File: strstr.c

package info (click to toggle)
glibc 2.29-3
  • links: PTS, VCS
  • area: main
  • in suites: bullseye, sid
  • size: 271,436 kB
  • sloc: ansic: 1,013,441; asm: 261,303; makefile: 11,428; sh: 10,300; python: 8,944; cpp: 5,062; awk: 1,956; perl: 514; yacc: 290; pascal: 182; sed: 73
file content (162 lines) | stat: -rw-r--r-- 5,514 bytes parent folder | download | duplicates (6)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
/* Return the offset of one string within another.
   Copyright (C) 1994-2019 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#ifndef _LIBC
# include <config.h>
#endif

#include <string.h>

#define RETURN_TYPE char *
#define AVAILABLE(h, h_l, j, n_l)			\
  (((j) + (n_l) <= (h_l)) \
   || ((h_l) += __strnlen ((void*)((h) + (h_l)), (n_l) + 512), \
       (j) + (n_l) <= (h_l)))
#include "str-two-way.h"

#undef strstr

#ifndef STRSTR
#define STRSTR strstr
#endif

static inline char *
strstr2 (const unsigned char *hs, const unsigned char *ne)
{
  uint32_t h1 = (ne[0] << 16) | ne[1];
  uint32_t h2 = 0;
  for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
      h2 = (h2 << 16) | c;
  return h1 == h2 ? (char *)hs - 2 : NULL;
}

static inline char *
strstr3 (const unsigned char *hs, const unsigned char *ne)
{
  uint32_t h1 = ((uint32_t)ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8);
  uint32_t h2 = 0;
  for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
      h2 = (h2 | c) << 8;
  return h1 == h2 ? (char *)hs - 3 : NULL;
}

/* Hash character pairs so a small shift table can be used.  All bits of
   p[0] are included, but not all bits from p[-1].  So if two equal hashes
   match on p[-1], p[0] matches too.  Hash collisions are harmless and result
   in smaller shifts.  */
#define hash2(p) (((size_t)(p)[0] - ((size_t)(p)[-1] << 3)) % sizeof (shift))

/* Fast strstr algorithm with guaranteed linear-time performance.
   Small needles up to size 3 use a dedicated linear search.  Longer needles
   up to size 256 use a novel modified Horspool algorithm.  It hashes pairs
   of characters to quickly skip past mismatches.  The main search loop only
   exits if the last 2 characters match, avoiding unnecessary calls to memcmp
   and allowing for a larger skip if there is no match.  A self-adapting
   filtering check is used to quickly detect mismatches in long needles.
   By limiting the needle length to 256, the shift table can be reduced to 8
   bits per entry, lowering preprocessing overhead and minimizing cache effects.
   The limit also implies worst-case performance is linear.
   Needles larger than 256 characters use the linear-time Two-Way algorithm.  */
char *
STRSTR (const char *haystack, const char *needle)
{
  const unsigned char *hs = (const unsigned char *) haystack;
  const unsigned char *ne = (const unsigned char *) needle;

  /* Handle short needle special cases first.  */
  if (ne[0] == '\0')
    return (char *)hs;
  hs = (const unsigned char *)strchr ((const char*)hs, ne[0]);
  if (hs == NULL || ne[1] == '\0')
    return (char*)hs;
  if (ne[2] == '\0')
    return strstr2 (hs, ne);
  if (ne[3] == '\0')
    return strstr3 (hs, ne);

  /* Ensure haystack length is at least as long as needle length.
     Since a match may occur early on in a huge haystack, use strnlen
     and read ahead a few cachelines for improved performance.  */
  size_t ne_len = strlen ((const char*)ne);
  size_t hs_len = __strnlen ((const char*)hs, ne_len | 512);
  if (hs_len < ne_len)
    return NULL;

  /* Check whether we have a match.  This improves performance since we
     avoid initialization overheads.  */
  if (memcmp (hs, ne, ne_len) == 0)
    return (char *) hs;

  /* Use Two-Way algorithm for very long needles.  */
  if (__glibc_unlikely (ne_len > 256))
    return two_way_long_needle (hs, hs_len, ne, ne_len);

  const unsigned char *end = hs + hs_len - ne_len;
  uint8_t shift[256];
  size_t tmp, shift1;
  size_t m1 = ne_len - 1;
  size_t offset = 0;

  /* Initialize bad character shift hash table.  */
  memset (shift, 0, sizeof (shift));
  for (int i = 1; i < m1; i++)
    shift[hash2 (ne + i)] = i;
  /* Shift1 is the amount we can skip after matching the hash of the
     needle end but not the full needle.  */
  shift1 = m1 - shift[hash2 (ne + m1)];
  shift[hash2 (ne + m1)] = m1;

  while (1)
    {
      if (__glibc_unlikely (hs > end))
	{
	  end += __strnlen ((const char*)end + m1 + 1, 2048);
	  if (hs > end)
	    return NULL;
	}

      /* Skip past character pairs not in the needle.  */
      do
	{
	  hs += m1;
	  tmp = shift[hash2 (hs)];
	}
      while (tmp == 0 && hs <= end);

      /* If the match is not at the end of the needle, shift to the end
	 and continue until we match the hash of the needle end.  */
      hs -= tmp;
      if (tmp < m1)
	continue;

      /* Hash of the last 2 characters matches.  If the needle is long,
	 try to quickly filter out mismatches.  */
      if (m1 < 15 || memcmp (hs + offset, ne + offset, 8) == 0)
	{
	  if (memcmp (hs, ne, m1) == 0)
	    return (void *) hs;

	  /* Adjust filter offset when it doesn't find the mismatch.  */
	  offset = (offset >= 8 ? offset : m1) - 8;
	}

      /* Skip based on matching the hash of the needle end.  */
      hs += shift1;
    }
}
libc_hidden_builtin_def (strstr)