lib/mbsstr.c

   1 /* Searching in a string.  -*- coding: utf-8 -*-
   2    Copyright (C) 2005-2020 Free Software Foundation, Inc.
   3    Written by Bruno Haible <bruno@clisp.org>, 2005.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    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
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 #include <config.h>
  19
  20 /* Specification.  */
  21 #include <string.h>
  22
  23 #include <stdbool.h>
  24 #include <stddef.h>  /* for NULL, in case a nonstandard string.h lacks it */
  25 #include <stdlib.h>
  26
  27 #include "malloca.h"
  28 #include "mbuiter.h"
  29
  30 /* Knuth-Morris-Pratt algorithm.  */
  31 #define UNIT unsigned char
  32 #define CANON_ELEMENT(c) c
  33 #include "str-kmp.h"
  34
  35 /* Knuth-Morris-Pratt algorithm.
  36    See https://en.wikipedia.org/wiki/Knuth-Morris-Pratt_algorithm
  37    Return a boolean indicating success:
  38    Return true and set *RESULTP if the search was completed.
  39    Return false if it was aborted because not enough memory was available.  */
  40 static bool
  41 knuth_morris_pratt_multibyte (const char *haystack, const char *needle,
  42                               const char **resultp)
  43 {
  44   size_t m = mbslen (needle);
  45   mbchar_t *needle_mbchars;
  46   size_t *table;
  47
  48   /* Allocate room for needle_mbchars and the table.  */
  49   void *memory = nmalloca (m, sizeof (mbchar_t) + sizeof (size_t));
  50   void *table_memory;
  51   if (memory == NULL)
  52     return false;
  53   needle_mbchars = memory;
  54   table_memory = needle_mbchars + m;
  55   table = table_memory;
  56
  57   /* Fill needle_mbchars.  */
  58   {
  59     mbui_iterator_t iter;
  60     size_t j;
  61
  62     j = 0;
  63     for (mbui_init (iter, needle); mbui_avail (iter); mbui_advance (iter), j++)
  64       mb_copy (&needle_mbchars[j], &mbui_cur (iter));
  65   }
  66
  67   /* Fill the table.
  68      For 0 < i < m:
  69        0 < table[i] <= i is defined such that
  70        forall 0 < x < table[i]: needle[x..i-1] != needle[0..i-1-x],
  71        and table[i] is as large as possible with this property.
  72      This implies:
  73      1) For 0 < i < m:
  74           If table[i] < i,
  75           needle[table[i]..i-1] = needle[0..i-1-table[i]].
  76      2) For 0 < i < m:
  77           rhaystack[0..i-1] == needle[0..i-1]
  78           and exists h, i <= h < m: rhaystack[h] != needle[h]
  79           implies
  80           forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1].
  81      table[0] remains uninitialized.  */
  82   {
  83     size_t i, j;
  84
  85     /* i = 1: Nothing to verify for x = 0.  */
  86     table[1] = 1;
  87     j = 0;
  88
  89     for (i = 2; i < m; i++)
  90       {
  91         /* Here: j = i-1 - table[i-1].
  92            The inequality needle[x..i-1] != needle[0..i-1-x] is known to hold
  93            for x < table[i-1], by induction.
  94            Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1].  */
  95         mbchar_t *b = &needle_mbchars[i - 1];
  96
  97         for (;;)
  98           {
  99             /* Invariants: The inequality needle[x..i-1] != needle[0..i-1-x]
 100                is known to hold for x < i-1-j.
 101                Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1].  */
 102             if (mb_equal (*b, needle_mbchars[j]))
 103               {
 104                 /* Set table[i] := i-1-j.  */
 105                 table[i] = i - ++j;
 106                 break;
 107               }
 108             /* The inequality needle[x..i-1] != needle[0..i-1-x] also holds
 109                for x = i-1-j, because
 110                  needle[i-1] != needle[j] = needle[i-1-x].  */
 111             if (j == 0)
 112               {
 113                 /* The inequality holds for all possible x.  */
 114                 table[i] = i;
 115                 break;
 116               }
 117             /* The inequality needle[x..i-1] != needle[0..i-1-x] also holds
 118                for i-1-j < x < i-1-j+table[j], because for these x:
 119                  needle[x..i-2]
 120                  = needle[x-(i-1-j)..j-1]
 121                  != needle[0..j-1-(x-(i-1-j))]  (by definition of table[j])
 122                     = needle[0..i-2-x],
 123                hence needle[x..i-1] != needle[0..i-1-x].
 124                Furthermore
 125                  needle[i-1-j+table[j]..i-2]
 126                  = needle[table[j]..j-1]
 127                  = needle[0..j-1-table[j]]  (by definition of table[j]).  */
 128             j = j - table[j];
 129           }
 130         /* Here: j = i - table[i].  */
 131       }
 132   }
 133
 134   /* Search, using the table to accelerate the processing.  */
 135   {
 136     size_t j;
 137     mbui_iterator_t rhaystack;
 138     mbui_iterator_t phaystack;
 139
 140     *resultp = NULL;
 141     j = 0;
 142     mbui_init (rhaystack, haystack);
 143     mbui_init (phaystack, haystack);
 144     /* Invariant: phaystack = rhaystack + j.  */
 145     while (mbui_avail (phaystack))
 146       if (mb_equal (needle_mbchars[j], mbui_cur (phaystack)))
 147         {
 148           j++;
 149           mbui_advance (phaystack);
 150           if (j == m)
 151             {
 152               /* The entire needle has been found.  */
 153               *resultp = mbui_cur_ptr (rhaystack);
 154               break;
 155             }
 156         }
 157       else if (j > 0)
 158         {
 159           /* Found a match of needle[0..j-1], mismatch at needle[j].  */
 160           size_t count = table[j];
 161           j -= count;
 162           for (; count > 0; count--)
 163             {
 164               if (!mbui_avail (rhaystack))
 165                 abort ();
 166               mbui_advance (rhaystack);
 167             }
 168         }
 169       else
 170         {
 171           /* Found a mismatch at needle[0] already.  */
 172           if (!mbui_avail (rhaystack))
 173             abort ();
 174           mbui_advance (rhaystack);
 175           mbui_advance (phaystack);
 176         }
 177   }
 178
 179   freea (memory);
 180   return true;
 181 }
 182
 183 /* Find the first occurrence of the character string NEEDLE in the character
 184    string HAYSTACK.  Return NULL if NEEDLE is not found in HAYSTACK.  */
 185 char *
 186 mbsstr (const char *haystack, const char *needle)
 187 {
 188   /* Be careful not to look at the entire extent of haystack or needle
 189      until needed.  This is useful because of these two cases:
 190        - haystack may be very long, and a match of needle found early,
 191        - needle may be very long, and not even a short initial segment of
 192          needle may be found in haystack.  */
 193   if (MB_CUR_MAX > 1)
 194     {
 195       mbui_iterator_t iter_needle;
 196
 197       mbui_init (iter_needle, needle);
 198       if (mbui_avail (iter_needle))
 199         {
 200           /* Minimizing the worst-case complexity:
 201              Let n = mbslen(haystack), m = mbslen(needle).
 202              The naïve algorithm is O(n*m) worst-case.
 203              The Knuth-Morris-Pratt algorithm is O(n) worst-case but it needs a
 204              memory allocation.
 205              To achieve linear complexity and yet amortize the cost of the
 206              memory allocation, we activate the Knuth-Morris-Pratt algorithm
 207              only once the naïve algorithm has already run for some time; more
 208              precisely, when
 209                - the outer loop count is >= 10,
 210                - the average number of comparisons per outer loop is >= 5,
 211                - the total number of comparisons is >= m.
 212              But we try it only once.  If the memory allocation attempt failed,
 213              we don't retry it.  */
 214           bool try_kmp = true;
 215           size_t outer_loop_count = 0;
 216           size_t comparison_count = 0;
 217           size_t last_ccount = 0;                  /* last comparison count */
 218           mbui_iterator_t iter_needle_last_ccount; /* = needle + last_ccount */
 219
 220           mbui_iterator_t iter_haystack;
 221
 222           mbui_init (iter_needle_last_ccount, needle);
 223           mbui_init (iter_haystack, haystack);
 224           for (;; mbui_advance (iter_haystack))
 225             {
 226               if (!mbui_avail (iter_haystack))
 227                 /* No match.  */
 228                 return NULL;
 229
 230               /* See whether it's advisable to use an asymptotically faster
 231                  algorithm.  */
 232               if (try_kmp
 233                   && outer_loop_count >= 10
 234                   && comparison_count >= 5 * outer_loop_count)
 235                 {
 236                   /* See if needle + comparison_count now reaches the end of
 237                      needle.  */
 238                   size_t count = comparison_count - last_ccount;
 239                   for (;
 240                        count > 0 && mbui_avail (iter_needle_last_ccount);
 241                        count--)
 242                     mbui_advance (iter_needle_last_ccount);
 243                   last_ccount = comparison_count;
 244                   if (!mbui_avail (iter_needle_last_ccount))
 245                     {
 246                       /* Try the Knuth-Morris-Pratt algorithm.  */
 247                       const char *result;
 248                       bool success =
 249                         knuth_morris_pratt_multibyte (haystack, needle,
 250                                                       &result);
 251                       if (success)
 252                         return (char *) result;
 253                       try_kmp = false;
 254                     }
 255                 }
 256
 257               outer_loop_count++;
 258               comparison_count++;
 259               if (mb_equal (mbui_cur (iter_haystack), mbui_cur (iter_needle)))
 260                 /* The first character matches.  */
 261                 {
 262                   mbui_iterator_t rhaystack;
 263                   mbui_iterator_t rneedle;
 264
 265                   memcpy (&rhaystack, &iter_haystack, sizeof (mbui_iterator_t));
 266                   mbui_advance (rhaystack);
 267
 268                   mbui_init (rneedle, needle);
 269                   if (!mbui_avail (rneedle))
 270                     abort ();
 271                   mbui_advance (rneedle);
 272
 273                   for (;; mbui_advance (rhaystack), mbui_advance (rneedle))
 274                     {
 275                       if (!mbui_avail (rneedle))
 276                         /* Found a match.  */
 277                         return (char *) mbui_cur_ptr (iter_haystack);
 278                       if (!mbui_avail (rhaystack))
 279                         /* No match.  */
 280                         return NULL;
 281                       comparison_count++;
 282                       if (!mb_equal (mbui_cur (rhaystack), mbui_cur (rneedle)))
 283                         /* Nothing in this round.  */
 284                         break;
 285                     }
 286                 }
 287             }
 288         }
 289       else
 290         return (char *) haystack;
 291     }
 292   else
 293     {
 294       if (*needle != '\0')
 295         {
 296           /* Minimizing the worst-case complexity:
 297              Let n = strlen(haystack), m = strlen(needle).
 298              The naïve algorithm is O(n*m) worst-case.
 299              The Knuth-Morris-Pratt algorithm is O(n) worst-case but it needs a
 300              memory allocation.
 301              To achieve linear complexity and yet amortize the cost of the
 302              memory allocation, we activate the Knuth-Morris-Pratt algorithm
 303              only once the naïve algorithm has already run for some time; more
 304              precisely, when
 305                - the outer loop count is >= 10,
 306                - the average number of comparisons per outer loop is >= 5,
 307                - the total number of comparisons is >= m.
 308              But we try it only once.  If the memory allocation attempt failed,
 309              we don't retry it.  */
 310           bool try_kmp = true;
 311           size_t outer_loop_count = 0;
 312           size_t comparison_count = 0;
 313           size_t last_ccount = 0;                  /* last comparison count */
 314           const char *needle_last_ccount = needle; /* = needle + last_ccount */
 315
 316           /* Speed up the following searches of needle by caching its first
 317              character.  */
 318           char b = *needle++;
 319
 320           for (;; haystack++)
 321             {
 322               if (*haystack == '\0')
 323                 /* No match.  */
 324                 return NULL;
 325
 326               /* See whether it's advisable to use an asymptotically faster
 327                  algorithm.  */
 328               if (try_kmp
 329                   && outer_loop_count >= 10
 330                   && comparison_count >= 5 * outer_loop_count)
 331                 {
 332                   /* See if needle + comparison_count now reaches the end of
 333                      needle.  */
 334                   if (needle_last_ccount != NULL)
 335                     {
 336                       needle_last_ccount +=
 337                         strnlen (needle_last_ccount,
 338                                  comparison_count - last_ccount);
 339                       if (*needle_last_ccount == '\0')
 340                         needle_last_ccount = NULL;
 341                       last_ccount = comparison_count;
 342                     }
 343                   if (needle_last_ccount == NULL)
 344                     {
 345                       /* Try the Knuth-Morris-Pratt algorithm.  */
 346                       const unsigned char *result;
 347                       bool success =
 348                         knuth_morris_pratt ((const unsigned char *) haystack,
 349                                             (const unsigned char *) (needle - 1),
 350                                             strlen (needle - 1),
 351                                             &result);
 352                       if (success)
 353                         return (char *) result;
 354                       try_kmp = false;
 355                     }
 356                 }
 357
 358               outer_loop_count++;
 359               comparison_count++;
 360               if (*haystack == b)
 361                 /* The first character matches.  */
 362                 {
 363                   const char *rhaystack = haystack + 1;
 364                   const char *rneedle = needle;
 365
 366                   for (;; rhaystack++, rneedle++)
 367                     {
 368                       if (*rneedle == '\0')
 369                         /* Found a match.  */
 370                         return (char *) haystack;
 371                       if (*rhaystack == '\0')
 372                         /* No match.  */
 373                         return NULL;
 374                       comparison_count++;
 375                       if (*rhaystack != *rneedle)
 376                         /* Nothing in this round.  */
 377                         break;
 378                     }
 379                 }
 380             }
 381         }
 382       else
 383         return (char *) haystack;
 384     }
 385 }