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@jabney jabney/entropy.js
Last active Feb 7, 2019

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Javascript implementation of a Shannon entropy calculation in bits per symbol
// 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 shannon.entropy(s) * s.length;
};
// Log the entropy of a string to the console.
shannon.log = function(s) {
console.log('Entropy of "' + s + '" in bits per symbol:', shannon.entropy(s));
};
})(window.shannon = window.shannon || Object.create(null));
shannon.log('1223334444'); // 1.8464393446710154
shannon.log('0'); // 0
shannon.log('01'); // 1
shannon.log('0123'); // 2
shannon.log('01234567'); // 3
shannon.log('0123456789abcdef'); // 4
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