1 /* mpz_scan0 -- search for a 0 bit.
3 Copyright 2000, 2001, 2002, 2004 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 u>0 search since there might not be a 0
26 bit before the end of the data. mpn_scan1 could be used for the inverted
27 search under u<0, but usually the search won't go very far so it seems
28 reasonable to inline that code. */
31 mpz_scan0 (mpz_srcptr u, unsigned long starting_bit)
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 unsigned long starting_limb = starting_bit / GMP_NUMB_BITS;
38 mp_srcptr p = u_ptr + starting_limb;
42 /* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for
43 u<0. Notice this test picks up all cases of u==0 too. */
44 if (starting_limb >= abs_size)
45 return (size >= 0 ? starting_bit : ULONG_MAX);
51 /* Mask to 1 all bits before starting_bit, thus ignoring them. */
52 limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
54 /* Search for a limb which isn't all ones. If the end is reached then
55 the zero bit immediately past the end is returned. */
56 while (limb == GMP_NUMB_MAX)
60 return (unsigned long) abs_size * GMP_NUMB_BITS;
64 /* Now seek low 1 bit. */
71 /* If there's a non-zero limb before ours then we're in the ones
72 complement region. Search from *(p-1) downwards since that might
73 give better cache locality, and since a non-zero in the middle of a
74 number is perhaps a touch more likely than at the end. */
83 /* Adjust so ~limb implied by searching for 1 bit below becomes -limb.
84 If limb==0 here then this isn't the beginning of twos complement
85 inversion, but that doesn't matter because limb==0 is a zero bit
86 immediately (-1 is all ones for below). */
90 /* Now seeking a 1 bit. */
92 /* Mask to 0 all bits before starting_bit, thus ignoring them. */
93 limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
97 /* If the high limb is zero after masking, then no 1 bits past
103 /* Search further for a non-zero limb. The high limb is non-zero,
117 count_trailing_zeros (cnt, limb);
118 return (p - u_ptr) * GMP_NUMB_BITS + cnt;