1:  /// -------- ToujoursEnBeta

   2:  /// Author & Copyright : Peter Luschny

   3:  /// License: LGPL version 3.0 or (at your option)

   4:  /// Creative Commons Attribution-ShareAlike 3.0

   5:  /// Comments mail to: peter(at)luschny.de

   6:  /// Created: 2010-03-01

   7:   

   8:  namespace Sharith.Math.Factorial 

   9:  {

  10:      using XInt = Sharith.Arithmetic.XInt;

  11:      using XMath = Sharith.Math.MathUtils.XMath;

  12:      

  13:      public class BoitenSplit : IFactorialFunction 

  14:      {

  15:          public BoitenSplit() { }

  16:   

  17:          public string Name

  18:          {

  19:              get { return "BoitenSplit         "; }

  20:          }

  21:   

  22:          public XInt Factorial(int n)

  23:          {

  24:              if (n < 0)

  25:              {

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

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

  28:              }

  29:   

  30:              if (n < 2) return XInt.One;

  31:   

  32:              XInt p = XInt.One;

  33:              XInt r = XInt.One;

  34:   

  35:              int h = 0, shift = 0, k = 1;

  36:              int log2n = XMath.FloorLog2(n);

  37:   

  38:              while (h != n)

  39:              {

  40:                  shift += h;

  41:                  h = n >> log2n--;

  42:                  int high = (h & 1) == 1 ? h : h - 1;

  43:   

  44:                  while (k != high)

  45:                  {

  46:                      k += 2;

  47:                      p *= k;

  48:                  }

  49:                  r *= p;

  50:              }

  51:   

  52:              return r << shift;

  53:          }

  54:      }

  55:  } // endOfFactorialBoitenSplit