1 /* mpz_scan1 -- search for a 1 bit. 2 3 Copyright 2000, 2001, 2002, 2004, 2012 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 #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 30 mp_bitcnt_t 31 mpz_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