// 戻り値の長さは |a| + |b| - 1 となります。
public static long[] Convolution(long[] a, long[] b)
{
if (a == null)
{
throw new ArgumentNullException(nameof(a));
}
if (b == null)
{
throw new ArgumentNullException(nameof(b));
}
var n = a.Length + b.Length - 1;
var ntt = new FNTT(n);
var fa = ntt.Transform(a, false);
var fb = ntt.Transform(b, false);
for (int k = 0; k < fa.Length; ++k)
{
fa[k] = fa[k] * fb[k] % p;
}
var c = ntt.Transform(fa, true);
if (n < c.Length)
{
Array.Resize(ref c, n);
}
return(c);
}
public void Transform_FNTT()
{
var ntt = new FNTT(n);
Test(f => ntt.Transform(f, false), f => ntt.Transform(f, true));
}