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/* mpfr_fac_ui -- factorial of a nonnegative integer
Copyright 2001, 2004-2020 Free Software Foundation, Inc.
Contributed by the AriC and Caramba projects, INRIA.
This file is part of the GNU MPFR Library.
The GNU 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 3 of the License, or (at your
option) any later version.
The GNU 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 GNU MPFR Library; see the file COPYING.LESSER. If not, see
https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
#define MPFR_NEED_LONGLONG_H
#include "mpfr-impl.h"
/* The computation of n! is done by
n!=prod^{n}_{i=1}i
*/
/* FIXME: efficient problems with large arguments; see comments in gamma.c. */
int
mpfr_fac_ui (mpfr_ptr y, unsigned long int x, mpfr_rnd_t rnd_mode)
{
mpfr_t t; /* Variable of Intermediary Calculation*/
unsigned long i;
int round, inexact;
mpfr_prec_t Ny; /* Precision of output variable */
mpfr_prec_t Nt; /* Precision of Intermediary Calculation variable */
mpfr_prec_t err; /* Precision of error */
mpfr_rnd_t rnd;
MPFR_SAVE_EXPO_DECL (expo);
MPFR_ZIV_DECL (loop);
/***** test x = 0 and x == 1******/
if (MPFR_UNLIKELY (x <= 1))
return mpfr_set_ui (y, 1, rnd_mode); /* 0! = 1 and 1! = 1 */
MPFR_SAVE_EXPO_MARK (expo);
/* Initialisation of the Precision */
Ny = MPFR_PREC (y);
/* compute the size of intermediary variable */
Nt = Ny + 2 * MPFR_INT_CEIL_LOG2 (x) + 7;
mpfr_init2 (t, Nt); /* initialize of intermediary variable */
rnd = MPFR_RNDZ;
MPFR_ZIV_INIT (loop, Nt);
for (;;)
{
/* compute factorial */
inexact = mpfr_set_ui (t, 1, rnd);
for (i = 2 ; i <= x ; i++)
{
round = mpfr_mul_ui (t, t, i, rnd);
/* assume the first inexact product gives the sign
of difference: is that always correct? */
if (inexact == 0)
inexact = round;
}
err = Nt - 1 - MPFR_INT_CEIL_LOG2 (Nt);
round = !inexact || MPFR_CAN_ROUND (t, err, Ny, rnd_mode);
if (MPFR_LIKELY (round))
{
/* If inexact = 0, then t is exactly x!, so round is the
correct inexact flag.
Otherwise, t != x! since we rounded to zero or away. */
round = mpfr_set (y, t, rnd_mode);
if (inexact == 0)
{
inexact = round;
break;
}
else if ((inexact < 0 && round <= 0)
|| (inexact > 0 && round >= 0))
break;
else /* inexact and round have opposite signs: we cannot
compute the inexact flag. Restart using the
symmetric rounding. */
rnd = (rnd == MPFR_RNDZ) ? MPFR_RNDU : MPFR_RNDZ;
}
MPFR_ZIV_NEXT (loop, Nt);
mpfr_set_prec (t, Nt);
}
MPFR_ZIV_FREE (loop);
mpfr_clear (t);
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_check_range (y, inexact, rnd_mode);
}
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