1:  using Xint = System.Numerics.BigInteger;

   2:   

   3:   

   4:  namespace Luschny.Math.Factorial

   5:  {

   6:      public class FactorialSquaredDiff : IFactorialFunction

   7:      {

   8:          public FactorialSquaredDiff() { }

   9:   

  10:          public string Name

  11:          {

  12:              get { return "SquaredDifference   "; }

  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 < 2)

  24:              {

  25:                  return Xint.One;

  26:              }

  27:   

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

  29:              long r = (n & 1) == 1 ? 2 * q * n : 2 * q;

  30:              Xint f = new Xint(r);

  31:   

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

  33:              {

  34:                  f *= q -= d;

  35:              }

  36:   

  37:              return f;

  38:          }

  39:      }

  40:  } //endOfFactorialSqrDiff