rosetta code: entropy

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/examples/rosettacode/entropy.nit b/examples/rosettacode/entropy.nit
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+#!/usr/bin/env nit
+#
+# This file is part of NIT ( http://www.nitlanguage.org ).
+# This program is public domain
+
+# Task: Entropy
+# SEE: <http://rosettacode.org/wiki/Entropy>
+module entropy
+
+import counter
+
+# Basic implementation with a hashmap of chars to count them
+fun entropy(string: String): Float
+do
+       var cpt = new HashMap[Char, Int]
+       for char in string.chars do
+               var occ = cpt.get_or_default(char, 0)
+               cpt[char] = occ + 1
+       end
+
+       var len = string.length.to_f
+       var e = 0.0
+       for char, occ in cpt do
+               var freq = occ.to_f / len
+               e = e - freq * freq.log_base(2.0)
+       end
+       return e
+end
+print entropy("1223334444")
+
+# Alternative one-liner implementation using the `Counter::entropy` method of the library `counter`
+print "1223334444".chars.to_counter.entropy

diff --git a/tests/sav/entropy.res b/tests/sav/entropy.res
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+1.846
+1.846