scan1.c revision 1.1.1.3 
1/* mpz_scan1 -- search for a 1 bit.
2
3Copyright 2000-2002, 2004, 2012, 2015 Free Software Foundation, Inc.
4
5This file is part of the GNU MP Library.
6
7The GNU MP Library is free software; you can redistribute it and/or modify
8it 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
14or
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
20or both in parallel, as here.
21
22The GNU MP Library is distributed in the hope that it will be useful, but
23WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
24or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
25for more details.
26
27You should have received copies of the GNU General Public License and the
28GNU Lesser General Public License along with the GNU MP Library.  If not,
29see 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 inverted u<0 search since there might not
37   be a 0 bit before the end of the data.  mpn_scan1 could be used under u>0
38   (except when in the high limb), but usually the search won't go very far
39   so it seems reasonable to inline that code.  */
40
41mp_bitcnt_t
42mpz_scan1 (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 - 1;
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  /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0.
54     Notice this test picks up any u==0 too. */
55  if (starting_limb >= abs_size)
56    return (size >= 0 ? ~(mp_bitcnt_t) 0 : starting_bit);
57
58  /* This is an important case, where sign is not relevant! */
59  if (starting_bit == 0)
60    goto short_cut;
61
62  limb = *p;
63
64  if (size >= 0)
65    {
66      /* Mask to 0 all bits before starting_bit, thus ignoring them. */
67      limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
68
69      if (limb == 0)
70	{
71	  /* If it's the high limb which is zero after masking, then there's
72	     no 1 bits after starting_bit.  */
73	  if (p == u_end)
74	    return ~(mp_bitcnt_t) 0;
75
76	  /* Otherwise search further for a non-zero limb.  The high limb is
77	     non-zero, if nothing else.  */
78	search_nonzero:
79	  do
80	    {
81	      ASSERT (p != u_end);
82	      p++;
83	    short_cut:
84	      limb = *p;
85	    }
86	  while (limb == 0);
87	}
88    }
89  else
90    {
91      /* If there's a non-zero limb before ours then we're in the ones
92	 complement region.  */
93      if (starting_limb == 0 || mpn_zero_p (u_ptr, starting_limb)) {
94	if (limb == 0)
95	  /* Seeking for the first non-zero bit, it is the same for u and -u. */
96	  goto search_nonzero;
97
98	/* Adjust so ~limb implied by searching for 0 bit becomes -limb.  */
99	limb--;
100      }
101
102      /* Now seeking a 0 bit. */
103
104      /* Mask to 1 all bits before starting_bit, thus ignoring them. */
105      limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
106
107      /* Search for a limb which is not all ones.  If the end is reached
108	 then the zero immediately past the end is the result.  */
109      while (limb == GMP_NUMB_MAX)
110	{
111	  if (p == u_end)
112	    return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
113	  p++;
114	  limb = *p;
115	}
116
117      /* Now seeking low 1 bit. */
118      limb = ~limb;
119    }
120
121  ASSERT (limb != 0);
122  count_trailing_zeros (cnt, limb);
123  return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
124}
125