scan0.c revision 1.1.1.1 
1/* mpz_scan0 -- search for a 0 bit.
2
3Copyright 2000, 2001, 2002, 2004 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 the GNU Lesser General Public License as published by
9the Free Software Foundation; either version 3 of the License, or (at your
10option) any later version.
11
12The GNU MP Library is distributed in the hope that it will be useful, but
13WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
14or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
15License for more details.
16
17You should have received a copy of the GNU Lesser General Public License
18along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
19
20#include "gmp.h"
21#include "gmp-impl.h"
22#include "longlong.h"
23
24
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.  */
29
30mp_bitcnt_t
31mpz_scan0 (mpz_srcptr u, mp_bitcnt_t starting_bit)
32{
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  mp_size_t      starting_limb = starting_bit / GMP_NUMB_BITS;
38  mp_srcptr      p = u_ptr + starting_limb;
39  mp_limb_t      limb;
40  int            cnt;
41
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);
46
47  limb = *p;
48
49  if (size >= 0)
50    {
51      /* Mask to 1 all bits before starting_bit, thus ignoring them. */
52      limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;
53
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)
57        {
58          p++;
59          if (p == u_end)
60            return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
61          limb = *p;
62        }
63
64      /* Now seek low 1 bit. */
65      limb = ~limb;
66    }
67  else
68    {
69      mp_srcptr  q;
70
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.  */
75      q = p;
76      while (q != u_ptr)
77        {
78          q--;
79          if (*q != 0)
80            goto inverted;
81        }
82
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).  */
87      limb--;
88
89    inverted:
90      /* Now seeking a 1 bit. */
91
92      /* Mask to 0 all bits before starting_bit, thus ignoring them. */
93      limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));
94
95      if (limb == 0)
96        {
97          /* If the high limb is zero after masking, then no 1 bits past
98             starting_bit.  */
99          p++;
100          if (p == u_end)
101            return ULONG_MAX;
102
103          /* Search further for a non-zero limb.  The high limb is non-zero,
104             if nothing else.  */
105          for (;;)
106            {
107              limb = *p;
108              if (limb != 0)
109                break;
110              p++;
111              ASSERT (p < u_end);
112            }
113        }
114    }
115
116  ASSERT (limb != 0);
117  count_trailing_zeros (cnt, limb);
118  return (mp_bitcnt_t) (p - u_ptr) * GMP_NUMB_BITS + cnt;
119}
120