1 /* mpz_scan1 -- search for a 1 bit.
2
3 Copyright 2000-2002, 2004, 2012, 2015 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-impl.h"
32 #include "longlong.h"
33
34
35 /* mpn_scan0 can't be used for the inverted u<0 search since there might not
36 be a 0 bit before the end of the data. mpn_scan1 could be used under u>0
37 (except when in the high limb), but usually the search won't go very far
38 so it seems reasonable to inline that code. */
39
40 mp_bitcnt_t
mpz_scan1(mpz_srcptr u,mp_bitcnt_t starting_bit)41 mpz_scan1 (mpz_srcptr u, mp_bitcnt_t starting_bit) __GMP_NOTHROW
42 {
43 mp_srcptr u_ptr = PTR(u);
44 mp_size_t size = SIZ(u);
45 mp_size_t abs_size = ABS(size);
46 mp_srcptr u_end = u_ptr + abs_size - 1;
47 mp_size_t starting_limb = starting_bit / GMP_NUMB_BITS;
48 mp_srcptr p = u_ptr + starting_limb;
49 mp_limb_t limb;
50 int cnt;
51
52 /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0.
53 Notice this test picks up any u==0 too. */
54 if (starting_limb >= abs_size)
55 return (size >= 0 ? ~(mp_bitcnt_t) 0 : starting_bit);
56
57 /* This is an important case, where sign is not relevant! */
58 if (starting_bit == 0)
59 goto short_cut;
60
61 limb = *p;
62
63 if (size >= 0)
64 {
65 /* Mask to 0 all bits before starting_bit, thus ignoring them. */
66 limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
67
68 if (limb == 0)
69 {
70 /* If it's the high limb which is zero after masking, then there's
71 no 1 bits after starting_bit. */
72 if (p == u_end)
73 return ~(mp_bitcnt_t) 0;
74
75 /* Otherwise search further for a non-zero limb. The high limb is
76 non-zero, if nothing else. */
77 search_nonzero:
78 do
79 {
80 ASSERT (p != u_end);
81 p++;
82 short_cut:
83 limb = *p;
84 }
85 while (limb == 0);
86 }
87 }
88 else
89 {
90 /* If there's a non-zero limb before ours then we're in the ones
91 complement region. */
92 if (starting_limb == 0 || mpn_zero_p (u_ptr, starting_limb)) {
93 if (limb == 0)
94 /* Seeking for the first non-zero bit, it is the same for u and -u. */
95 goto search_nonzero;
96
97 /* Adjust so ~limb implied by searching for 0 bit becomes -limb. */
98 limb--;
99 }
100
101 /* Now seeking a 0 bit. */
102
103 /* Mask to 1 all bits before starting_bit, thus ignoring them. */
104 limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
105
106 /* Search for a limb which is not all ones. If the end is reached
107 then the zero immediately past the end is the result. */
108 while (limb == GMP_NUMB_MAX)
109 {
110 if (p == u_end)
111 return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
112 p++;
113 limb = *p;
114 }
115
116 /* Now seeking low 1 bit. */
117 limb = ~limb;
118 }
119
120 ASSERT (limb != 0);
121 count_trailing_zeros (cnt, limb);
122 return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
123 }
124