Javascript implementation of a Shannon entropy calculation in bits per symbol

entropy.js

// entropy.js MIT License © 2014 James Abney http://github.com/jabney

// Calculate the Shannon entropy of a string in bits per symbol.
(function(shannon) {
'use strict';

  // Create a dictionary of character frequencies and iterate over it.
  function process(s, evaluator) {
    var h = Object.create(null), k;
    s.split('').forEach(function(c) {
      h[c] && h[c]++ || (h[c] = 1); });
    if (evaluator) for (k in h) evaluator(k, h[k]);
    return h;
  };
  
  // Measure the entropy of a string in bits per symbol.
  shannon.entropy = function(s) {
    var sum = 0,len = s.length;
    process(s, function(k, f) {
      var p = f/len;
      sum -= p * Math.log(p) / Math.log(2);
    });
    return sum;
  };
  
  // Measure the entropy of a string in total bits.
  shannon.bits = function(s) {
    return Math.ceil(shannon.entropy(s)) * s.length;
  };

  // Get entropy level
  shannon.level = function (s) {
    var H = shannon.bits(s);
    var l = 'dummy'; //Text level

    if(H < 35){
      l = 'week';
    } else if (H < 55){
      l = 'medium';
    } else {
      l = 'strong';
    }

    return l;
  };
  
  // Log the entropy of a string to the console.
  shannon.log = function(s) {
    console.log('Entropy of "' + s + '" in bits per symbol:', shannon.bits(s));
  };
})(window.shannon = window.shannon || Object.create(null));