1:  using Xint = System.Numerics.BigInteger;

   2:  using Xmath = Luschny.Math.MathUtils.Xmath;

   3:   

   4:  namespace Luschny.Math.Factorial

   5:  {

   6:      public class FactorialSquaredDiffProd : IFactorialFunction

   7:      {

   8:          public FactorialSquaredDiffProd() { }

   9:   

  10:          public string Name

  11:          {

  12:              get { return "SquaredDiffProduct  "; }

  13:          }                

  14:   

  15:          public Xint Factorial(int n)

  16:          {

  17:              if (n < 0)

  18:              {

  19:                  throw new System.ArgumentOutOfRangeException("n",

  20:                      Name + ": n >= 0 required, but was " + n);

  21:              }

  22:   

  23:              if (n < 4)

  24:              {

  25:                  return new Xint(n < 2 ? 1 : n == 2 ? 2 : 6);

  26:              }

  27:   

  28:              long h = n / 2, q = h * h;

  29:              long[] f = new long[(int)h];

  30:              f[0] = (n & 1) == 1 ? 2 * q * n : 2 * q;

  31:              int i = 1;

  32:   

  33:              for (int d = 1; d < n - 2; d += 2)

  34:              {

  35:                  f[i++] = q -= d;

  36:              }

  37:   

  38:              return Xmath.Product(f);

  39:          }

  40:      }

  41:  } //endOfFactorialSquaredDiffProd