X-Git-Url: http://nitlanguage.org

diff --git a/lib/counter.nit b/lib/counter.nit
index 6545b73..644c115 100644
--- a/lib/counter.nit
+++ b/lib/counter.nit
@@ -53,14 +53,14 @@ class Counter[E]
 	redef fun iterator do return map.iterator
 
 	# The number of counted occurrences of `e`
-	redef fun [](e: E): Int
+	redef fun [](e)
 	do
 		var map = self.map
 		if map.has_key(e) then return map[e]
 		return 0
 	end
 
-	redef fun []=(e: E, value: Int)
+	redef fun []=(e, value)
 	do
 		sum -= self[e]
 		self.map[e] = value
@@ -93,6 +93,22 @@ class Counter[E]
 		for e in es do inc(e)
 	end
 
+	# Decrement the value of `e` by 1
+	fun dec(e: E) do
+		if not has_key(e) then
+			self.map[e] = 0
+		else
+			self.map[e] = self[e] - 1
+			sum += - 1
+		end
+	end
+
+	# Decrement the value for each element of `es`
+	fun dec_all(es: Collection[E])
+	do
+		for e in es do dec(e)
+	end
+
 	# A new Counter initialized with `inc_all`.
 	init from(es: Collection[E])
 	do
@@ -238,6 +254,36 @@ class Counter[E]
 		end
 		return (sum / map.length.to_f).sqrt
 	end
+
+	# The information entropy (Shannon entropy) of the elements in the counter (in bits).
+	fun entropy: Float
+	do
+		var res = 0.0
+		var sum = self.sum.to_f
+		for k, v in self do
+			var f = v.to_f / sum
+			res = res - f * f.log_base(2.0)
+		end
+		return res
+	end
+end
+
+redef class Collection[E]
+	# Create and fill up a counter with the elements of `self.
+	#
+	# ~~~
+	# var cpt = "abaa".chars.to_counter
+	# assert cpt['a'] == 3
+	# assert cpt['b'] == 1
+	# assert cpt.length == 2
+	# assert cpt.sum == 4
+	# ~~~
+	fun to_counter: Counter[E]
+	do
+		var res = new Counter[E]
+		res.inc_all(self)
+		return res
+	end
 end
 
 private class CounterComparator[E]