Time for an 2014 update…

Task/Entropy/Emacs-Lisp/entropy-1.l

@@ -0,0 +1,15 @@
+(defun shannon-entropy (input)
+  (let ((freq-table (make-hash-table))
+	(entropy 0)
+	(length (+ (length input) 0.0)))
+    (mapcar (lambda (x)
+	      (puthash x
+		       (+ 1 (gethash x freq-table 0))
+		       freq-table))
+	    input)
+    (maphash (lambda (k v)
+	       (set 'entropy (+ entropy
+			     (* (/ v length)
+				(log (/ v length) 2)))))
+	     freq-table)
+  (- entropy)))

Task/Entropy/Emacs-Lisp/entropy-2.l

@@ -0,0 +1,2 @@
+(shannon-entropy "1223334444")
+1.8464393446710154