path: root/lib/mbsstr.c

/* Searching in a string.
   Copyright (C) 2005-2007 Free Software Foundation, Inc.
   Written by Bruno Haible <bruno@clisp.org>, 2005.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   This program 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 General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software Foundation,
   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */

#include <config.h>

/* Specification.  */
#include <string.h>

#include <stdbool.h>
#include <stddef.h>  /* for NULL, in case a nonstandard string.h lacks it */

#include "malloca.h"
#if HAVE_MBRTOWC
# include "mbuiter.h"
#endif

/* Knuth-Morris-Pratt algorithm.
   See http://en.wikipedia.org/wiki/Knuth-Morris-Pratt_algorithm
   Return a boolean indicating success.  */

static bool
knuth_morris_pratt_unibyte (const char *haystack, const char *needle,
			    const char **resultp)
{
  size_t m = strlen (needle);

  /* Allocate the table.  */
  size_t *table = (size_t *) malloca (m * sizeof (size_t));
  if (table == NULL)
    return false;
  /* Fill the table.
     For 0 < i < m:
       0 < table[i] <= i is defined such that
       rhaystack[0..i-1] == needle[0..i-1] and rhaystack[i] != needle[i]
       implies
       forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1],
       and table[i] is as large as possible with this property.
     table[0] remains uninitialized.  */
  {
    size_t i, j;

    table[1] = 1;
    j = 0;
    for (i = 2; i < m; i++)
      {
	unsigned char b = (unsigned char) needle[i - 1];

	for (;;)
	  {
	    if (b == (unsigned char) needle[j])
	      {
		table[i] = i - ++j;
		break;
	      }
	    if (j == 0)
	      {
		table[i] = i;
		break;
	      }
	    j = j - table[j];
	  }
      }
  }

  /* Search, using the table to accelerate the processing.  */
  {
    size_t j;
    const char *rhaystack;
    const char *phaystack;

    *resultp = NULL;
    j = 0;
    rhaystack = haystack;
    phaystack = haystack;
    /* Invariant: phaystack = rhaystack + j.  */
    while (*phaystack != '\0')
      if ((unsigned char) needle[j] == (unsigned char) *phaystack)
	{
	  j++;
	  phaystack++;
	  if (j == m)
	    {
	      /* The entire needle has been found.  */
	      *resultp = rhaystack;
	      break;
	    }
	}
      else if (j > 0)
	{
	  /* Found a match of needle[0..j-1], mismatch at needle[j].  */
	  rhaystack += table[j];
	  j -= table[j];
	}
      else
	{
	  /* Found a mismatch at needle[0] already.  */
	  rhaystack++;
	  phaystack++;
	}
  }

  freea (table);
  return true;
}

#if HAVE_MBRTOWC
static bool
knuth_morris_pratt_multibyte (const char *haystack, const char *needle,
			      const char **resultp)
{
  size_t m = mbslen (needle);
  mbchar_t *needle_mbchars;
  size_t *table;

  /* Allocate room for needle_mbchars and the table.  */
  char *memory = (char *) malloca (m * (sizeof (mbchar_t) + sizeof (size_t)));
  if (memory == NULL)
    return false;
  needle_mbchars = (mbchar_t *) memory;
  table = (size_t *) (memory + m * sizeof (mbchar_t));

  /* Fill needle_mbchars.  */
  {
    mbui_iterator_t iter;
    size_t j;

    j = 0;
    for (mbui_init (iter, needle); mbui_avail (iter); mbui_advance (iter), j++)
      mb_copy (&needle_mbchars[j], &mbui_cur (iter));
  }

  /* Fill the table.
     For 0 < i < m:
       0 < table[i] <= i is defined such that
       rhaystack[0..i-1] == needle[0..i-1] and rhaystack[i] != needle[i]
       implies
       forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1],
       and table[i] is as large as possible with this property.
     table[0] remains uninitialized.  */
  {
    size_t i, j;

    table[1] = 1;
    j = 0;
    for (i = 2; i < m; i++)
      {
	mbchar_t *b = &needle_mbchars[i - 1];

	for (;;)
	  {
	    if (mb_equal (*b, needle_mbchars[j]))
	      {
		table[i] = i - ++j;
		break;
	      }
	    if (j == 0)
	      {
		table[i] = i;
		break;
	      }
	    j = j - table[j];
	  }
      }
  }

  /* Search, using the table to accelerate the processing.  */
  {
    size_t j;
    mbui_iterator_t rhaystack;
    mbui_iterator_t phaystack;

    *resultp = NULL;
    j = 0;
    mbui_init (rhaystack, haystack);
    mbui_init (phaystack, haystack);
    /* Invariant: phaystack = rhaystack + j.  */
    while (mbui_avail (phaystack))
      if (mb_equal (needle_mbchars[j], mbui_cur (phaystack)))
	{
	  j++;
	  mbui_advance (phaystack);
	  if (j == m)
	    {
	      /* The entire needle has been found.  */
	      *resultp = mbui_cur_ptr (rhaystack);
	      break;
	    }
	}
      else if (j > 0)
	{
	  /* Found a match of needle[0..j-1], mismatch at needle[j].  */
	  size_t count = table[j];
	  j -= count;
	  for (; count > 0; count--)
	    {
	      if (!mbui_avail (rhaystack))
		abort ();
	      mbui_advance (rhaystack);
	    }
	}
      else
	{
	  /* Found a mismatch at needle[0] already.  */
	  if (!mbui_avail (rhaystack))
	    abort ();
	  mbui_advance (rhaystack);
	  mbui_advance (phaystack);
	}
  }

  freea (memory);
  return true;
}
#endif

/* Find the first occurrence of the character string NEEDLE in the character
   string HAYSTACK.  Return NULL if NEEDLE is not found in HAYSTACK.  */
char *
mbsstr (const char *haystack, const char *needle)
{
  /* Be careful not to look at the entire extent of haystack or needle
     until needed.  This is useful because of these two cases:
       - haystack may be very long, and a match of needle found early,
       - needle may be very long, and not even a short initial segment of
         needle may be found in haystack.  */
#if HAVE_MBRTOWC
  if (MB_CUR_MAX > 1)
    {
      mbui_iterator_t iter_needle;

      mbui_init (iter_needle, needle);
      if (mbui_avail (iter_needle))
	{
	  /* Minimizing the worst-case complexity:
	     Let n = mbslen(haystack), m = mbslen(needle).
	     The naïve algorithm is O(n*m) worst-case.
	     The Knuth-Morris-Pratt algorithm is O(n) worst-case but it needs a
	     memory allocation.
	     To achieve linear complexity and yet amortize the cost of the
	     memory allocation, we activate the Knuth-Morris-Pratt algorithm
	     only once the naïve algorithm has already run for some time; more
	     precisely, when
	       - the outer loop count is >= 10,
	       - the average number of comparisons per outer loop is >= 5,
	       - the total number of comparisons is >= m.
	     But we try it only once.  If the memory allocation attempt failed,
	     we don't retry it.  */
	  bool try_kmp = true;
	  size_t outer_loop_count = 0;
	  size_t comparison_count = 0;
	  size_t last_ccount = 0;		   /* last comparison count */
	  mbui_iterator_t iter_needle_last_ccount; /* = needle + last_ccount */

	  mbui_iterator_t iter_haystack;

	  mbui_init (iter_needle_last_ccount, needle);
	  mbui_init (iter_haystack, haystack);
	  for (;; mbui_advance (iter_haystack))
	    {
	      if (!mbui_avail (iter_haystack))
		/* No match.  */
		return NULL;

	      /* See whether it's advisable to use an asymptotically faster
		 algorithm.  */
	      if (try_kmp
		  && outer_loop_count >= 10
		  && comparison_count >= 5 * outer_loop_count)
		{
		  /* See if needle + comparison_count now reaches the end of
		     needle.  */
		  size_t count = comparison_count - last_ccount;
		  for (;
		       count > 0 && mbui_avail (iter_needle_last_ccount);
		       count--)
		    mbui_advance (iter_needle_last_ccount);
		  last_ccount = comparison_count;
		  if (!mbui_avail (iter_needle_last_ccount))
		    {
		      /* Try the Knuth-Morris-Pratt algorithm.  */
		      const char *result;
		      bool success =
			knuth_morris_pratt_multibyte (haystack, needle,
						      &result);
		      if (success)
			return (char *) result;
		      try_kmp = false;
		    }
		}

	      outer_loop_count++;
	      comparison_count++;
	      if (mb_equal (mbui_cur (iter_haystack), mbui_cur (iter_needle)))
		/* The first character matches.  */
		{
		  mbui_iterator_t rhaystack;
		  mbui_iterator_t rneedle;

		  memcpy (&rhaystack, &iter_haystack, sizeof (mbui_iterator_t));
		  mbui_advance (rhaystack);

		  mbui_init (rneedle, needle);
		  if (!mbui_avail (rneedle))
		    abort ();
		  mbui_advance (rneedle);

		  for (;; mbui_advance (rhaystack), mbui_advance (rneedle))
		    {
		      if (!mbui_avail (rneedle))
			/* Found a match.  */
			return (char *) mbui_cur_ptr (iter_haystack);
		      if (!mbui_avail (rhaystack))
			/* No match.  */
			return NULL;
		      comparison_count++;
		      if (!mb_equal (mbui_cur (rhaystack), mbui_cur (rneedle)))
			/* Nothing in this round.  */
			break;
		    }
		}
	    }
	}
      else
	return (char *) haystack;
    }
  else
#endif
    {
      if (*needle != '\0')
	{
	  /* Minimizing the worst-case complexity:
	     Let n = strlen(haystack), m = strlen(needle).
	     The naïve algorithm is O(n*m) worst-case.
	     The Knuth-Morris-Pratt algorithm is O(n) worst-case but it needs a
	     memory allocation.
	     To achieve linear complexity and yet amortize the cost of the
	     memory allocation, we activate the Knuth-Morris-Pratt algorithm
	     only once the naïve algorithm has already run for some time; more
	     precisely, when
	       - the outer loop count is >= 10,
	       - the average number of comparisons per outer loop is >= 5,
	       - the total number of comparisons is >= m.
	     But we try it only once.  If the memory allocation attempt failed,
	     we don't retry it.  */
	  bool try_kmp = true;
	  size_t outer_loop_count = 0;
	  size_t comparison_count = 0;
	  size_t last_ccount = 0;		   /* last comparison count */
	  const char *needle_last_ccount = needle; /* = needle + last_ccount */

	  /* Speed up the following searches of needle by caching its first
	     character.  */
	  char b = *needle++;

	  for (;; haystack++)
	    {
	      if (*haystack == '\0')
		/* No match.  */
		return NULL;

	      /* See whether it's advisable to use an asymptotically faster
		 algorithm.  */
	      if (try_kmp
		  && outer_loop_count >= 10
		  && comparison_count >= 5 * outer_loop_count)
		{
		  /* See if needle + comparison_count now reaches the end of
		     needle.  */
		  if (needle_last_ccount != NULL)
		    {
		      needle_last_ccount +=
			strnlen (needle_last_ccount,
				 comparison_count - last_ccount);
		      if (*needle_last_ccount == '\0')
			needle_last_ccount = NULL;
		      last_ccount = comparison_count;
		    }
		  if (needle_last_ccount == NULL)
		    {
		      /* Try the Knuth-Morris-Pratt algorithm.  */
		      const char *result;
		      bool success =
			knuth_morris_pratt_unibyte (haystack, needle - 1,
						    &result);
		      if (success)
			return (char *) result;
		      try_kmp = false;
		    }
		}

	      outer_loop_count++;
	      comparison_count++;
	      if (*haystack == b)
		/* The first character matches.  */
		{
		  const char *rhaystack = haystack + 1;
		  const char *rneedle = needle;

		  for (;; rhaystack++, rneedle++)
		    {
		      if (*rneedle == '\0')
			/* Found a match.  */
			return (char *) haystack;
		      if (*rhaystack == '\0')
			/* No match.  */
			return NULL;
		      comparison_count++;
		      if (*rhaystack != *rneedle)
			/* Nothing in this round.  */
			break;
		    }
		}
	    }
	}
      else
	return (char *) haystack;
    }
}