lib/unistr/u-strstr.h

   1 /* Substring test for UTF-8/UTF-16/UTF-32 strings.  -*- coding: utf-8 -*-
   2    Copyright (C) 1999, 2002, 2006, 2010-2021 Free Software Foundation, Inc.
   3    Written by Bruno Haible <bruno@clisp.org>, 2002, 2005.
   4
   5    This program is free software: you can redistribute it and/or modify it
   6    under the terms of the GNU Lesser General Public License as published
   7    by the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program 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 License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 UNIT *
  19 FUNC (const UNIT *haystack, const UNIT *needle)
  20 {
  21   UNIT first = needle[0];
  22
  23   /* Is needle empty?  */
  24   if (first == 0)
  25     return (UNIT *) haystack;
  26
  27   /* Is needle nearly empty (only one unit)?  */
  28   if (needle[1] == 0)
  29     return U_STRCHR (haystack, first);
  30
  31 #ifdef U_STRMBTOUC
  32   /* Is needle nearly empty (only one character)?  */
  33   {
  34     ucs4_t first_uc;
  35     int count = U_STRMBTOUC (&first_uc, needle);
  36     if (count > 0 && needle[count] == 0)
  37       return U_STRCHR (haystack, first_uc);
  38   }
  39 #endif
  40
  41 #if UNIT_IS_UINT8_T
  42   return (uint8_t *) strstr ((const char *) haystack, (const char *) needle);
  43 #else
  44   {
  45     /* Minimizing the worst-case complexity:
  46        Let n = U_STRLEN(haystack), m = U_STRLEN(needle).
  47        The naïve algorithm is O(n*m) worst-case.
  48        The Knuth-Morris-Pratt algorithm is O(n) worst-case but it needs a
  49        memory allocation.
  50        To achieve linear complexity and yet amortize the cost of the
  51        memory allocation, we activate the Knuth-Morris-Pratt algorithm
  52        only once the naïve algorithm has already run for some time; more
  53        precisely, when
  54          - the outer loop count is >= 10,
  55          - the average number of comparisons per outer loop is >= 5,
  56          - the total number of comparisons is >= m.
  57        But we try it only once.  If the memory allocation attempt failed,
  58        we don't retry it.  */
  59     bool try_kmp = true;
  60     size_t outer_loop_count = 0;
  61     size_t comparison_count = 0;
  62     size_t last_ccount = 0;                  /* last comparison count */
  63     const UNIT *needle_last_ccount = needle; /* = needle + last_ccount */
  64
  65     /* Speed up the following searches of needle by caching its first
  66        character.  */
  67     UNIT b = *needle++;
  68
  69     for (;; haystack++)
  70       {
  71         if (*haystack == 0)
  72           /* No match.  */
  73           return NULL;
  74
  75         /* See whether it's advisable to use an asymptotically faster
  76            algorithm.  */
  77         if (try_kmp
  78             && outer_loop_count >= 10
  79             && comparison_count >= 5 * outer_loop_count)
  80           {
  81             /* See if needle + comparison_count now reaches the end of
  82                needle.  */
  83             if (needle_last_ccount != NULL)
  84               {
  85                 needle_last_ccount +=
  86                   U_STRNLEN (needle_last_ccount,
  87                              comparison_count - last_ccount);
  88                 if (*needle_last_ccount == 0)
  89                   needle_last_ccount = NULL;
  90                 last_ccount = comparison_count;
  91               }
  92             if (needle_last_ccount == NULL)
  93               {
  94                 /* Try the Knuth-Morris-Pratt algorithm.  */
  95                 const UNIT *result;
  96                 bool success =
  97                   knuth_morris_pratt (haystack,
  98                                       needle - 1, U_STRLEN (needle - 1),
  99                                       &result);
 100                 if (success)
 101                   return (UNIT *) result;
 102                 try_kmp = false;
 103               }
 104           }
 105
 106         outer_loop_count++;
 107         comparison_count++;
 108         if (*haystack == b)
 109           /* The first character matches.  */
 110           {
 111             const UNIT *rhaystack = haystack + 1;
 112             const UNIT *rneedle = needle;
 113
 114             for (;; rhaystack++, rneedle++)
 115               {
 116                 if (*rneedle == 0)
 117                   /* Found a match.  */
 118                   return (UNIT *) haystack;
 119                 if (*rhaystack == 0)
 120                   /* No match.  */
 121                   return NULL;
 122                 comparison_count++;
 123                 if (*rhaystack != *rneedle)
 124                   /* Nothing in this round.  */
 125                   break;
 126               }
 127           }
 128       }
 129   }
 130 #endif
 131 }