September Morn Update

Task/Entropy/JavaScript/entropy.js

@@ -1,31 +1,15 @@
-(function(shannon) {
-  // 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;
-  };
-})(window.shannon = window.shannon || {});
+const entropy = (s) => {
+  const split = s.split('');
+  const counter = {};
+  split.forEach(ch => {
+    if (!counter[ch]) counter[ch] = 1;
+    else counter[ch]++;
+  });
 
-// Log the Shannon entropy of a string.
-function logEntropy(s) {
-  console.log('Entropy of "' + s + '" in bits per symbol:', shannon.entropy(s));
-}
 
-logEntropy('1223334444');
-logEntropy('0');
-logEntropy('01');
-logEntropy('0123');
-logEntropy('01234567');
-logEntropy('0123456789abcdef');
+  const lengthf = s.length * 1.0;
+  const counts = Object.values(counter);
+  return -1 * counts
+    .map(count => count / lengthf * Math.log2(count / lengthf))
+    .reduce((a, b) => a + b);
+};