Implementation and test of Enumerable#with_position

lib/core/facets/enumerable/with_position.rb

@@ -0,0 +1,58 @@
+
+module Enumerable
+	class Position
+		def initialize
+			@first = true
+			@last = false
+		end
+
+		def after_first!
+			@first = false
+		end
+
+		def at_end!
+			@last = true
+		end
+
+		def first?
+			@first
+		end
+
+		def middle?
+			not (@first or @last)
+		end
+
+		def last?
+			@last
+		end
+	end
+
+	# Allows one to know the position of the enumeration easily.
+	#
+	#    5.times.collect.with_position do |i, position|
+	#        $stdout.write "#{i}#{position.middle? ? ',' : ''}"
+	#    end
+	#
+	# Where this becomes more useful, is when it's easy to determine the position of elements, e.g.
+	#
+	#    10.times.select(&:odd?).with_position{|n,p| ... }
+	#
+	def with_position
+		return to_enum(:with_position) unless block_given?
+
+		position = Position.new
+
+		while item = self.next
+			begin
+				self.peek
+			rescue StopIteration
+				position.at_end!
+			end
+
+			yield item, position.dup
+
+			position.after_first!
+		end
+	rescue StopIteration
+	end
+end

test/core/enumerable/test_with_position.rb

@@ -0,0 +1,42 @@
+covers 'facets/enumerable/with_position'
+
+test_case Enumerable do
+
+  method :with_position do
+
+    test do
+      a = 5.times.with_position.to_a
+
+      item, position = a.first
+      item.assert == 0
+      assert position.first?
+      refute position.middle?
+      refute position.last?
+
+
+      item, position = a[2]
+      item.assert == 2
+      assert position.middle?
+      refute position.first?
+      refute position.last?
+
+      item, position = a.last
+      item.assert == 4
+      refute position.first?
+      refute position.middle?
+      assert position.last?
+    end
+
+    test do
+      a = 1.times.with_position.to_a
+
+      item, position = a.first
+      item.assert == 0
+      assert position.first?
+      refute position.middle?
+      assert position.last?
+    end
+
+  end
+
+end