string/strstr.c

   1 /* Return the offset of one string within another.
   2    Copyright (C) 1994-2023 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _LIBC
  20 # include <config.h>
  21 #endif
  22
  23 #include <string.h>
  24
  25 #define RETURN_TYPE char *
  26 #define AVAILABLE(h, h_l, j, n_l)                       \
  27   (((j) + (n_l) <= (h_l)) \
  28    || ((h_l) += __strnlen ((void*)((h) + (h_l)), (n_l) + 512), \
  29        (j) + (n_l) <= (h_l)))
  30 #include "str-two-way.h"
  31
  32 #undef strstr
  33
  34 #ifndef STRSTR
  35 #define STRSTR strstr
  36 #endif
  37
  38 static inline char *
  39 strstr2 (const unsigned char *hs, const unsigned char *ne)
  40 {
  41   uint32_t h1 = (ne[0] << 16) | ne[1];
  42   uint32_t h2 = 0;
  43   for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
  44       h2 = (h2 << 16) | c;
  45   return h1 == h2 ? (char *)hs - 2 : NULL;
  46 }
  47
  48 static inline char *
  49 strstr3 (const unsigned char *hs, const unsigned char *ne)
  50 {
  51   uint32_t h1 = ((uint32_t)ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8);
  52   uint32_t h2 = 0;
  53   for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
  54       h2 = (h2 | c) << 8;
  55   return h1 == h2 ? (char *)hs - 3 : NULL;
  56 }
  57
  58 /* Hash character pairs so a small shift table can be used.  All bits of
  59    p[0] are included, but not all bits from p[-1].  So if two equal hashes
  60    match on p[-1], p[0] matches too.  Hash collisions are harmless and result
  61    in smaller shifts.  */
  62 #define hash2(p) (((size_t)(p)[0] - ((size_t)(p)[-1] << 3)) % sizeof (shift))
  63
  64 /* Fast strstr algorithm with guaranteed linear-time performance.
  65    Small needles up to size 3 use a dedicated linear search.  Longer needles
  66    up to size 256 use a novel modified Horspool algorithm.  It hashes pairs
  67    of characters to quickly skip past mismatches.  The main search loop only
  68    exits if the last 2 characters match, avoiding unnecessary calls to memcmp
  69    and allowing for a larger skip if there is no match.  A self-adapting
  70    filtering check is used to quickly detect mismatches in long needles.
  71    By limiting the needle length to 256, the shift table can be reduced to 8
  72    bits per entry, lowering preprocessing overhead and minimizing cache effects.
  73    The limit also implies worst-case performance is linear.
  74    Needles larger than 256 characters use the linear-time Two-Way algorithm.  */
  75 char *
  76 STRSTR (const char *haystack, const char *needle)
  77 {
  78   const unsigned char *hs = (const unsigned char *) haystack;
  79   const unsigned char *ne = (const unsigned char *) needle;
  80
  81   /* Handle short needle special cases first.  */
  82   if (ne[0] == '\0')
  83     return (char *)hs;
  84   hs = (const unsigned char *)strchr ((const char*)hs, ne[0]);
  85   if (hs == NULL || ne[1] == '\0')
  86     return (char*)hs;
  87   if (ne[2] == '\0')
  88     return strstr2 (hs, ne);
  89   if (ne[3] == '\0')
  90     return strstr3 (hs, ne);
  91
  92   /* Ensure haystack length is at least as long as needle length.
  93      Since a match may occur early on in a huge haystack, use strnlen
  94      and read ahead a few cachelines for improved performance.  */
  95   size_t ne_len = strlen ((const char*)ne);
  96   size_t hs_len = __strnlen ((const char*)hs, ne_len | 512);
  97   if (hs_len < ne_len)
  98     return NULL;
  99
 100   /* Check whether we have a match.  This improves performance since we
 101      avoid initialization overheads.  */
 102   if (memcmp (hs, ne, ne_len) == 0)
 103     return (char *) hs;
 104
 105   /* Use Two-Way algorithm for very long needles.  */
 106   if (__glibc_unlikely (ne_len > 256))
 107     return two_way_long_needle (hs, hs_len, ne, ne_len);
 108
 109   const unsigned char *end = hs + hs_len - ne_len;
 110   uint8_t shift[256];
 111   size_t tmp, shift1;
 112   size_t m1 = ne_len - 1;
 113   size_t offset = 0;
 114
 115   /* Initialize bad character shift hash table.  */
 116   memset (shift, 0, sizeof (shift));
 117   for (int i = 1; i < m1; i++)
 118     shift[hash2 (ne + i)] = i;
 119   /* Shift1 is the amount we can skip after matching the hash of the
 120      needle end but not the full needle.  */
 121   shift1 = m1 - shift[hash2 (ne + m1)];
 122   shift[hash2 (ne + m1)] = m1;
 123
 124   while (1)
 125     {
 126       if (__glibc_unlikely (hs > end))
 127         {
 128           end += __strnlen ((const char*)end + m1 + 1, 2048);
 129           if (hs > end)
 130             return NULL;
 131         }
 132
 133       /* Skip past character pairs not in the needle.  */
 134       do
 135         {
 136           hs += m1;
 137           tmp = shift[hash2 (hs)];
 138         }
 139       while (tmp == 0 && hs <= end);
 140
 141       /* If the match is not at the end of the needle, shift to the end
 142          and continue until we match the hash of the needle end.  */
 143       hs -= tmp;
 144       if (tmp < m1)
 145         continue;
 146
 147       /* Hash of the last 2 characters matches.  If the needle is long,
 148          try to quickly filter out mismatches.  */
 149       if (m1 < 15 || memcmp (hs + offset, ne + offset, 8) == 0)
 150         {
 151           if (memcmp (hs, ne, m1) == 0)
 152             return (void *) hs;
 153
 154           /* Adjust filter offset when it doesn't find the mismatch.  */
 155           offset = (offset >= 8 ? offset : m1) - 8;
 156         }
 157
 158       /* Skip based on matching the hash of the needle end.  */
 159       hs += shift1;
 160     }
 161 }
 162 libc_hidden_builtin_def (strstr)