efee5258bc
The MPFR library is a C library for multiple-precision floating-point computations with exact rounding (also called correct rounding). It is based on the GMP multiple-precision library and should replace the MPF class in further releases of GMP. GCC >= 4.2 requires MPFR.
124 lines
3.7 KiB
C
124 lines
3.7 KiB
C
/* mpfr_round_p -- check if an approximation is roundable.
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Copyright 2005, 2006, 2007, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
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Contributed by the Arenaire and Cacao projects, INRIA.
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This file is part of the GNU MPFR Library.
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The GNU MPFR Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 3 of the License, or (at your
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option) any later version.
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The GNU MPFR Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MPFR Library; see the file COPYING.LESSER. If not, see
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http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
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51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
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#include "mpfr-impl.h"
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/* Check against mpfr_can_round ? */
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#ifdef WANT_ASSERT
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# if WANT_ASSERT >= 2
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int mpfr_round_p_2 (mp_limb_t *, mp_size_t, mpfr_exp_t, mpfr_prec_t);
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int
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mpfr_round_p (mp_limb_t *bp, mp_size_t bn, mpfr_exp_t err0, mpfr_prec_t prec)
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{
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int i1, i2;
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i1 = mpfr_round_p_2 (bp, bn, err0, prec);
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i2 = mpfr_can_round_raw (bp, bn, MPFR_SIGN_POS, err0,
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MPFR_RNDN, MPFR_RNDZ, prec);
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if (i1 != i2)
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{
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fprintf (stderr, "mpfr_round_p(%d) != mpfr_can_round(%d)!\n"
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"bn = %lu, err0 = %ld, prec = %lu\nbp = ", i1, i2,
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(unsigned long) bn, (long) err0, (unsigned long) prec);
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gmp_fprintf (stderr, "%NX\n", bp, bn);
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MPFR_ASSERTN (0);
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}
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return i1;
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}
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# define mpfr_round_p mpfr_round_p_2
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# endif
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#endif
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/*
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* Assuming {bp, bn} is an approximation of a non-singular number
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* with error at most equal to 2^(EXP(b)-err0) (`err0' bits of b are known)
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* of direction unknown, check if we can round b toward zero with
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* precision prec.
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*/
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int
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mpfr_round_p (mp_limb_t *bp, mp_size_t bn, mpfr_exp_t err0, mpfr_prec_t prec)
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{
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mpfr_prec_t err;
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mp_size_t k, n;
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mp_limb_t tmp, mask;
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int s;
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err = (mpfr_prec_t) bn * GMP_NUMB_BITS;
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if (MPFR_UNLIKELY (err0 <= 0 || (mpfr_uexp_t) err0 <= prec || prec >= err))
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return 0; /* can't round */
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err = MIN (err, (mpfr_uexp_t) err0);
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k = prec / GMP_NUMB_BITS;
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s = GMP_NUMB_BITS - prec%GMP_NUMB_BITS;
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n = err / GMP_NUMB_BITS - k;
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MPFR_ASSERTD (n >= 0);
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MPFR_ASSERTD (bn > k);
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/* Check first limb */
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bp += bn-1-k;
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tmp = *bp--;
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mask = s == GMP_NUMB_BITS ? MP_LIMB_T_MAX : MPFR_LIMB_MASK (s);
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tmp &= mask;
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if (MPFR_LIKELY (n == 0))
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{
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/* prec and error are in the same limb */
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s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
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MPFR_ASSERTD (s < GMP_NUMB_BITS);
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tmp >>= s;
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mask >>= s;
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return tmp != 0 && tmp != mask;
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}
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else if (MPFR_UNLIKELY (tmp == 0))
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{
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/* Check if all (n-1) limbs are 0 */
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while (--n)
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if (*bp-- != 0)
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return 1;
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/* Check if final error limb is 0 */
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s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
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if (s == GMP_NUMB_BITS)
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return 0;
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tmp = *bp >> s;
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return tmp != 0;
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}
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else if (MPFR_UNLIKELY (tmp == mask))
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{
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/* Check if all (n-1) limbs are 11111111111111111 */
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while (--n)
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if (*bp-- != MP_LIMB_T_MAX)
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return 1;
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/* Check if final error limb is 0 */
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s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
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if (s == GMP_NUMB_BITS)
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return 0;
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tmp = *bp >> s;
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return tmp != (MP_LIMB_T_MAX >> s);
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}
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else
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{
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/* First limb is different from 000000 or 1111111 */
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return 1;
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}
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}
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