1 /* mpz_clrbit -- clear a specified bit. 2 3 Copyright 1991, 1993, 1994, 1995, 2001, 2002 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_clrbit (mpz_ptr d, mp_bitcnt_t bit_index) 25 { 26 mp_size_t dsize = d->_mp_size; 27 mp_ptr dp = d->_mp_d; 28 mp_size_t limb_index; 29 30 limb_index = bit_index / GMP_NUMB_BITS; 31 if (dsize >= 0) 32 { 33 if (limb_index < dsize) 34 { 35 mp_limb_t dlimb; 36 dlimb = dp[limb_index]; 37 dlimb &= ~((mp_limb_t) 1 << (bit_index % GMP_NUMB_BITS)); 38 dp[limb_index] = dlimb; 39 40 if (UNLIKELY (dlimb == 0 && limb_index == dsize-1)) 41 { 42 /* high limb became zero, must normalize */ 43 do { 44 dsize--; 45 } while (dsize > 0 && dp[dsize-1] == 0); 46 d->_mp_size = dsize; 47 } 48 } 49 else 50 ; 51 } 52 else 53 { 54 mp_size_t zero_bound; 55 56 /* Simulate two's complement arithmetic, i.e. simulate 57 1. Set OP = ~(OP - 1) [with infinitely many leading ones]. 58 2. clear the bit. 59 3. Set OP = ~OP + 1. */ 60 61 dsize = -dsize; 62 63 /* No upper bound on this loop, we're sure there's a non-zero limb 64 sooner ot later. */ 65 for (zero_bound = 0; ; zero_bound++) 66 if (dp[zero_bound] != 0) 67 break; 68 69 if (limb_index > zero_bound) 70 { 71 if (limb_index < dsize) 72 dp[limb_index] |= (mp_limb_t) 1 << (bit_index % GMP_NUMB_BITS); 73 else 74 { 75 /* Ugh. The bit should be cleared outside of the end of the 76 number. We have to increase the size of the number. */ 77 if (UNLIKELY (d->_mp_alloc < limb_index + 1)) 78 dp = _mpz_realloc (d, limb_index + 1); 79 80 MPN_ZERO (dp + dsize, limb_index - dsize); 81 dp[limb_index] = (mp_limb_t) 1 << (bit_index % GMP_NUMB_BITS); 82 d->_mp_size = -(limb_index + 1); 83 } 84 } 85 else if (limb_index == zero_bound) 86 { 87 dp[limb_index] = ((((dp[limb_index] - 1) 88 | ((mp_limb_t) 1 << (bit_index % GMP_NUMB_BITS))) + 1) 89 & GMP_NUMB_MASK); 90 if (dp[limb_index] == 0) 91 { 92 mp_size_t i; 93 for (i = limb_index + 1; i < dsize; i++) 94 { 95 dp[i] = (dp[i] + 1) & GMP_NUMB_MASK; 96 if (dp[i] != 0) 97 goto fin; 98 } 99 /* We got carry all way out beyond the end of D. Increase 100 its size (and allocation if necessary). */ 101 dsize++; 102 if (UNLIKELY (d->_mp_alloc < dsize)) 103 dp = _mpz_realloc (d, dsize); 104 105 dp[i] = 1; 106 d->_mp_size = -dsize; 107 fin:; 108 } 109 } 110 else 111 ; 112 } 113 } 114