string/memmem.c

   1 /* Copyright (C) 1991-2021 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #ifndef _LIBC
  19 # include <config.h>
  20 #endif
  21
  22 #include <string.h>
  23
  24 #ifndef _LIBC
  25 # define __memmem       memmem
  26 #endif
  27
  28 #define RETURN_TYPE void *
  29 #define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
  30 #define FASTSEARCH(S,C,N) (void*) memchr ((void *)(S), (C), (N))
  31 #include "str-two-way.h"
  32
  33 #undef memmem
  34
  35 /* Hash character pairs so a small shift table can be used.  All bits of
  36    p[0] are included, but not all bits from p[-1].  So if two equal hashes
  37    match on p[-1], p[0] matches too.  Hash collisions are harmless and result
  38    in smaller shifts.  */
  39 #define hash2(p) (((size_t)(p)[0] - ((size_t)(p)[-1] << 3)) % sizeof (shift))
  40
  41 /* Fast memmem algorithm with guaranteed linear-time performance.
  42    Small needles up to size 2 use a dedicated linear search.  Longer needles
  43    up to size 256 use a novel modified Horspool algorithm.  It hashes pairs
  44    of characters to quickly skip past mismatches.  The main search loop only
  45    exits if the last 2 characters match, avoiding unnecessary calls to memcmp
  46    and allowing for a larger skip if there is no match.  A self-adapting
  47    filtering check is used to quickly detect mismatches in long needles.
  48    By limiting the needle length to 256, the shift table can be reduced to 8
  49    bits per entry, lowering preprocessing overhead and minimizing cache effects.
  50    The limit also implies worst-case performance is linear.
  51    Needles larger than 256 characters use the linear-time Two-Way algorithm.  */
  52 void *
  53 __memmem (const void *haystack, size_t hs_len,
  54           const void *needle, size_t ne_len)
  55 {
  56   const unsigned char *hs = (const unsigned char *) haystack;
  57   const unsigned char *ne = (const unsigned char *) needle;
  58
  59   if (ne_len == 0)
  60     return (void *) hs;
  61   if (ne_len == 1)
  62     return (void *) memchr (hs, ne[0], hs_len);
  63
  64   /* Ensure haystack length is >= needle length.  */
  65   if (hs_len < ne_len)
  66     return NULL;
  67
  68   const unsigned char *end = hs + hs_len - ne_len;
  69
  70   if (ne_len == 2)
  71     {
  72       uint32_t nw = ne[0] << 16 | ne[1], hw = hs[0] << 16 | hs[1];
  73       for (hs++; hs <= end && hw != nw; )
  74         hw = hw << 16 | *++hs;
  75       return hw == nw ? (void *)hs - 1 : NULL;
  76     }
  77
  78   /* Use Two-Way algorithm for very long needles.  */
  79   if (__builtin_expect (ne_len > 256, 0))
  80     return two_way_long_needle (hs, hs_len, ne, ne_len);
  81
  82   uint8_t shift[256];
  83   size_t tmp, shift1;
  84   size_t m1 = ne_len - 1;
  85   size_t offset = 0;
  86
  87   memset (shift, 0, sizeof (shift));
  88   for (int i = 1; i < m1; i++)
  89     shift[hash2 (ne + i)] = i;
  90   /* Shift1 is the amount we can skip after matching the hash of the
  91      needle end but not the full needle.  */
  92   shift1 = m1 - shift[hash2 (ne + m1)];
  93   shift[hash2 (ne + m1)] = m1;
  94
  95   for ( ; hs <= end; )
  96     {
  97       /* Skip past character pairs not in the needle.  */
  98       do
  99         {
 100           hs += m1;
 101           tmp = shift[hash2 (hs)];
 102         }
 103       while (tmp == 0 && hs <= end);
 104
 105       /* If the match is not at the end of the needle, shift to the end
 106          and continue until we match the hash of the needle end.  */
 107       hs -= tmp;
 108       if (tmp < m1)
 109         continue;
 110
 111       /* Hash of the last 2 characters matches.  If the needle is long,
 112          try to quickly filter out mismatches.  */
 113       if (m1 < 15 || memcmp (hs + offset, ne + offset, 8) == 0)
 114         {
 115           if (memcmp (hs, ne, m1) == 0)
 116             return (void *) hs;
 117
 118           /* Adjust filter offset when it doesn't find the mismatch.  */
 119           offset = (offset >= 8 ? offset : m1) - 8;
 120         }
 121
 122       /* Skip based on matching the hash of the needle end.  */
 123       hs += shift1;
 124     }
 125   return NULL;
 126 }
 127 libc_hidden_def (__memmem)
 128 weak_alias (__memmem, memmem)
 129 libc_hidden_weak (memmem)