1:  package de.luschny.math.factorial;

   2:   

   3:  import de.luschny.math.arithmetic.Xint;

   4:   

   5:  public class FactorialSwingSimple implements IFactorialFunction

   6:  {

   7:   

   8:      public FactorialSwingSimple()

   9:      {

  10:      }

  11:   

  12:      public String getName()

  13:      {

  14:          return "SwingSimple         ";

  15:      }

  16:   

  17:      public Xint factorial(int n)

  18:      {

  19:          if (n < 0)

  20:          {

  21:              throw new ArithmeticException("Factorial: n has to be >= 0, but was " + n);

  22:          }

  23:   

  24:          return recFactorial(n);

  25:      }

  26:   

  27:      private Xint recFactorial(int n)

  28:      {

  29:          if (n < 2)

  30:          {

  31:              return Xint.ONE;

  32:          }

  33:   

  34:          return recFactorial(n / 2).square().multiply(swing(n));

  35:      }

  36:   

  37:      private Xint swing(int n)

  38:      {

  39:          int z;

  40:   

  41:          switch (n % 4)

  42:              {

  43:              case 1:

  44:                  z = n / 2 + 1;

  45:                  break;

  46:              case 2:

  47:                  z = 2;

  48:                  break;

  49:              case 3:

  50:                  z = 2 * (n / 2 + 2);

  51:                  break;

  52:              default:

  53:                  z = 1;

  54:                  break;

  55:              }

  56:   

  57:          Xint b = Xint.valueOf(z);

  58:          z = 2 * (n - ((n + 1) & 1));

  59:   

  60:          for (int i = 1; i <= n / 4; i++, z -= 4)

  61:          {

  62:              b = b.multiply(z).divide(i);

  63:          }

  64:   

  65:          return b;

  66:      }

  67:  } // endOfFactorialSwing