pcfg-generator/index.js

A module to convert JSON-formated treebank data into a Stochastic Context-Free Grammar

var strip_duplicate_indicator = function(property_name) {
  var stripped_property_name = property_name.replace(/_\d+$/, '');
  return stripped_property_name;
};

var add_rule = function(pcfg, rule) {
  var rhs_matched = false;

  for (var i=0; i<pcfg.length; i++) {
    if (rule.lhs === pcfg[i].lhs) {
      for (var j=0; j<pcfg[i].rhs.length; j++) {
        rhs_matched = true;
        for (var k=0; k<rule.rhs.length; k++) {
          if (rule.rhs[k] !== pcfg[i].rhs[j].values[k]) {
            rhs_matched = false;
          } 
        }
        if (rhs_matched) {
          pcfg[i].rhs[j].count++;
          return pcfg;
        }
      }
      pcfg[i].rhs.push({count: 1, values: rule.rhs});
      return pcfg;
    }
  }

  pcfg.push({lhs: rule.lhs, rhs: [{count: 1, values: rule.rhs}]});
  return pcfg;
};

var generate_probabilities = function(pcfg, index) {
  var i, j, start = index || 0, past_end = index ? index+1 : pcfg.length;

  for (i=start; i<past_end; i++) {
    var count = 0;
    for (j=0; j<pcfg[i].rhs.length; j++) {
      count += pcfg[i].rhs[j].count;
    }

    for (j=0; j<pcfg[i].rhs.length; j++) {
      pcfg[i].rhs[j].probability = pcfg[i].rhs[j].count / count;
    }
  }

  return pcfg;
};

var generate_helper = function(input, pcfg, parent) {
  var rule = null, property = null;

  if (parent !== null) {
    rule = {lhs: parent, rhs: []};
    if (typeof input === 'string') {
      rule.rhs.push('\''+input+'\'');
    }
    else {
      for (property in input) {
        rule.rhs.push(strip_duplicate_indicator(property));
      }
    }
    pcfg = add_rule(pcfg, rule);
  }

  if (typeof input === 'object') {
    for (property in input) {
      pcfg = generate_helper(input[property], pcfg, strip_duplicate_indicator(property));
    }
  }
  
  return pcfg;
};

exports.generate = function(input) {
  var pcfg = [];
  pcfg = generate_probabilities(generate_helper(input, pcfg, null));
  return pcfg;
};

exports.add_to_pcfg = function(pcfg, lhs, rhs, count) {
  count = count || 1;
  for (var i=0; i<pcfg.length; i++) {
    if (pcfg[i].lhs === lhs) {
      pcfg[i].rhs.push({count: count, values: rhs});
      return generate_probabilities(pcfg, i);
    }
  }

  throw "LHS not found!";
};

exports.toCNF = function(pcfg) {
  var cnf = [];

  for (var i=0; i<pcfg.length; i++) {
    var lhs = pcfg[i].lhs;

    for (var j=0; j<pcfg[i].rhs.length; j++) {
      var rhs = pcfg[i].rhs[j].values.join(' ');
      var prob = pcfg[i].rhs[j].probability;
      cnf.push(lhs + ' -> ' + rhs + ' [' + prob + ']');
    }
  }

  return cnf.join('\n');
};

exports.finalize = function(pcfg) {
  var finalized_pcfg = [];

  for (var i=0; i<pcfg.length; i++) {
    var lhs = pcfg[i].lhs;

    for (var j=0; j<pcfg[i].rhs.length; j++) {
      var rhs = pcfg[i].rhs[j].values;
      var prob = pcfg[i].rhs[j].probability;
      finalized_pcfg.push({lhs: lhs, rhs: rhs, prob: prob});
    }
  }

  return finalized_pcfg;
};