source/libs/mpfr/mpfr-3.1.3/src/round_p.c

   1 /* mpfr_round_p -- check if an approximation is roundable.
   2
   3 Copyright 2005-2015 Free Software Foundation, Inc.
   4 Contributed by the AriC and Caramel projects, INRIA.
   5
   6 This file is part of the GNU MPFR Library.
   7
   8 The GNU MPFR Library is free software; you can redistribute it and/or modify
   9 it under the terms of the GNU Lesser General Public License as published by
  10 the Free Software Foundation; either version 3 of the License, or (at your
  11 option) any later version.
  12
  13 The GNU MPFR Library is distributed in the hope that it will be useful, but
  14 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  15 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  16 License for more details.
  17
  18 You should have received a copy of the GNU Lesser General Public License
  19 along with the GNU MPFR Library; see the file COPYING.LESSER.  If not, see
  20 http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
  21 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
  22
  23 #include "mpfr-impl.h"
  24
  25 /* Check against mpfr_can_round? */
  26 #ifdef MPFR_WANT_ASSERT
  27 # if MPFR_WANT_ASSERT >= 2
  28 int mpfr_round_p_2 (mp_limb_t *, mp_size_t, mpfr_exp_t, mpfr_prec_t);
  29 int
  30 mpfr_round_p (mp_limb_t *bp, mp_size_t bn, mpfr_exp_t err0, mpfr_prec_t prec)
  31 {
  32   int i1, i2;
  33
  34   i1 = mpfr_round_p_2 (bp, bn, err0, prec);
  35   i2 = mpfr_can_round_raw (bp, bn, MPFR_SIGN_POS, err0,
  36                            MPFR_RNDN, MPFR_RNDZ, prec);
  37   if (i1 != i2)
  38     {
  39       fprintf (stderr, "mpfr_round_p(%d) != mpfr_can_round(%d)!\n"
  40                "bn = %lu, err0 = %ld, prec = %lu\nbp = ", i1, i2,
  41                (unsigned long) bn, (long) err0, (unsigned long) prec);
  42       gmp_fprintf (stderr, "%NX\n", bp, bn);
  43       MPFR_ASSERTN (0);
  44     }
  45   return i1;
  46 }
  47 # define mpfr_round_p mpfr_round_p_2
  48 # endif
  49 #endif
  50
  51 /*
  52  * Assuming {bp, bn} is an approximation of a non-singular number
  53  * with error at most equal to 2^(EXP(b)-err0) (`err0' bits of b are known)
  54  * of direction unknown, check if we can round b toward zero with
  55  * precision prec.
  56  */
  57 int
  58 mpfr_round_p (mp_limb_t *bp, mp_size_t bn, mpfr_exp_t err0, mpfr_prec_t prec)
  59 {
  60   mpfr_prec_t err;
  61   mp_size_t k, n;
  62   mp_limb_t tmp, mask;
  63   int s;
  64
  65   err = (mpfr_prec_t) bn * GMP_NUMB_BITS;
  66   if (MPFR_UNLIKELY (err0 <= 0 || (mpfr_uexp_t) err0 <= prec || prec >= err))
  67     return 0;  /* can't round */
  68   err = MIN (err, (mpfr_uexp_t) err0);
  69
  70   k = prec / GMP_NUMB_BITS;
  71   s = GMP_NUMB_BITS - prec%GMP_NUMB_BITS;
  72   n = err / GMP_NUMB_BITS - k;
  73
  74   MPFR_ASSERTD (n >= 0);
  75   MPFR_ASSERTD (bn > k);
  76
  77   /* Check first limb */
  78   bp += bn-1-k;
  79   tmp = *bp--;
  80   mask = s == GMP_NUMB_BITS ? MP_LIMB_T_MAX : MPFR_LIMB_MASK (s);
  81   tmp &= mask;
  82
  83   if (MPFR_LIKELY (n == 0))
  84     {
  85       /* prec and error are in the same limb */
  86       s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
  87       MPFR_ASSERTD (s < GMP_NUMB_BITS);
  88       tmp  >>= s;
  89       mask >>= s;
  90       return tmp != 0 && tmp != mask;
  91     }
  92   else if (MPFR_UNLIKELY (tmp == 0))
  93     {
  94       /* Check if all (n-1) limbs are 0 */
  95       while (--n)
  96         if (*bp-- != 0)
  97           return 1;
  98       /* Check if final error limb is 0 */
  99       s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
 100       if (s == GMP_NUMB_BITS)
 101         return 0;
 102       tmp = *bp >> s;
 103       return tmp != 0;
 104     }
 105   else if (MPFR_UNLIKELY (tmp == mask))
 106     {
 107       /* Check if all (n-1) limbs are 11111111111111111 */
 108       while (--n)
 109         if (*bp-- != MP_LIMB_T_MAX)
 110           return 1;
 111       /* Check if final error limb is 0 */
 112       s = GMP_NUMB_BITS - err % GMP_NUMB_BITS;
 113       if (s == GMP_NUMB_BITS)
 114         return 0;
 115       tmp = *bp >> s;
 116       return tmp != (MP_LIMB_T_MAX >> s);
 117     }
 118   else
 119     {
 120       /* First limb is different from 000000 or 1111111 */
 121       return 1;
 122     }
 123 }