contrib/gmp/mpz/combit.c

   1 /* mpz_combit -- complement a specified bit.
   2
   3 Copyright 2002, 2003 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 the GNU Lesser General Public License as published by
   9 the Free Software Foundation; either version 3 of the License, or (at your
  10 option) any later version.
  11
  12 The GNU MP Library is distributed in the hope that it will be useful, but
  13 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  14 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  15 License for more details.
  16
  17 You should have received a copy of the GNU Lesser General Public License
  18 along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
  19
  20 #include "gmp.h"
  21 #include "gmp-impl.h"
  22
  23 void
  24 mpz_combit (mpz_ptr d, mp_bitcnt_t bit_index)
  25 {
  26   mp_size_t dsize = ABSIZ(d);
  27   mp_ptr dp = LIMBS(d);
  28
  29   mp_size_t limb_index = bit_index / GMP_NUMB_BITS;
  30   mp_limb_t bit = ((mp_limb_t) 1 << (bit_index % GMP_NUMB_BITS));
  31
  32   if (limb_index >= dsize)
  33     {
  34       MPZ_REALLOC(d, limb_index + 1);
  35       dp = LIMBS(d);
  36
  37       MPN_ZERO(dp + dsize, limb_index + 1 - dsize);
  38       dsize = limb_index + 1;
  39     }
  40
  41   if (SIZ(d) >= 0)
  42     {
  43       dp[limb_index] ^= bit;
  44       MPN_NORMALIZE (dp, dsize);
  45       SIZ(d) = dsize;
  46     }
  47   else
  48     {
  49       mp_limb_t x = -dp[limb_index];
  50       mp_size_t i;
  51
  52       /* non-zero limb below us means ones-complement */
  53       for (i = limb_index-1; i >= 0; i--)
  54         if (dp[i] != 0)
  55           {
  56             x--;  /* change twos comp to ones comp */
  57             break;
  58           }
  59
  60       if (x & bit)
  61         {
  62           mp_limb_t  c;
  63
  64           /* Clearing the bit increases the magitude. We might need a carry. */
  65           MPZ_REALLOC(d, dsize + 1);
  66           dp = LIMBS(d);
  67
  68           __GMPN_ADD_1 (c, dp+limb_index, dp+limb_index,
  69                         dsize - limb_index, bit);
  70           dp[dsize] = c;
  71           dsize += c;
  72         }
  73       else
  74         /* Setting the bit decreases the magnitude */
  75         mpn_sub_1(dp+limb_index, dp+limb_index, dsize + limb_index, bit);
  76
  77       MPN_NORMALIZE (dp, dsize);
  78       SIZ(d) = -dsize;
  79     }
  80 }