1:  using Xint = System.Numerics.BigInteger;

   2:   

   3:   

   4:  namespace Luschny.Math.Factorial

   5:  {

   6:      public class FactorialProductRecursive : IFactorialFunction

   7:      {

   8:          public FactorialProductRecursive() { }

   9:   

  10:          public string Name

  11:          {

  12:              get { return "ProductRecursive    "; }

  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 (1 < n)

  24:              {

  25:                  return RecProduct(1, n);

  26:              }

  27:   

  28:              return Xint.One;

  29:          }

  30:   

  31:          private Xint RecProduct(int n, int len)

  32:          {

  33:              if (1 < len)

  34:              {

  35:                  int l = len >> 1;

  36:                  return RecProduct(n, l) * RecProduct(n + l, len - l);

  37:              }

  38:   

  39:              return new Xint(n);

  40:          }

  41:      }

  42:  }   // endOfFactorialProductRecursive