1:  package de.luschny.math.factorial;

   2:   

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

   4:   

   5:  public class FactorialSquaredDiffProd implements IFactorialFunction

   6:  {

   7:   

   8:      public FactorialSquaredDiffProd()

   9:      {

  10:      }

  11:   

  12:      public String getName()

  13:      {

  14:          return "SquaredDiffProduct";

  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:          if (n < 4)

  25:          {

  26:              return Xint.valueOf(n < 2 ? 1 : n == 2 ? 2 : 6);

  27:          }

  28:   

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

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

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

  32:          int i = 1;

  33:   

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

  35:          {

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

  37:          }

  38:   

  39:          return Xint.product(f);

  40:      }

  41:   

  42:  } // endOfFactorialSquaredrDiffProd