1  /* mpz_combit -- complement a specified bit.
       2  
       3  Copyright 2002, 2003, 2012, 2015 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-impl.h"
      32  
      33  void
      34  mpz_combit (mpz_ptr d, mp_bitcnt_t bit_index)
      35  {
      36    mp_size_t dsize = SIZ(d);
      37    mp_ptr dp = PTR(d);
      38  
      39    mp_size_t limb_index = bit_index / GMP_NUMB_BITS;
      40    mp_limb_t bit = (CNST_LIMB (1) << (bit_index % GMP_NUMB_BITS));
      41  
      42    /* Check for the most common case: Positive input, no realloc or
      43       normalization needed. */
      44    if (limb_index + 1 < dsize)
      45      dp[limb_index] ^= bit;
      46  
      47    /* Check for the hairy case. d < 0, and we have all zero bits to the
      48       right of the bit to toggle. */
      49    else if (limb_index < -dsize
      50  	   && (limb_index == 0 || mpn_zero_p (dp, limb_index))
      51  	   && (dp[limb_index] & (bit - 1)) == 0)
      52      {
      53        ASSERT (dsize < 0);
      54        dsize = -dsize;
      55  
      56        if (dp[limb_index] & bit)
      57  	{
      58  	  /* We toggle the least significant one bit. Corresponds to
      59  	     an add, with potential carry propagation, on the absolute
      60  	     value. */
      61  	  dp = MPZ_REALLOC (d, 1 + dsize);
      62  	  dp[dsize] = 0;
      63  	  MPN_INCR_U (dp + limb_index, 1 + dsize - limb_index, bit);
      64  	  SIZ(d) = - dsize - dp[dsize];
      65  	}
      66        else
      67  	{
      68  	  /* We toggle a zero bit, subtract from the absolute value. */
      69  	  MPN_DECR_U (dp + limb_index, dsize - limb_index, bit);
      70  	  /* The absolute value shrinked by at most one bit. */
      71  	  dsize -= dp[dsize - 1] == 0;
      72  	  ASSERT (dsize > 0 && dp[dsize - 1] != 0);
      73  	  SIZ (d) = -dsize;
      74  	}
      75      }
      76    else
      77      {
      78        /* Simple case: Toggle the bit in the absolute value. */
      79        dsize = ABS(dsize);
      80        if (limb_index < dsize)
      81  	{
      82  	  mp_limb_t	 dlimb;
      83  	  dlimb = dp[limb_index] ^ bit;
      84  	  dp[limb_index] = dlimb;
      85  
      86  	  /* Can happen only when limb_index = dsize - 1. Avoid SIZ(d)
      87  	     bookkeeping in the common case. */
      88  	  if (UNLIKELY ((dlimb == 0) + limb_index == dsize)) /* dsize == limb_index + 1 */
      89  	    {
      90  	      /* high limb became zero, must normalize */
      91  	      MPN_NORMALIZE (dp, limb_index);
      92  	      SIZ (d) = SIZ (d) >= 0 ? limb_index : -limb_index;
      93  	    }
      94  	}
      95        else
      96  	{
      97  	  dp = MPZ_REALLOC (d, limb_index + 1);
      98  	  MPN_ZERO(dp + dsize, limb_index - dsize);
      99  	  dp[limb_index++] = bit;
     100  	  SIZ(d) = SIZ(d) >= 0 ? limb_index : -limb_index;
     101  	}
     102      }
     103  }