1 /* mpz_tstbit -- test a specified bit. 2 3 Copyright 2000, 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 24 /* For negatives the effective twos complement is achieved by negating the 25 limb tested, either with a ones or twos complement. Twos complement 26 ("-") is used if there's only zero limbs below the one being tested. 27 Ones complement ("~") is used if there's a non-zero below. Note that "-" 28 is correct even if the limb examined is 0 (and the true beginning of twos 29 complement is further up). 30 31 Testing the limbs below p is unavoidable on negatives, but will usually 32 need to examine only *(p-1). The search is done from *(p-1) down to 33 *u_ptr, since that might give better cache locality, and because a 34 non-zero limb is perhaps a touch more likely in the middle of a number 35 than at the low end. 36 37 Bits past the end of available data simply follow sign of u. Notice that 38 the limb_index >= abs_size test covers u=0 too. */ 39 40 int 41 mpz_tstbit (mpz_srcptr u, mp_bitcnt_t bit_index) 42 { 43 mp_srcptr u_ptr = PTR(u); 44 mp_size_t size = SIZ(u); 45 unsigned abs_size = ABS(size); 46 mp_size_t limb_index = bit_index / GMP_NUMB_BITS; 47 mp_srcptr p = u_ptr + limb_index; 48 mp_limb_t limb; 49 50 if (limb_index >= abs_size) 51 return (size < 0); 52 53 limb = *p; 54 if (size < 0) 55 { 56 limb = -limb; /* twos complement */ 57 58 while (p != u_ptr) 59 { 60 p--; 61 if (*p != 0) 62 { 63 limb--; /* make it a ones complement instead */ 64 break; 65 } 66 } 67 } 68 69 return (limb >> (bit_index % GMP_NUMB_BITS)) & 1; 70 } 71