retrieval/src/Bm25/idf_map/lib/idf.js

Full text search engine in js. Features BM25 ranking function that can be tuned.

/*
 * Fcn to compute the inverse document frequency of some input term.
 */
module.exports = (function() {
  /**
   * Calc idf by looping over docs in corpus; tallying the number of docs the given term appears in.
   * @param  {String} term
   * @param  {Array} termFreqMaps
   * @return {Number} the inverse document frequency of the input term
   */
  return function(term, termFreqMaps) {
            var numDocs = termFreqMaps.length;
            var docsAppear = 0;
            for(let i in termFreqMaps) {
              // Loop over docs in corpus, increment by 1 if term is seen 
              // in a given doc.
              docsAppear += ( termFreqMaps[i].hasOwnProperty(term) ? 1 : 0 );
            }
            // Add 1 to log to shift log curve above 0 so that terms are 
            // guaranteed strictly positive weight.
            // Add 1 in denominator to prevent division by 0 when a term 
            // doesn't appear in a document.
            return 1 + Math.log(numDocs / (1 + docsAppear));
         };
})();