1 /* mpz_scan0 -- search for a 0 bit. 2 3 Copyright 2000-2002, 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 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 14 or 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 20 or both in parallel, as here. 21 22 The GNU MP Library is distributed in the hope that it will be useful, but 23 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 24 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 25 for more details. 26 27 You should have received copies of the GNU General Public License and the 28 GNU Lesser General Public License along with the GNU MP Library. If not, 29 see https://www.gnu.org/licenses/. */ 30 31 #include "gmp.h" 32 #include "gmp-impl.h" 33 #include "longlong.h" 34 35 36 /* mpn_scan0 can't be used for the u>0 search since there might not be a 0 37 bit before the end of the data. mpn_scan1 could be used for the inverted 38 search under u<0, but usually the search won't go very far so it seems 39 reasonable to inline that code. */ 40 41 mp_bitcnt_t 42 mpz_scan0 (mpz_srcptr u, mp_bitcnt_t starting_bit) __GMP_NOTHROW 43 { 44 mp_srcptr u_ptr = PTR(u); 45 mp_size_t size = SIZ(u); 46 mp_size_t abs_size = ABS(size); 47 mp_srcptr u_end = u_ptr + abs_size; 48 mp_size_t starting_limb = starting_bit / GMP_NUMB_BITS; 49 mp_srcptr p = u_ptr + starting_limb; 50 mp_limb_t limb; 51 int cnt; 52 53 /* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for 54 u<0. Notice this test picks up all cases of u==0 too. */ 55 if (starting_limb >= abs_size) 56 return (size >= 0 ? starting_bit : ~(mp_bitcnt_t) 0); 57 58 limb = *p; 59 60 if (size >= 0) 61 { 62 /* Mask to 1 all bits before starting_bit, thus ignoring them. */ 63 limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1; 64 65 /* Search for a limb which isn't all ones. If the end is reached then 66 the zero bit immediately past the end is returned. */ 67 while (limb == GMP_NUMB_MAX) 68 { 69 p++; 70 if (p == u_end) 71 return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS; 72 limb = *p; 73 } 74 75 /* Now seek low 1 bit. */ 76 limb = ~limb; 77 } 78 else 79 { 80 mp_srcptr q; 81 82 /* If there's a non-zero limb before ours then we're in the ones 83 complement region. Search from *(p-1) downwards since that might 84 give better cache locality, and since a non-zero in the middle of a 85 number is perhaps a touch more likely than at the end. */ 86 q = p; 87 while (q != u_ptr) 88 { 89 q--; 90 if (*q != 0) 91 goto inverted; 92 } 93 94 /* Adjust so ~limb implied by searching for 1 bit below becomes -limb. 95 If limb==0 here then this isn't the beginning of twos complement 96 inversion, but that doesn't matter because limb==0 is a zero bit 97 immediately (-1 is all ones for below). */ 98 limb--; 99 100 inverted: 101 /* Now seeking a 1 bit. */ 102 103 /* Mask to 0 all bits before starting_bit, thus ignoring them. */ 104 limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS)); 105 106 if (limb == 0) 107 { 108 /* If the high limb is zero after masking, then no 1 bits past 109 starting_bit. */ 110 p++; 111 if (p == u_end) 112 return ~(mp_bitcnt_t) 0; 113 114 /* Search further for a non-zero limb. The high limb is non-zero, 115 if nothing else. */ 116 for (;;) 117 { 118 limb = *p; 119 if (limb != 0) 120 break; 121 p++; 122 ASSERT (p < u_end); 123 } 124 } 125 } 126 127 ASSERT (limb != 0); 128 count_trailing_zeros (cnt, limb); 129 return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt; 130 } 131