jabney · August 26, 2025 23:46 · Nov 22, 2019 · Dec 7, 2014 · Nov 24, 2014 · Nov 24, 2014
diff --git a/entropy.js b/entropy.js
@@ -1,5 +1,34 @@
 // entropy.js MIT License © 2014 James Abney https://github.com/jabney
 
+/***************************************
+ * ES2015
+ ***************************************/
+
+// Shannon entropy in bits per symbol.
+function entropy(str) {
+  const len = str.length
+
+  // Build a frequency map from the string.
+  const frequencies = Array.from(str)
+    .reduce((freq, c) => (freq[c] = (freq[c] || 0) + 1) && freq, {})
+
+  // Sum the frequency of each character.
+  return Object.values(frequencies)
+    .reduce((sum, f) => sum - f/len * Math.log2(f/len), 0)
+}
+
+console.log(entropy('1223334444'))        // 1.8464393446710154
+console.log(entropy('0'))                 // 0
+console.log(entropy('01'))                // 1
+console.log(entropy('0123'))              // 2
+console.log(entropy('01234567'))          // 3
+console.log(entropy('0123456789abcdef'))  // 4
+
+
+/***************************************
+ * ES5
+ ***************************************/
+
 // Calculate the Shannon entropy of a string in bits per symbol.
 (function(shannon) {
 'use strict';
@@ -39,4 +68,4 @@ shannon.log('0');                   // 0
 shannon.log('01');                  // 1
 shannon.log('0123');                // 2
 shannon.log('01234567');            // 3
-shannon.log('0123456789abcdef');    // 4
+shannon.log('0123456789abcdef');    // 4
diff --git a/entropy.js b/entropy.js
@@ -1,5 +1,5 @@
-// ---------------------------------------------------------------
-// entropy.js MIT License © 2014 James Abney
+// entropy.js MIT License © 2014 James Abney https://github.com/jabney
+
 // Calculate the Shannon entropy of a string in bits per symbol.
 (function(shannon) {
 'use strict';

diff --git a/entropy.js b/entropy.js
@@ -32,7 +32,7 @@
   shannon.log = function(s) {
     console.log('Entropy of "' + s + '" in bits per symbol:', shannon.entropy(s));
   };
-})(window.shannon = window.shannon || {});
+})(window.shannon = window.shannon || Object.create(null));
 
 shannon.log('1223334444');          // 1.8464393446710154
 shannon.log('0');                   // 0

diff --git a/entropy.js b/entropy.js
@@ -2,6 +2,8 @@
 // entropy.js MIT License © 2014 James Abney
 // 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;
@@ -10,6 +12,7 @@
     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;
@@ -19,10 +22,12 @@
     });
     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));

diff --git a/entropy.js b/entropy.js
@@ -1,4 +1,5 @@
-
+// ---------------------------------------------------------------
+// entropy.js MIT License © 2014 James Abney
 // Calculate the Shannon entropy of a string in bits per symbol.
 (function(shannon) {
   // Create a dictionary of character frequencies and iterate over it.

diff --git a/entropy.js b/entropy.js
@@ -28,9 +28,9 @@
   };
 })(window.shannon = window.shannon || {});
 
-shannon.log('1223334444');
-shannon.log('0');
-shannon.log('01');
-shannon.log('0123');
-shannon.log('01234567');
-shannon.log('0123456789abcdef');
+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
diff --git a/entropy.js b/entropy.js
@@ -18,6 +18,10 @@
     });
     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));

diff --git a/entropy.js b/entropy.js
@@ -1,3 +1,5 @@
+
+// Calculate the Shannon entropy of a string in bits per symbol.
 (function(shannon) {
   // Create a dictionary of character frequencies and iterate over it.
   function process(s, evaluator) {
@@ -27,4 +29,4 @@ shannon.log('0');
 shannon.log('01');
 shannon.log('0123');
 shannon.log('01234567');
-shannon.log('0123456789abcdef');
+shannon.log('0123456789abcdef');
diff --git a/entropy.js b/entropy.js
@@ -0,0 +1,30 @@
+(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;
+  };
+  // 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 || {});
+
+shannon.log('1223334444');
+shannon.log('0');
+shannon.log('01');
+shannon.log('0123');
+shannon.log('01234567');
+shannon.log('0123456789abcdef');
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