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-impl.h"
      32  
      33  void
      34  mpz_clrbit (mpz_ptr d, mp_bitcnt_t bit_idx)
      35  {
      36    mp_size_t dsize = SIZ (d);
      37    mp_ptr dp = PTR (d);
      38    mp_size_t limb_idx;
      39    mp_limb_t mask;
      40  
      41    limb_idx = bit_idx / GMP_NUMB_BITS;
      42    mask = CNST_LIMB(1) << (bit_idx % GMP_NUMB_BITS);
      43    if (dsize >= 0)
      44      {
      45        if (limb_idx < dsize)
      46  	{
      47  	  mp_limb_t  dlimb;
      48  	  dlimb = dp[limb_idx] & ~mask;
      49  	  dp[limb_idx] = dlimb;
      50  
      51  	  if (UNLIKELY ((dlimb == 0) + limb_idx == dsize)) /* dsize == limb_idx + 1 */
      52  	    {
      53  	      /* high limb became zero, must normalize */
      54  	      MPN_NORMALIZE (dp, limb_idx);
      55  	      SIZ (d) = limb_idx;
      56  	    }
      57  	}
      58        else
      59  	;
      60      }
      61    else
      62      {
      63        /* Simulate two's complement arithmetic, i.e. simulate
      64  	 1. Set OP = ~(OP - 1) [with infinitely many leading ones].
      65  	 2. clear the bit.
      66  	 3. Set OP = ~OP + 1.  */
      67  
      68        dsize = -dsize;
      69  
      70        if (limb_idx < dsize)
      71  	{
      72  	  mp_size_t zero_bound;
      73  
      74  	  /* No index upper bound on this loop, we're sure there's a non-zero limb
      75  	     sooner or later.  */
      76  	  zero_bound = 0;
      77  	  while (dp[zero_bound] == 0)
      78  	    zero_bound++;
      79  
      80  	  if (limb_idx > zero_bound)
      81  	    {
      82  	      dp[limb_idx] |= mask;
      83  	    }
      84  	  else if (limb_idx == zero_bound)
      85  	    {
      86  	      mp_limb_t  dlimb;
      87  	      dlimb = (((dp[limb_idx] - 1) | mask) + 1) & GMP_NUMB_MASK;
      88  	      dp[limb_idx] = dlimb;
      89  
      90  	      if (dlimb == 0)
      91  		{
      92  		  /* Increment at limb_idx + 1.  Extend the number with a zero limb
      93  		     for simplicity.  */
      94  		  dp = MPZ_REALLOC (d, dsize + 1);
      95  		  dp[dsize] = 0;
      96  		  MPN_INCR_U (dp + limb_idx + 1, dsize - limb_idx, 1);
      97  		  dsize += dp[dsize];
      98  
      99  		  SIZ (d) = -dsize;
     100  		}
     101  	    }
     102  	  else
     103  	    ;
     104  	}
     105        else
     106  	{
     107  	  /* Ugh.  The bit should be cleared outside of the end of the
     108  	     number.  We have to increase the size of the number.  */
     109  	  dp = MPZ_REALLOC (d, limb_idx + 1);
     110  	  SIZ (d) = -(limb_idx + 1);
     111  	  MPN_ZERO (dp + dsize, limb_idx - dsize);
     112  	  dp[limb_idx] = mask;
     113  	}
     114      }
     115  }