stem_porter.c

使用具有增量学习的监控式学习方法。包括几个不同的分类算法。

      {
      switch ( state )
         {
         case 0: state = (IsVowel(*word)) ? 1 : 2;
                 break;
         case 1: state = (IsVowel(*word)) ? 1 : 2;
                 if ( 2 == state ) result++;
                 break;
         case 2: state = (IsVowel(*word) || ('y' == *word)) ? 1 : 2;
                 break;
         }
      word++;
      }

   return( result );

   } /* WordSize */

/*FN**************************************************************************

       ContainsVowel( word )

   Returns: int -- TRUE (1) if the word parameter contains a vowel,
            FALSE (0) otherwise.

   Purpose: Some of the rewrite rules apply only to a root containing
            a vowel, where a vowel is one of "aeiou" or y with a
            consonant in front of it.

   Plan:    Obviously, under the definition of a vowel, a word contains
            a vowel iff either its first letter is one of "aeiou", or
            any of its other letters are "aeiouy".  The plan is to
            test this condition.

   Notes:   None
**/

static int
ContainsVowel( word )
   char *word;   /* in: buffer with word checked */
   {

   if ( EOS == *word )
      return( FALSE );
   else
      return( IsVowel(*word) || (NULL != strpbrk(word+1,"aeiouy")) );


   } /* ContainsVowel */

/*FN**************************************************************************

       EndsWithCVC( word )

   Returns: int -- TRUE (1) if the current word ends with a
            consonant-vowel-consonant combination, and the second
            consonant is not w, x, or y, FALSE (0) otherwise.

   Purpose: Some of the rewrite rules apply only to a root with
            this characteristic.

   Plan:    Look at the last three characters.

   Notes:   None
**/

static int
EndsWithCVC( word )
   char *word;   /* in: buffer with the word checked */
   {
   int length;         /* for finding the last three characters */

   if ( (length = strlen(word)) < 3 )
      return( FALSE );
   else
      {
      end = word + length - 1;
      return(    (NULL == strchr("aeiouwxy",*end--))      /* consonant */
              && (NULL != strchr("aeiouy",  *end--))        /* vowel */
              && (NULL == strchr("aeiou",   *end  )) );   /* consonant */
      }

   } /* EndsWithCVC */

/*FN**************************************************************************

       AddAnE( word )

   Returns: int -- TRUE (1) if the current word meets special conditions
            for adding an e.

   Purpose: Rule 122 applies only to a root with this characteristic.

   Plan:    Check for size of 1 and a consonant-vowel-consonant ending.

   Notes:   None
**/

static int
AddAnE( word )
   char *word;
   {

   return( (1 == WordSize(word)) && EndsWithCVC(word) );

   } /* AddAnE */

/*FN**************************************************************************

       RemoveAnE( word )

   Returns: int -- TRUE (1) if the current word meets special conditions
            for removing an e.

   Purpose: Rule 502 applies only to a root with this characteristic.

   Plan:    Check for size of 1 and no consonant-vowel-consonant ending.

   Notes:   None
**/

static int
RemoveAnE( word )
   char *word;
   {

   return( (1 == WordSize(word)) && !EndsWithCVC(word) );

   } /* RemoveAnE */

/*FN**************************************************************************

       ReplaceEnd( word, rule )

   Returns: int -- the id for the rule fired, 0 is none is fired

   Purpose: Apply a set of rules to replace the suffix of a word

   Plan:    Loop through the rule set until a match meeting all conditions
            is found.  If a rule fires, return its id, otherwise return 0.
            Connditions on the length of the root are checked as part of this
            function's processing because this check is so often made.

   Notes:   This is the main routine driving the stemmer.  It goes through
            a set of suffix replacement rules looking for a match on the
            current suffix.  When it finds one, if the root of the word
            is long enough, and it meets whatever other conditions are
            required, then the suffix is replaced, and the function returns.
**/

static int
ReplaceEnd( word, rule )
   char *word;        /* in/out: buffer with the stemmed word */
   RuleList *rule;    /* in: data structure with replacement rules */
   {
   register char *ending;   /* set to start of possible stemmed suffix */
   char tmp_ch;             /* save replaced character when testing */

   while ( 0 != rule->id )
      {
      ending = end - rule->old_offset;
      if ( word <= ending )
         if ( 0 == strcmp(ending,rule->old_end) )
            {
            tmp_ch = *ending;
            *ending = EOS;
            if ( rule->min_root_size < WordSize(word) )
               if ( !rule->condition || (*rule->condition)(word) )
                  {
                  (void)strcat( word, rule->new_end );
                  end = ending + rule->new_offset;
                  break;
                  }
            *ending = tmp_ch;
            }
      rule++;
      }

   return( rule->id );

   } /* ReplaceEnd */

/*****************************************************************************/
/*********************   Public Function Declarations   **********************/

/*FN***************************************************************************

       porter_stem( word )

   Returns: int -- FALSE (0) if the word contains non-alphabetic characters
            and hence is not stemmed, TRUE (1) otherwise

   Purpose: Stem a word

   Plan:    Part 1: Check to ensure the word is all alphabetic
            Part 2: Run through the Porter algorithm
            Part 3: Return an indication of successful stemming

   Notes:   This function implements the Porter stemming algorithm, with
            a few additions here and there.  See:

               Porter, M.F., "An Algorithm For Suffix Stripping,"
               Program 14 (3), July 1980, pp. 130-137.

            Porter's algorithm is an ad hoc set of rewrite rules with
            various conditions on rule firing.  The terminology of
            "step 1a" and so on, is taken directly from Porter's
            article, which unfortunately gives almost no justification
            for the various steps.  Thus this function more or less
            faithfully refects the opaque presentation in the article.
            Changes from the article amount to a few additions to the
            rewrite rules;  these are marked in the RuleList data
            structures with comments.
**/

int
porter_stem( word )
   char *word;  /* in/out: the word stemmed */
   {
   int rule;    /* which rule is fired in replacing an end */

            /* Part 1: Check to ensure the word is all alphabetic */
   for ( end = word; *end != EOS; end++ )
      if ( !isalpha((int)*end) ) return( FALSE );
      else *end = tolower( *end );
   end--;

                /*  Part 2: Run through the Porter algorithm */
   (void)ReplaceEnd( word, step1a_rules );
   rule = ReplaceEnd( word, step1b_rules );
   if ( (106 == rule) || (107 == rule) )
      (void)ReplaceEnd( word, step1b1_rules );
   (void)ReplaceEnd( word, step1c_rules );

   (void)ReplaceEnd( word, step2_rules );

   (void)ReplaceEnd( word, step3_rules );

   (void)ReplaceEnd( word, step4_rules );

   (void)ReplaceEnd( word, step5a_rules );
   (void)ReplaceEnd( word, step5b_rules );

           /* Part 3: Return an indication of successful stemming */
   return( TRUE );

   } /* porter_stem */