Module: Stemmable

Included in:
String
Defined in:
lib/stemmer/porter.rb

Overview

! /local/ruby/bin/ruby

$Id: stemmable.rb,v 1.2 2003/02/01 02:07:30 condit Exp $

See example usage at the end of this file.

Constant Summary collapse

STEP_2_LIST = 
{
  'ational'=>'ate', 'tional'=>'tion', 'enci'=>'ence', 'anci'=>'ance',
  'izer'=>'ize', 'bli'=>'ble',
  'alli'=>'al', 'entli'=>'ent', 'eli'=>'e', 'ousli'=>'ous',
  'ization'=>'ize', 'ation'=>'ate',
  'ator'=>'ate', 'alism'=>'al', 'iveness'=>'ive', 'fulness'=>'ful',
  'ousness'=>'ous', 'aliti'=>'al',
  'iviti'=>'ive', 'biliti'=>'ble', 'logi'=>'log'
}
STEP_3_LIST = 
{
  'icate'=>'ic', 'ative'=>'', 'alize'=>'al', 'iciti'=>'ic',
  'ical'=>'ic', 'ful'=>'', 'ness'=>''
}
SUFFIX_1_REGEXP = 
/(
ational  |
tional   |
enci     |
anci     |
izer     |
bli      |
alli     |
entli    |
eli      |
ousli    |
ization  |
ation    |
ator     |
alism    |
iveness  |
fulness  |
ousness  |
aliti    |
iviti    |
biliti   |
logi)$/x
SUFFIX_2_REGEXP = 
/(
al       |
ance     |
ence     |
er       |
ic       | 
able     |
ible     |
ant      |
ement    |
ment     |
ent      |
ou       |
ism      |
ate      |
iti      |
ous      |
ive      |
ize)$/x
C =

consonant

"[^aeiou]"
V =

vowel

"[aeiouy]"
CC =

consonant sequence

"#{C}(?>[^aeiouy]*)"
VV =

vowel sequence

"#{V}(?>[aeiou]*)"
MGR0 =

[cc]vvcc… is m>0

/^(#{CC})?#{VV}#{CC}/o
MEQ1 =
cc]vvcc[vv

is m=1

/^(#{CC})?#{VV}#{CC}(#{VV})?$/o
MGR1 =

[cc]vvccvvcc… is m>1

/^(#{CC})?#{VV}#{CC}#{VV}#{CC}/o
VOWEL_IN_STEM =

vowel in stem

/^(#{CC})?#{V}/o

Instance Method Summary collapse

Instance Method Details

#stem_porter ⇒ Object Also known as: stem

Porter stemmer in Ruby.

This is the Porter stemming algorithm, ported to Ruby from the version coded up in Perl. It’s easy to follow against the rules in the original paper in:

Porter, 1980, An algorithm for suffix stripping, Program, Vol. 14,
no. 3, pp 130-137,

See also www.tartarus.org/~martin/PorterStemmer

Send comments to [email protected]



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# File 'lib/stemmer/porter.rb', line 96

def stem_porter

  # make a copy of the given object and convert it to a string.
  w = self.dup.to_str
  
  return w if w.length < 3
  
  # now map initial y to Y so that the patterns never treat it as vowel
  w[0] = 'Y' if w[0] == ?y
  
  # Step 1a
  if w =~ /(ss|i)es$/
    w = $` + $1
  elsif w =~ /([^s])s$/ 
    w = $` + $1
  end

  # Step 1b
  if w =~ /eed$/
    w.chop! if $` =~ MGR0 
  elsif w =~ /(ed|ing)$/
    stem = $`
    if stem =~ VOWEL_IN_STEM 
      w = stem
	case w
      when /(at|bl|iz)$/             then w << "e"
      when /([^aeiouylsz])\1$/       then w.chop!
      when /^#{CC}#{V}[^aeiouwxy]$/o then w << "e"
      end
    end
  end

  if w =~ /y$/ 
    stem = $`
    w = stem + "i" if stem =~ VOWEL_IN_STEM 
  end

  # Step 2
  if w =~ SUFFIX_1_REGEXP
    stem = $`
    suffix = $1
    # print "stem= " + stem + "\n" + "suffix=" + suffix + "\n"
    if stem =~ MGR0
      w = stem + STEP_2_LIST[suffix]
    end
  end

  # Step 3
  if w =~ /(icate|ative|alize|iciti|ical|ful|ness)$/
    stem = $`
    suffix = $1
    if stem =~ MGR0
      w = stem + STEP_3_LIST[suffix]
    end
  end

  # Step 4
  if w =~ SUFFIX_2_REGEXP
    stem = $`
    if stem =~ MGR1
      w = stem
    end
  elsif w =~ /(s|t)(ion)$/
    stem = $` + $1
    if stem =~ MGR1
      w = stem
    end
  end

  #  Step 5
  if w =~ /e$/ 
    stem = $`
    if (stem =~ MGR1) ||
        (stem =~ MEQ1 && stem !~ /^#{CC}#{V}[^aeiouwxy]$/o)
      w = stem
    end
  end

  if w =~ /ll$/ && w =~ MGR1
    w.chop!
  end

  # and turn initial Y back to y
  w[0] = 'y' if w[0] == ?Y

  w
end