1/* mpz_clrbit -- clear a specified bit.
2
3Copyright 1991, 1993-1995, 2001, 2002, 2012 Free Software Foundation, Inc.
4
5This file is part of the GNU MP Library.
6
7The GNU MP Library is free software; you can redistribute it and/or modify
8it 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
14or
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
20or both in parallel, as here.
21
22The GNU MP Library is distributed in the hope that it will be useful, but
23WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
24or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
25for more details.
26
27You should have received copies of the GNU General Public License and the
28GNU Lesser General Public License along with the GNU MP Library.  If not,
29see https://www.gnu.org/licenses/.  */
30
31#include "gmp-impl.h"
32
33void
34mpz_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}
116