Module: English::PorterStemming

Defined in:
lib/gems/english-0.3.1/lib/english/porter_stemming.rb

Overview

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,

Taken from www.tartarus.org/~martin/PorterStemmer (Public Domain)

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

Class Method Summary collapse

Class Method Details

.stem(word) ⇒ Object 



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# File 'lib/gems/english-0.3.1/lib/english/porter_stemming.rb', line 98

def self.stem(word)

  # make a copy of the given object and convert it to a string.
  word = word.dup.to_str

  return word if word.length < 3

  # now map initial y to Y so that the patterns never treat it as vowel
  word[0] = 'Y' if word[0] == ?y

  # Step 1a
  if word =~ /(ss|i)es$/
    word = $` + $1
  elsif word =~ /([^s])s$/
    word = $` + $1
  end

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

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

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

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

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

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

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

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

  word
end