path: root/wcsmbs/wcs-two-way.h

/* Wide character substring search, using the Two-Way algorithm.
   Copyright (C) 2008-2024 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

/* Before including this file, you need to include <string.h> (and
   <config.h> before that, if not part of libc), and define:
     AVAILABLE(h, h_l, j, n_l)
			     A macro that returns nonzero if there are
			     at least N_L characters left starting at H[J].
			     H is 'wchar_t *', H_L, J, and N_L are 'size_t';
			     H_L is an lvalue.  For NUL-terminated searches,
			     H_L can be modified each iteration to avoid
			     having to compute the end of H up front.

  For case-insensitivity, you may optionally define:
     CMP_FUNC(p1, p2, l)     A macro that returns 0 iff the first L
			     characters of P1 and P2 are equal.
     CANON_ELEMENT(c)        A macro that canonicalizes an element right after
			     it has been fetched from one of the two strings.
			     The argument is an 'wchar_t'; the result must
			     be an 'wchar_t' as well.
*/

#include <limits.h>
#include <stdint.h>
#include <sys/param.h>                  /* Defines MAX.  */

/* We use the Two-Way string matching algorithm, which guarantees
   linear complexity with constant space.

   See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260
   and http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm
*/

#ifndef CANON_ELEMENT
# define CANON_ELEMENT(c) c
#endif
#ifndef CMP_FUNC
# define CMP_FUNC __wmemcmp
#endif

/* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN.
   Return the index of the first character in the right half, and set
   *PERIOD to the global period of the right half.

   The global period of a string is the smallest index (possibly its
   length) at which all remaining bytes in the string are repetitions
   of the prefix (the last repetition may be a subset of the prefix).

   When NEEDLE is factored into two halves, a local period is the
   length of the smallest word that shares a suffix with the left half
   and shares a prefix with the right half.  All factorizations of a
   non-empty NEEDLE have a local period of at least 1 and no greater
   than NEEDLE_LEN.

   A critical factorization has the property that the local period
   equals the global period.  All strings have at least one critical
   factorization with the left half smaller than the global period.

   Given an ordered alphabet, a critical factorization can be computed
   in linear time, with 2 * NEEDLE_LEN comparisons, by computing the
   larger of two ordered maximal suffixes.  The ordered maximal
   suffixes are determined by lexicographic comparison of
   periodicity.  */
static size_t
critical_factorization (const wchar_t *needle, size_t needle_len,
			size_t *period)
{
  /* Index of last character of left half, or SIZE_MAX.  */
  size_t max_suffix, max_suffix_rev;
  size_t j; /* Index into NEEDLE for current candidate suffix.  */
  size_t k; /* Offset into current period.  */
  size_t p; /* Intermediate period.  */
  wchar_t a, b; /* Current comparison bytes.  */

  /* Special case NEEDLE_LEN of 1 or 2 (all callers already filtered
     out 0-length needles.  */
  if (needle_len < 3)
    {
      *period = 1;
      return needle_len - 1;
    }

  /* Invariants:
     0 <= j < NEEDLE_LEN - 1
     -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed)
     min(max_suffix, max_suffix_rev) < global period of NEEDLE
     1 <= p <= global period of NEEDLE
     p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j]
     1 <= k <= p
  */

  /* Perform lexicographic search.  */
  max_suffix = SIZE_MAX;
  j = 0;
  k = p = 1;
  while (j + k < needle_len)
    {
      a = CANON_ELEMENT (needle[j + k]);
      b = CANON_ELEMENT (needle[max_suffix + k]);
      if (a < b)
	{
	  /* Suffix is smaller, period is entire prefix so far.  */
	  j += k;
	  k = 1;
	  p = j - max_suffix;
	}
      else if (a == b)
	{
	  /* Advance through repetition of the current period.  */
	  if (k != p)
	    ++k;
	  else
	    {
	      j += p;
	      k = 1;
	    }
	}
      else /* b < a */
	{
	  /* Suffix is larger, start over from current location.  */
	  max_suffix = j++;
	  k = p = 1;
	}
    }
  *period = p;

  /* Perform reverse lexicographic search.  */
  max_suffix_rev = SIZE_MAX;
  j = 0;
  k = p = 1;
  while (j + k < needle_len)
    {
      a = CANON_ELEMENT (needle[j + k]);
      b = CANON_ELEMENT (needle[max_suffix_rev + k]);
      if (b < a)
	{
	  /* Suffix is smaller, period is entire prefix so far.  */
	  j += k;
	  k = 1;
	  p = j - max_suffix_rev;
	}
      else if (a == b)
	{
	  /* Advance through repetition of the current period.  */
	  if (k != p)
	    ++k;
	  else
	    {
	      j += p;
	      k = 1;
	    }
	}
      else /* a < b */
	{
	  /* Suffix is larger, start over from current location.  */
	  max_suffix_rev = j++;
	  k = p = 1;
	}
    }

  /* Choose the shorter suffix.  Return the first character of the right
     half, rather than the last character of the left half.  */
  if (max_suffix_rev + 1 < max_suffix + 1)
    return max_suffix + 1;
  *period = p;
  return max_suffix_rev + 1;
}

/* Return the first location of non-empty NEEDLE within HAYSTACK, or
   NULL.  HAYSTACK_LEN is the minimum known length of HAYSTACK.

   If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at
   most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.
   If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
   HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.  */
static inline wchar_t *
two_way_short_needle (const wchar_t *haystack, size_t haystack_len,
		      const wchar_t *needle, size_t needle_len)
{
  size_t i; /* Index into current character of NEEDLE.  */
  size_t j; /* Index into current window of HAYSTACK.  */
  size_t period; /* The period of the right half of needle.  */
  size_t suffix; /* The index of the right half of needle.  */

  /* Factor the needle into two halves, such that the left half is
     smaller than the global period, and the right half is
     periodic (with a period as large as NEEDLE_LEN - suffix).  */
  suffix = critical_factorization (needle, needle_len, &period);

  /* Perform the search.  Each iteration compares the right half
     first.  */
  if (CMP_FUNC (needle, needle + period, suffix) == 0)
    {
      /* Entire needle is periodic; a mismatch can only advance by the
	 period, so use memory to avoid rescanning known occurrences
	 of the period.  */
      size_t memory = 0;
      j = 0;
      while (AVAILABLE (haystack, haystack_len, j, needle_len))
	{
	  const wchar_t *pneedle;
	  const wchar_t *phaystack;

	  /* Scan for matches in right half.  */
	  i = MAX (suffix, memory);
	  pneedle = &needle[i];
	  phaystack = &haystack[i + j];
	  while (i < needle_len && (CANON_ELEMENT (*pneedle++)
				    == CANON_ELEMENT (*phaystack++)))
	    ++i;
	  if (needle_len <= i)
	    {
	      /* Scan for matches in left half.  */
	      i = suffix - 1;
	      pneedle = &needle[i];
	      phaystack = &haystack[i + j];
	      while (memory < i + 1 && (CANON_ELEMENT (*pneedle--)
					== CANON_ELEMENT (*phaystack--)))
		--i;
	      if (i + 1 < memory + 1)
		return (wchar_t *) (haystack + j);
	      /* No match, so remember how many repetitions of period
		 on the right half were scanned.  */
	      j += period;
	      memory = needle_len - period;
	    }
	  else
	    {
	      j += i - suffix + 1;
	      memory = 0;
	    }
	}
    }
  else
    {
      const wchar_t *phaystack;
      /* The comparison always starts from needle[suffix], so cache it
	 and use an optimized first-character loop.  */
      wchar_t needle_suffix = CANON_ELEMENT (needle[suffix]);

      /* The two halves of needle are distinct; no extra memory is
	 required, and any mismatch results in a maximal shift.  */
      period = MAX (suffix, needle_len - suffix) + 1;
      j = 0;
      while (AVAILABLE (haystack, haystack_len, j, needle_len))
	{
	  wchar_t haystack_char;
	  const wchar_t *pneedle;

	  phaystack = &haystack[suffix + j];

	  while (needle_suffix
	      != (haystack_char = CANON_ELEMENT (*phaystack++)))
	    {
	      ++j;
	      if (!AVAILABLE (haystack, haystack_len, j, needle_len))
		goto ret0;
	    }

	  /* Scan for matches in right half.  */
	  i = suffix + 1;
	  pneedle = &needle[i];
	  while (i < needle_len)
	    {
	      if (CANON_ELEMENT (*pneedle++)
		  != (haystack_char = CANON_ELEMENT (*phaystack++)))
		break;
	      ++i;
	    }
	  if (needle_len <= i)
	    {
	      /* Scan for matches in left half.  */
	      i = suffix - 1;
	      pneedle = &needle[i];
	      phaystack = &haystack[i + j];
	      while (i != SIZE_MAX)
		{
		  if (CANON_ELEMENT (*pneedle--)
		      != (haystack_char = CANON_ELEMENT (*phaystack--)))
		    break;
		  --i;
		}
	      if (i == SIZE_MAX)
		return (wchar_t *) (haystack + j);
	      j += period;
	    }
	  else
	    j += i - suffix + 1;
	}
    }
ret0: __attribute__ ((unused))
  return NULL;
}

#undef AVAILABLE
#undef CANON_ELEMENT
#undef CMP_FUNC