public static Modular Combination(int n, int r, int mod = _defaultMod)
{
if (!FactorialInverseCache.ContainsKey(mod))
{
InitializeCombinationTable(maxFactorial, mod);
}
CheckNR(n, r);
r = Math.Min(r, n - r);
return(n == r ? new Modular(1, mod) : new Modular(FactorialCache[mod][n], mod) * new Modular(FactorialInverseCache[mod][r], mod) * new Modular(FactorialInverseCache[mod][n - r], mod));
}
private static void InitializeCombinationTable(int max, int mod)
{
Factorial(max);
FactorialInverseCache.Add(mod, new int[max + 1]);
long fInv = (new Modular(1, mod) / Factorial(max, mod)).Value;
FactorialInverseCache[mod][max] = (int)fInv;
for (int i = max - 1; i >= 0; i--)
{
fInv = (fInv * (i + 1)) % mod;
FactorialInverseCache[mod][i] = (int)fInv;
}
}
