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 } 116