kc3-lang/gnulib/lib/str-kmp.h

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• Hash : 5c26c7d4
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  Date : 2008-01-08T23:54:30
  

  Document the knuth_morris_pratt calling convention better.
  

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• lib/str-kmp.h

• /* Substring search in a NUL terminated string of 'char' elements,
     using the Knuth-Morris-Pratt algorithm.
     Copyright (C) 2005-2008 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.  */
  
  /* Before including this file, you need to define:
       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 'unsigned char'; the result
                               must be an 'unsigned char' as well.  */
  
  /* Knuth-Morris-Pratt algorithm.
     See http://en.wikipedia.org/wiki/Knuth-Morris-Pratt_algorithm
     Return a boolean indicating success:
     Return true and set *RESULTP if the search was completed.
     Return false if it was aborted because not enough memory was available.  */
  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 *) nmalloca (m, sizeof (size_t));
    if (table == NULL)
      return false;
    /* Fill the table.
       For 0 < i < m:
         0 < table[i] <= i is defined such that
         forall 0 < x < table[i]: needle[x..i-1] != needle[0..i-1-x],
         and table[i] is as large as possible with this property.
       This implies:
       1) For 0 < i < m:
            If table[i] < i,
            needle[table[i]..i-1] = needle[0..i-1-table[i]].
       2) For 0 < i < m:
            rhaystack[0..i-1] == needle[0..i-1]
            and exists h, i <= h < m: rhaystack[h] != needle[h]
            implies
            forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1].
       table[0] remains uninitialized.  */
    {
      size_t i, j;
  
      /* i = 1: Nothing to verify for x = 0.  */
      table[1] = 1;
      j = 0;
  
      for (i = 2; i < m; i++)
        {
  	/* Here: j = i-1 - table[i-1].
  	   The inequality needle[x..i-1] != needle[0..i-1-x] is known to hold
  	   for x < table[i-1], by induction.
  	   Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1].  */
  	unsigned char b = CANON_ELEMENT ((unsigned char) needle[i - 1]);
  
  	for (;;)
  	  {
  	    /* Invariants: The inequality needle[x..i-1] != needle[0..i-1-x]
  	       is known to hold for x < i-1-j.
  	       Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1].  */
  	    if (b == CANON_ELEMENT ((unsigned char) needle[j]))
  	      {
  		/* Set table[i] := i-1-j.  */
  		table[i] = i - ++j;
  		break;
  	      }
  	    /* The inequality needle[x..i-1] != needle[0..i-1-x] also holds
  	       for x = i-1-j, because
  	         needle[i-1] != needle[j] = needle[i-1-x].  */
  	    if (j == 0)
  	      {
  		/* The inequality holds for all possible x.  */
  		table[i] = i;
  		break;
  	      }
  	    /* The inequality needle[x..i-1] != needle[0..i-1-x] also holds
  	       for i-1-j < x < i-1-j+table[j], because for these x:
  		 needle[x..i-2]
  		 = needle[x-(i-1-j)..j-1]
  		 != needle[0..j-1-(x-(i-1-j))]  (by definition of table[j])
  		    = needle[0..i-2-x],
  	       hence needle[x..i-1] != needle[0..i-1-x].
  	       Furthermore
  		 needle[i-1-j+table[j]..i-2]
  		 = needle[table[j]..j-1]
  		 = needle[0..j-1-table[j]]  (by definition of table[j]).  */
  	    j = j - table[j];
  	  }
  	/* Here: j = i - table[i].  */
        }
    }
  
    /* 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 (CANON_ELEMENT ((unsigned char) needle[j])
  	  == CANON_ELEMENT ((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;
  }
  
  #undef CANON_ELEMENT
  

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