Data commit

Task/Entropy/JavaScript/entropy-1.js

@@ -0,0 +1,19 @@
+// 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