Time for an 2014 update…

Task/Entropy/Groovy/entropy-1.groovy

@@ -0,0 +1,6 @@
+String.metaClass.getShannonEntrophy = {
+    -delegate.inject([:]) { map, v -> map[v] = (map[v] ?: 0) + 1; map }.values().inject(0.0) { sum, v ->
+        def p = (BigDecimal)v / delegate.size()
+        sum + p * Math.log(p) / Math.log(2)
+    }
+}

Task/Entropy/Groovy/entropy-2.groovy

@@ -0,0 +1,11 @@
+[ '1223334444': '1.846439344671',
+  '1223334444555555555': '1.969811065121',
+  '122333': '1.459147917061',
+  '1227774444': '1.846439344671',
+  aaBBcccDDDD: '1.936260027482',
+  '1234567890abcdefghijklmnopqrstuvwxyz': '5.169925004424',
+  'Rosetta Code': '3.084962500407' ].each { s, expected ->
+
+    println "Checking $s has a shannon entrophy of $expected"
+    assert sprintf('%.12f', s.shannonEntrophy) == expected
+}