PorterStemmer.scala

/*
 Copyright 2009 David Hall, Daniel Ramage

 Licensed under the Apache License, Version 2.0 (the "License")
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
*/
package epic.features

/**
 * Converts words to their stemmed form using the classic Porter stemming
 * algorithm.
 *
 * @author dlwh
 * @author dramage
 */
class PorterStemmer() extends (String=>String) {
  import PorterStemmer._
  def apply(w: String) = {
    if (w.length < 3) w.toLowerCase
    else {
      val ret = {
        w.toLowerCase.replaceAll("([aeiou])y", "$1Y").replaceAll("^y", "Y")
      }
      step5(step4(step3(step2(step1(ret))))).toLowerCase
    }
  }
}

/**
 * Converts words to their stemmed form using the classic Porter stemming
 * algorithm.
 *
 * @author dlwh
 */
object PorterStemmer extends PorterStemmer {

  def apply() = this

  private def step1(w: String) = step1c(step1b(step1a(w)))

  // get rid of s's
  private def step1a(w: String) = {
    if (w.endsWith("sses") || w.endsWith("ies"))
      w.substring(0, w.length - 2)
    else if (w.endsWith("s") && w.charAt(w.length - 2) != 's')
      w.substring(0, w.length - 1)
    else w
  }

  private def step1b(w: String) = {
    //println(w + " " + m(w));
    def extra(w: String) = {
      if (w.endsWith("at") || w.endsWith("bl") || w.endsWith("iz")) w + 'e'
      // double consonant:
      else if (doublec(w) && !"lsz".contains(w.last)) w.substring(0, w.length - 1)
      else if (m(w) == 1 && cvc(w)) w + "e"
      else w
    }

    if (w.endsWith("eed")) {
      if (m(w.substring(0, w.length - 3)) > 0)
        w.substring(0, w.length - 1)
      else w
    } else if (w.endsWith("ed")) {
      if (w.indexWhere(isVowel) < (w.length - 2)) extra(w.substring(0, w.length - 2))
      else w
    } else if (w.endsWith("ing")) {
      if (w.indexWhere(isVowel) < (w.length - 3)) extra(w.substring(0, w.length - 3))
      else w
    } else w
  }

  def step1c(w: String) = {
    //println(w + " " + m(w));
    if ((w.last == 'y' || w.last == 'Y') && w.indexWhere(isVowel) < w.length - 1) {
      w.substring(0, w.length - 1) + 'i'
    } else w
  }

  private def replaceSuffix(w: String, suffix: String, repl: String) = {
    if (w endsWith suffix) Some((w.substring(0, w.length - suffix.length), repl))
    else None
  }

  private val mgt0 = {
    (w: (String, String)) => m(w._1) > 0
  }
  private val mgt1 = {
    (w: (String, String)) => m(w._1) > 1
  }

  private def step2(w: String) = {
    //println(w + " " + m(w));
    if (w.length < 3) w
    else {
      val opt = w(w.length - 2) match {
        case 'a' => replaceSuffix(w, "ational", "ate").orElse(replaceSuffix(w, "tional", "tion"))
        case 'c' =>
          replaceSuffix(w, "enci", "ence").orElse(replaceSuffix(w, "anci", "ance"))
        case 'e' => replaceSuffix(w, "izer", "ize")
        case 'g' => replaceSuffix(w, "logi", "log")
        case 'l' => replaceSuffix(w, "bli", "ble") orElse {
          replaceSuffix(w, "alli", "al")
        } orElse {
          replaceSuffix(w, "entli", "ent")
        } orElse {
          replaceSuffix(w, "eli", "e")
        } orElse {
          replaceSuffix(w, "ousli", "ous")
        }
        case 'o' => replaceSuffix(w, "ization", "ize") orElse {
          replaceSuffix(w, "ator", "ate")
        } orElse {
          replaceSuffix(w, "ation", "ate")
        }
        case 's' => replaceSuffix(w, "alism", "al") orElse {
          replaceSuffix(w, "iveness", "ive")
        } orElse {
          replaceSuffix(w, "fulness", "ful")
        } orElse {
          replaceSuffix(w, "ousness", "ous")
        }
        case 't' =>
          replaceSuffix(w, "aliti", "al") orElse {
            replaceSuffix(w, "iviti", "ive")
          } orElse {
            replaceSuffix(w, "biliti", "ble")
          }
        case _ => None
      }
      opt.filter(mgt0).map {
        case (a, b) => a + b
      }.getOrElse(w)
    }
  }

  private def step3(w: String) = {
    //println(w + " " + m(w));
    if (w.length < 3) w
    else {
      val opt = w.last match {
        case 'e' =>
          replaceSuffix(w, "icate", "ic") orElse {
            replaceSuffix(w, "alize", "al")
          } orElse {
            replaceSuffix(w, "ative", "")
          }
        case 'i' => replaceSuffix(w, "iciti", "ic")
        case 'l' => replaceSuffix(w, "ical", "ic").orElse(replaceSuffix(w, "ful", ""))
        case 's' => replaceSuffix(w, "ness", "")
        case _ => None
      }
      opt.filter(mgt0).map {
        case (a, b) => a + b
      }.getOrElse(w)
    }
  }

  private def step4(w: String) = {
    //println(w + " " + m(w));
    if (w.length < 3)
      w
    else {
      val opt = w(w.length - 2) match {
        case 'a' => replaceSuffix(w, "al", "")
        case 'c' => replaceSuffix(w, "ance", "").orElse(replaceSuffix(w, "ence", ""))
        case 'e' => replaceSuffix(w, "er", "")
        case 'i' => replaceSuffix(w, "ic", "")
        case 'l' => replaceSuffix(w, "able", "").orElse(replaceSuffix(w, "ible", ""))
        case 'n' => replaceSuffix(w, "ant", "") orElse {
          replaceSuffix(w, "ement", "")
        } orElse {
          //println("here")
          replaceSuffix(w, "ment", "")
        } orElse {
          //println("hereX")
          replaceSuffix(w, "ent", "")
        }
        case 'o' => replaceSuffix(w, "ion", "").filter(a => a._1.endsWith("t") || a._1.endsWith("s")).
          orElse(replaceSuffix(w, "ou", ""))
        case 's' => replaceSuffix(w, "ism", "")
        case 't' => replaceSuffix(w, "ate", "").orElse(replaceSuffix(w, "iti", ""))
        case 'u' => replaceSuffix(w, "ous", "")
        case 'v' => replaceSuffix(w, "ive", "")
        case 'z' => replaceSuffix(w, "ize", "")
        case _ => None
      }
      opt.filter(mgt1).map {
        case (a, b) => a + b
      }.getOrElse(w)
    }
  }

  private def step5(w: String) = {
    //println(w + " " + m(w));
    if (w.length < 3) w
    else
      step5b(step5a(w))
  }

  private def step5a(w: String) = {
    if (w.length < 3) w
    else
    if (w.last == 'e') {
      val n = m(w)
      if (n > 1) w.substring(0, w.length - 1)
      else if (n == 1 && !cvc(w.substring(0, w.length - 1))) w.substring(0, w.length - 1)
      else w
    }
    else {
      w
    }
  }

  private def step5b(w: String) = {
    if (w.last == 'l' && doublec(w) && m(w) > 1) w.substring(0, w.length - 1)
    else w
  }

  def m(w: String): Int = {
    val firstV = w.indexWhere(isVowel)
    if (firstV == -1) 0
    else {
      var m = 0
      var x: Seq[Char] = w.substring(firstV)
      if (x.isEmpty) m
      else {
        while (x.nonEmpty) {
          x = x.dropWhile(isVowel)
          if (x.isEmpty) return m
          m += 1
          if (m > 1) return m; // don't need anything bigger than this.
          x = x.dropWhile(isConsonant)
        }
        m
      }
    }
  }

  private def cvc(w: String) = (
    w.length > 2
      && isConsonant(w.last)
      && !("wxY" contains w.last)
      && isVowel(w(w.length - 2))
      && isConsonant(w.charAt(w.length - 3))
    )

  private def doublec(w: String) = {
    w.length > 2 && w.last == w.charAt(w.length - 2) && isConsonant(w.last)
  }

  def isConsonant(letter: Char) = !isVowel(letter)

  def isVowel(letter: Char) = "aeiouy" contains letter
}