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/* mpfr_round_p -- check if an approximation is roundable.
Copyright 2005-2019 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. */
#include "mpfr-impl.h"
/* Check against mpfr_can_round? */
#if MPFR_WANT_ASSERT >= 2
int mpfr_round_p_2 (mp_limb_t *, mp_size_t, mpfr_exp_t, mpfr_prec_t);
int
mpfr_round_p (mp_limb_t *bp, mp_size_t bn, mpfr_exp_t err0, mpfr_prec_t prec)
{
int i1, i2;
MPFR_ASSERTN(bp[bn - 1] & MPFR_LIMB_HIGHBIT);
i1 = mpfr_round_p_2 (bp, bn, err0, prec);
/* Note: since revision 10747, mpfr_can_round_raw is supposed to be always
correct, whereas mpfr_round_p_2 might return 0 in some cases where one
could round, for example with err0=67 and prec=54:
b = 1111101101010001100011111011100010100011101111011011101111111111
thus we cannot compare i1 and i2, we only can check that we don't have
i1 <> 0 and i2 = 0.
*/
i2 = mpfr_can_round_raw (bp, bn, MPFR_SIGN_POS, err0,
MPFR_RNDN, MPFR_RNDZ, prec);
if (i1 && (i2 == 0))
{
fprintf (stderr, "mpfr_round_p(%d) != mpfr_can_round(%d)!\n"
"bn = %lu, err0 = %ld, prec = %lu\nbp = ", i1, i2,
(unsigned long) bn, (long) err0, (unsigned long) prec);
gmp_fprintf (stderr, "%NX\n", bp, bn);
MPFR_ASSERTN (0);
}
return i1;
}
# define mpfr_round_p mpfr_round_p_2
#endif /* MPFR_WANT_ASSERT >= 2 */
/*
* Assuming {bp, bn} is an approximation of a non-singular number
* with error at most equal to 2^(EXP(b)-err0) (`err0' bits of b are known)
* of direction unknown, check if we can round b toward zero with
* precision prec.
*/
int
mpfr_round_p (mp_limb_t *bp, mp_size_t bn, mpfr_exp_t err0, mpfr_prec_t prec)
{
mpfr_prec_t err;
mp_size_t k, n;
mp_limb_t tmp, mask;
int s;
MPFR_ASSERTD(bp[bn - 1] & MPFR_LIMB_HIGHBIT);
err = (mpfr_prec_t) bn * GMP_NUMB_BITS;
if (MPFR_UNLIKELY (err0 <= 0 || (mpfr_uexp_t) err0 <= prec || prec >= err))
return 0; /* can't round */
err = MIN (err, (mpfr_uexp_t) err0);
k = prec / GMP_NUMB_BITS;
s = GMP_NUMB_BITS - prec % GMP_NUMB_BITS;
n = err / GMP_NUMB_BITS - k;
MPFR_ASSERTD (n >= 0);
MPFR_ASSERTD (bn > k);
/* Check first limb */
bp += bn - 1 - k;
tmp = *bp--;
mask = s == GMP_NUMB_BITS ? MPFR_LIMB_MAX : MPFR_LIMB_MASK (s);
tmp &= mask;
if (MPFR_LIKELY (n == 0))
{
/* prec and error are in the same limb */
s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
MPFR_ASSERTD (s < GMP_NUMB_BITS);
tmp >>= s;
mask >>= s;
return tmp != 0 && tmp != mask;
}
else if (MPFR_UNLIKELY (tmp == 0))
{
/* Check if all (n-1) limbs are 0 */
while (--n)
if (*bp-- != 0)
return 1;
/* Check if final error limb is 0 */
s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
if (s == GMP_NUMB_BITS)
return 0;
tmp = *bp >> s;
return tmp != 0;
}
else if (MPFR_UNLIKELY (tmp == mask))
{
/* Check if all (n-1) limbs are 11111111111111111 */
while (--n)
if (*bp-- != MPFR_LIMB_MAX)
return 1;
/* Check if final error limb is 0 */
s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
if (s == GMP_NUMB_BITS)
return 0;
tmp = *bp >> s;
return tmp != (MPFR_LIMB_MAX >> s);
}
else
{
/* First limb is different from 000000 or 1111111 */
return 1;
}
}
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