa, true));
}
public void Ntt_Fft()
{
var n = 1 << 4;
var a = Enumerable.Range(3, n).Select(x => (long)x).ToArray();
var t0 = Ntt0.Naive(a);
var r0 = Ntt0.Naive(t0, true);
var t1 = Ntt0.Fft(a);
var r1 = Ntt0.Fft(t1, true);
var ntt = new Ntt(n);
var t2 = ntt.Fft(a);
var r2 = ntt.Fft(t2, true);
CollectionAssert.AreEqual(t0, t1);
CollectionAssert.AreEqual(t0, t2);
CollectionAssert.AreEqual(a, r0);
CollectionAssert.AreEqual(a, r1);
CollectionAssert.AreEqual(a, r2);
}