standard/collection: add `to_step` as a decorator of iterators

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

diff --git a/lib/standard/collection/abstract_collection.nit b/lib/standard/collection/abstract_collection.nit
index c33416f..370305f 100644 (file)
--- a/lib/standard/collection/abstract_collection.nit
+++ b/lib/standard/collection/abstract_collection.nit
@@ -237,6 +237,43 @@ interface Iterator[E]
        #
        # Do nothing by default.
        fun finish do end
+
+       # A decorator around `self` that advance self a given number of steps instead of one.
+       #
+       # ~~~
+       # var i = [11, 22, 33, 44, 55].iterator
+       # var i2 = i.to_step(2)
+       #
+       # assert i2.item == 11
+       # i2.next
+       # assert i2.item == 33
+       #
+       # assert i.item == 33
+       # ~~~
+       fun to_step(step: Int): Iterator[E] do return new StepIterator[E](self, step)
+end
+
+# A basic helper class to specialize specific Iterator decorators
+abstract class IteratorDecorator[E]
+       super Iterator[E]
+
+       # The underling iterator
+       protected var real: Iterator[E]
+
+       redef fun is_ok do return real.is_ok
+       redef fun item do return real.item
+       redef fun finish do real.finish
+       redef fun next do real.next
+       redef fun next_by(step) do real.next_by(step)
+end
+
+# A decorator that advance a given number of steps
+private class StepIterator[E]
+       super IteratorDecorator[E]
+       var step: Int
+
+       redef fun next do real.next_by(step)
+       redef fun next_by(step) do real.next_by(step * self.step)
 end
 
 # A collection that contains only one item.