1 /* mpz_combit -- complement a specified bit. 2 3 Copyright 2002, 2003 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_combit (mpz_ptr d, mp_bitcnt_t bit_index) 25 { 26 mp_size_t dsize = ABSIZ(d); 27 mp_ptr dp = LIMBS(d); 28 29 mp_size_t limb_index = bit_index / GMP_NUMB_BITS; 30 mp_limb_t bit = ((mp_limb_t) 1 << (bit_index % GMP_NUMB_BITS)); 31 32 if (limb_index >= dsize) 33 { 34 MPZ_REALLOC(d, limb_index + 1); 35 dp = LIMBS(d); 36 37 MPN_ZERO(dp + dsize, limb_index + 1 - dsize); 38 dsize = limb_index + 1; 39 } 40 41 if (SIZ(d) >= 0) 42 { 43 dp[limb_index] ^= bit; 44 MPN_NORMALIZE (dp, dsize); 45 SIZ(d) = dsize; 46 } 47 else 48 { 49 mp_limb_t x = -dp[limb_index]; 50 mp_size_t i; 51 52 /* non-zero limb below us means ones-complement */ 53 for (i = limb_index-1; i >= 0; i--) 54 if (dp[i] != 0) 55 { 56 x--; /* change twos comp to ones comp */ 57 break; 58 } 59 60 if (x & bit) 61 { 62 mp_limb_t c; 63 64 /* Clearing the bit increases the magitude. We might need a carry. */ 65 MPZ_REALLOC(d, dsize + 1); 66 dp = LIMBS(d); 67 68 __GMPN_ADD_1 (c, dp+limb_index, dp+limb_index, 69 dsize - limb_index, bit); 70 dp[dsize] = c; 71 dsize += c; 72 } 73 else 74 /* Setting the bit decreases the magnitude */ 75 mpn_sub_1(dp+limb_index, dp+limb_index, dsize + limb_index, bit); 76 77 MPN_NORMALIZE (dp, dsize); 78 SIZ(d) = -dsize; 79 } 80 } 81