module Stemmable

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

See example usage at the end of this file.

Constants

C
CC
MEQ1
MGR0
MGR1
STEP_2_LIST
STEP_3_LIST
SUFFIX_1_REGEXP
SUFFIX_2_REGEXP
V
VOWEL_IN_STEM
VV

Public Instance Methods

stem()

make the #stem_porter the default stem method, just in case we feel like having multiple stemmers available later.

Alias for: stem_porter

stem_porter() click to toggle source

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,

Send comments to raypereda@hotmail.com

# 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

Also aliased as: stem