1/* mpz_tstbit -- test a specified bit. 2 3Copyright 2000, 2002 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 33 34/* For negatives the effective twos complement is achieved by negating the 35 limb tested, either with a ones or twos complement. Twos complement 36 ("-") is used if there's only zero limbs below the one being tested. 37 Ones complement ("~") is used if there's a non-zero below. Note that "-" 38 is correct even if the limb examined is 0 (and the true beginning of twos 39 complement is further up). 40 41 Testing the limbs below p is unavoidable on negatives, but will usually 42 need to examine only *(p-1). The search is done from *(p-1) down to 43 *u_ptr, since that might give better cache locality, and because a 44 non-zero limb is perhaps a touch more likely in the middle of a number 45 than at the low end. 46 47 Bits past the end of available data simply follow sign of u. Notice that 48 the limb_index >= abs_size test covers u=0 too. */ 49 50int 51mpz_tstbit (mpz_srcptr u, mp_bitcnt_t bit_index) __GMP_NOTHROW 52{ 53 mp_srcptr u_ptr = PTR(u); 54 mp_size_t size = SIZ(u); 55 unsigned abs_size = ABS(size); 56 mp_size_t limb_index = bit_index / GMP_NUMB_BITS; 57 mp_srcptr p = u_ptr + limb_index; 58 mp_limb_t limb; 59 60 if (limb_index >= abs_size) 61 return (size < 0); 62 63 limb = *p; 64 if (size < 0) 65 { 66 limb = -limb; /* twos complement */ 67 68 while (p != u_ptr) 69 { 70 p--; 71 if (*p != 0) 72 { 73 limb--; /* make it a ones complement instead */ 74 break; 75 } 76 } 77 } 78 79 return (limb >> (bit_index % GMP_NUMB_BITS)) & 1; 80} 81