release/src/router/gmp/mpz/clrbit.c

   1 /* mpz_clrbit -- clear a specified bit.
   2
   3 Copyright 1991, 1993-1995, 2001, 2002, 2012 Free Software Foundation, Inc.
   4
   5 This file is part of the GNU MP Library.
   6
   7 The GNU MP Library is free software; you can redistribute it and/or modify
   8 it under the terms of either:
   9
  10   * the GNU Lesser General Public License as published by the Free
  11     Software Foundation; either version 3 of the License, or (at your
  12     option) any later version.
  13
  14 or
  15
  16   * the GNU General Public License as published by the Free Software
  17     Foundation; either version 2 of the License, or (at your option) any
  18     later version.
  19
  20 or both in parallel, as here.
  21
  22 The GNU MP Library is distributed in the hope that it will be useful, but
  23 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  24 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  25 for more details.
  26
  27 You should have received copies of the GNU General Public License and the
  28 GNU Lesser General Public License along with the GNU MP Library.  If not,
  29 see https://www.gnu.org/licenses/.  */
  30
  31 #include "gmp.h"
  32 #include "gmp-impl.h"
  33
  34 void
  35 mpz_clrbit (mpz_ptr d, mp_bitcnt_t bit_idx)
  36 {
  37   mp_size_t dsize = SIZ (d);
  38   mp_ptr dp = PTR (d);
  39   mp_size_t limb_idx;
  40   mp_limb_t mask;
  41
  42   limb_idx = bit_idx / GMP_NUMB_BITS;
  43   mask = CNST_LIMB(1) << (bit_idx % GMP_NUMB_BITS);
  44   if (dsize >= 0)
  45     {
  46       if (limb_idx < dsize)
  47         {
  48           mp_limb_t  dlimb;
  49           dlimb = dp[limb_idx] & ~mask;
  50           dp[limb_idx] = dlimb;
  51
  52           if (UNLIKELY ((dlimb == 0) + limb_idx == dsize)) /* dsize == limb_idx + 1 */
  53             {
  54               /* high limb became zero, must normalize */
  55               MPN_NORMALIZE (dp, limb_idx);
  56               SIZ (d) = limb_idx;
  57             }
  58         }
  59       else
  60         ;
  61     }
  62   else
  63     {
  64       /* Simulate two's complement arithmetic, i.e. simulate
  65          1. Set OP = ~(OP - 1) [with infinitely many leading ones].
  66          2. clear the bit.
  67          3. Set OP = ~OP + 1.  */
  68
  69       dsize = -dsize;
  70
  71       if (limb_idx < dsize)
  72         {
  73           mp_size_t zero_bound;
  74
  75           /* No index upper bound on this loop, we're sure there's a non-zero limb
  76              sooner or later.  */
  77           zero_bound = 0;
  78           while (dp[zero_bound] == 0)
  79             zero_bound++;
  80
  81           if (limb_idx > zero_bound)
  82             {
  83               dp[limb_idx] |= mask;
  84             }
  85           else if (limb_idx == zero_bound)
  86             {
  87               mp_limb_t  dlimb;
  88               dlimb = (((dp[limb_idx] - 1) | mask) + 1) & GMP_NUMB_MASK;
  89               dp[limb_idx] = dlimb;
  90
  91               if (dlimb == 0)
  92                 {
  93                   /* Increment at limb_idx + 1.  Extend the number with a zero limb
  94                      for simplicity.  */
  95                   dp = MPZ_REALLOC (d, dsize + 1);
  96                   dp[dsize] = 0;
  97                   MPN_INCR_U (dp + limb_idx + 1, dsize - limb_idx, 1);
  98                   dsize += dp[dsize];
  99
 100                   SIZ (d) = -dsize;
 101                 }
 102             }
 103           else
 104             ;
 105         }
 106       else
 107         {
 108           /* Ugh.  The bit should be cleared outside of the end of the
 109              number.  We have to increase the size of the number.  */
 110           dp = MPZ_REALLOC (d, limb_idx + 1);
 111           SIZ (d) = -(limb_idx + 1);
 112           MPN_ZERO (dp + dsize, limb_idx - dsize);
 113           dp[limb_idx] = mask;
 114         }
 115     }
 116 }