pos/BrillTransformationRules.js

fasttag part of speech tagger implementation

/*
  Transformation rules for Brill's POS tagger
  Copyright (C) 2015 Hugo W.L. ter Doest

  This program is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

// Usage: 
// transformationRules = new BrillTransformationRules();
// transformationRules.rules.forEach(function(ruleFunction) {
//   ruleFunction(taggedSentence, i);
// });
// where taggedSentence is an array of arrays of the form:
// [[the, DET], [red, JJ], [book, NN]] and i the position to be processed

function BrillTransformationRules() {
  this.rules = [rule1, rule2, rule3, rule4, rule5, rule6, rule7, rule8];
}

BrillTransformationRules.prototype.getRule = function(index) {
  return(this.rules[index]);
};

BrillTransformationRules.prototype.setRule = function(index, rule) {
  this.rules[index] = rule;
};

BrillTransformationRules.prototype.appendRule = function(rule) {
  this.rules[this.rules.length] = rule;
};

BrillTransformationRules.prototype.setRules = function(newRules) {
  this.rules = newRules;
};

BrillTransformationRules.prototype.getRules = function() {
  return(this.rules);
};

/**
 * Indicates whether or not this string starts with the specified string.
 * @param {Object} string
 */
function startsWith($this, string) {
  if (!string) {
    return false;
  }
  return $this.indexOf(string) == 0;
}

/**
 * Indicates whether or not this string ends with the specified string.
 * @param {Object} string
 */
function endsWith($this, string) {
  if (!string || string.length > $this.length) {
    return false;
  }
  return $this.indexOf(string) == $this.length - string.length;
}

//  rule 1: DT, {VBD | VBP} --> DT, NN
function rule1(taggedSentence, index) {
  if ((index > 0) && (taggedSentence[index - 1][1] === "DT")) {
    if ((taggedSentence[index][1] === "VBD") ||
      (taggedSentence[index][1] === "VBP") ||
      (taggedSentence[index][1] === "VB")) {
      taggedSentence[index][1] = "NN";
    }
  }
}

// rule 2: convert a noun to a number (CD) if "." appears in the word
function rule2(taggedSentence, index) {
  if (startsWith(taggedSentence[index][1], "N")) {
    if (taggedSentence[index][0].indexOf(".") > -1) {
      // url if there are two contiguous alpha characters
      if (/[a-zA-Z]{2}/.test(taggedSentence[index][0])) {
        taggedSentence[index][1] = "URL";
      }
      else {
        taggedSentence[index][1] = "CD";
      }
    }
    // Attempt to convert into a number
    if (!isNaN(parseFloat(taggedSentence[index][0]))) {
      taggedSentence[index][1] = "CD";
    }
  }
}

// rule 3: convert a noun to a past participle if words[i] ends with "ed"
function rule3(taggedSentence, index) {
  if (startsWith(taggedSentence[index][1], "N") && endsWith(taggedSentence[index][0], "ed")) {
    taggedSentence[index][1] = "VBN";
  }
}

// rule 4: convert any type to adverb if it ends in "ly";
function rule4(taggedSentence, index) {
  if (endsWith(taggedSentence[index][0], "ly")) {
    taggedSentence[index][1] = "RB";
  }
}

// rule 5: convert a common noun (NN or NNS) to a adjective if it ends with "al"
function rule5(taggedSentence, index) {
  if (startsWith(taggedSentence[index][1], "NN") && endsWith(taggedSentence[index][0], "al")) {
    taggedSentence[index][1] = "JJ";
  }
}

// rule 6: convert a noun to a verb if the preceding work is "would"
function rule6(taggedSentence, index) {
  if ((index > 0) && startsWith(taggedSentence[index][1], "NN") && (taggedSentence[index - 1][0].toLowerCase() === "would")) {
    taggedSentence[index][1] = "VB";
  }
}

// rule 7: if a word has been categorized as a common noun and it ends with "s",
//         then set its type to plural common noun (NNS)
function rule7(taggedSentence, index) {
  if ((taggedSentence[index][1] === "NN") && (endsWith(taggedSentence[index][0], "s"))) {
    taggedSentence[index][1] = "NNS";
  }
}

// rule 8: convert a common noun to a present participle verb (i.e., a gerund)
function rule8(taggedSentence, index) {
  if (startsWith(taggedSentence[index][1], "NN") && endsWith(taggedSentence[index][0], "ing")) {
    taggedSentence[index][1] = "VBG";
  }
}

module.exports = BrillTransformationRules;