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/* mpfr_fac_ui -- factorial of a non-negative integer

Copyright 2001 Free Software Foundation, Inc.

This file is part of the MPFR Library.

The MPFR Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at your
option) any later version.

The MPFR Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the MPFR Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */

#include "gmp.h"
#include "gmp-impl.h"
#include "mpfr.h"
#include "mpfr-impl.h"

 /* The computation of n! is done by

    n!=prod^{n}_{i=1}i
 */

int
mpfr_fac_ui (mpfr_ptr y, unsigned long int x , mp_rnd_t rnd_mode) 
{

  /****** Declaration ******/

    mpfr_t t;       /* Variable of Intermediary Calculation*/
    int i;
    int round, inexact = 0;
    int boucle = 1;

    mp_prec_t Ny;   /* Precision of output variable */
    mp_prec_t Nt;   /* Precision of Intermediary Calculation variable */
    mp_prec_t err;  /* Precision of error */

  /***** test x = 0  ******/
  	  
    if (x == 0)
      {
	mpfr_set_ui (y, 1, GMP_RNDN); /* 0! = 1 */
	return 0;
      }
    else
      {
        /* Initialisation of the Precision */
	Ny=MPFR_PREC(y);
        
	Nt=Ny+2*(int)_mpfr_ceil_log2((double)x)+10; /*compute the size of intermediary variable */

	
        mpfr_init2(t, Nt);/* initialise of intermediary variable */
        
        while (boucle)
          {
            inexact = mpfr_set_ui (t, 1, GMP_RNDZ);
            
            for(i=2;i<=x;i++)              /* compute factorial */
              {
                round = mpfr_mul_ui (t, t, i, GMP_RNDZ);
                /* assume the first inexact product gives the sign
                   of difference: is that always correct? */
                if (inexact == 0)
                  inexact = round;
              }
	    
            err = Nt - 1 - (int) _mpfr_ceil_log2 ((double) Nt);

            round = !inexact || mpfr_can_round (t,err,GMP_RNDZ,rnd_mode,Ny);
            
            if (round)
              {
                round = mpfr_set (y, t, rnd_mode);
		  if (inexact == 0)
                    inexact = round;
		  boucle=0;
		}
	      else
		{
		  Nt=Nt+10;
		  /*initialise of intermediary variable */
		  mpfr_set_prec(t, Nt);
		}
	  }
   
	  mpfr_clear(t);
          return inexact;
      
    }
}