1 /* mpz_scan1 -- search for a 1 bit.
3 Copyright 2000, 2001, 2002, 2004, 2012 Free Software Foundation, Inc.
5 This file is part of the GNU MP Library.
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.
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.
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/. */
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. */
31 mpz_scan1 (mpz_srcptr u, mp_bitcnt_t starting_bit) __GMP_NOTHROW
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 - 1;
37 mp_size_t starting_limb = starting_bit / GMP_NUMB_BITS;
38 mp_srcptr p = u_ptr + starting_limb;
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);
47 /* This is an important case, where sign is not relevant! */
48 if (starting_bit == 0)
55 /* Mask to 0 all bits before starting_bit, thus ignoring them. */
56 limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
60 /* If it's the high limb which is zero after masking, then there's
61 no 1 bits after starting_bit. */
63 return ~(mp_bitcnt_t) 0;
65 /* Otherwise search further for a non-zero limb. The high limb is
66 non-zero, if nothing else. */
80 /* If there's a non-zero limb before ours then we're in the ones
82 if (mpn_zero_p (u_ptr, starting_limb)) {
84 /* Seeking for the first non-zero bit, it is the same for u and -u. */
87 /* Adjust so ~limb implied by searching for 0 bit becomes -limb. */
91 /* Now seeking a 0 bit. */
93 /* Mask to 1 all bits before starting_bit, thus ignoring them. */
94 limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
96 /* Search for a limb which is not all ones. If the end is reached
97 then the zero immediately past the end is the result. */
98 while (limb == GMP_NUMB_MAX)
101 return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
106 /* Now seeking low 1 bit. */
111 count_trailing_zeros (cnt, limb);
112 return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;