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/* mpfr_fac_ui -- factorial of a non-negative integer
Copyright (C) 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;
}
}
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