1/* mpz_scan1 -- search for a 1 bit. 2 3Copyright 2000, 2001, 2002, 2004, 2012 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 the GNU Lesser General Public License as published by 9the Free Software Foundation; either version 3 of the License, or (at your 10option) any later version. 11 12The GNU MP Library is distributed in the hope that it will be useful, but 13WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 14or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public 15License for more details. 16 17You should have received a copy of the GNU Lesser General Public License 18along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */ 19 20#include "gmp.h" 21#include "gmp-impl.h" 22#include "longlong.h" 23 24 25/* mpn_scan0 can't be used for the inverted u<0 search since there might not 26 be a 0 bit before the end of the data. mpn_scan1 could be used under u>0 27 (except when in the high limb), but usually the search won't go very far 28 so it seems reasonable to inline that code. */ 29 30mp_bitcnt_t 31mpz_scan1 (mpz_srcptr u, mp_bitcnt_t starting_bit) __GMP_NOTHROW 32{ 33 mp_srcptr u_ptr = PTR(u); 34 mp_size_t size = SIZ(u); 35 mp_size_t abs_size = ABS(size); 36 mp_srcptr u_end = u_ptr + abs_size; 37 mp_size_t starting_limb = starting_bit / GMP_NUMB_BITS; 38 mp_srcptr p = u_ptr + starting_limb; 39 mp_limb_t limb; 40 int cnt; 41 42 /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0. 43 Notice this test picks up any u==0 too. */ 44 if (starting_limb >= abs_size) 45 return (size >= 0 ? ~(mp_bitcnt_t) 0 : starting_bit); 46 47 limb = *p; 48 49 if (size >= 0) 50 { 51 /* Mask to 0 all bits before starting_bit, thus ignoring them. */ 52 limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS)); 53 54 if (limb == 0) 55 { 56 /* If it's the high limb which is zero after masking, then there's 57 no 1 bits after starting_bit. */ 58 p++; 59 if (p == u_end) 60 return ~(mp_bitcnt_t) 0; 61 62 /* Otherwise search further for a non-zero limb. The high limb is 63 non-zero, if nothing else. */ 64 for (;;) 65 { 66 limb = *p; 67 if (limb != 0) 68 break; 69 p++; 70 ASSERT (p < u_end); 71 } 72 } 73 } 74 else 75 { 76 mp_srcptr q; 77 78 /* If there's a non-zero limb before ours then we're in the ones 79 complement region. Search from *(p-1) downwards since that might 80 give better cache locality, and since a non-zero in the middle of a 81 number is perhaps a touch more likely than at the end. */ 82 q = p; 83 while (q != u_ptr) 84 { 85 q--; 86 if (*q != 0) 87 goto inverted; 88 } 89 90 if (limb == 0) 91 { 92 /* Skip zero limbs, to find the start of twos complement. The 93 high limb is non-zero, if nothing else. This search is 94 necessary so the -limb is applied at the right spot. */ 95 do 96 { 97 p++; 98 ASSERT (p < u_end); 99 limb = *p; 100 } 101 while (limb == 0); 102 103 /* Apply twos complement, and look for a 1 bit in that. Since 104 limb!=0 here, also have (-limb)!=0 so there's certainly a 1 105 bit. */ 106 limb = -limb; 107 goto got_limb; 108 } 109 110 /* Adjust so ~limb implied by searching for 0 bit becomes -limb. */ 111 limb--; 112 113 inverted: 114 /* Now seeking a 0 bit. */ 115 116 /* Mask to 1 all bits before starting_bit, thus ignoring them. */ 117 limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1; 118 119 /* Search for a limb which is not all ones. If the end is reached 120 then the zero immediately past the end is the result. */ 121 while (limb == GMP_NUMB_MAX) 122 { 123 p++; 124 if (p == u_end) 125 return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS; 126 limb = *p; 127 } 128 129 /* Now seeking low 1 bit. */ 130 limb = ~limb; 131 } 132 133 got_limb: 134 ASSERT (limb != 0); 135 count_trailing_zeros (cnt, limb); 136 return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt; 137} 138