lib/core/collection: add `CachedIterator` to factorize the logic of iterators with...

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

diff --git a/lib/core/collection/abstract_collection.nit b/lib/core/collection/abstract_collection.nit
index b51cd22..9e2179b 100644 (file)
--- a/lib/core/collection/abstract_collection.nit
+++ b/lib/core/collection/abstract_collection.nit
@@ -307,6 +307,45 @@ private class StepIterator[E]
        redef fun next_by(step) do real.next_by(step * self.step)
 end
 
+# An iterator that lazyly cache the current item.
+#
+# This class can be used as an helper to build simple iterator with a single and simplier `next_item` method.
+# The only constraint is that `next_item` returns null on the last item, so `null` cannot be a valid element.
+abstract class CachedIterator[E: Object]
+       super Iterator[E]
+
+       # Get the next item if any.
+       # Returns null if there is no next item.
+       fun next_item: nullable E is abstract
+
+       # The last item effectively read.
+       # `null` if on start, after a next of if no more items are available.
+       protected var cache: nullable E = null
+
+       # The current item, if any.
+       # If not, the cache is effectively filled (with `next_item`).
+       # Return `null` iff there is no more elements.
+       protected fun current_item: nullable E
+       do
+               var cache = self.cache
+               if cache != null then return cache
+               cache = next_item
+               self.cache = cache
+               return cache
+       end
+
+       redef fun item do return current_item.as(not null)
+
+       redef fun is_ok do return current_item != null
+
+       redef fun next do
+               # If needed, fill the cache (an consume the current element)
+               current_item
+               # Empty the cache (so the next element will be read)
+               cache = null
+       end
+end
+
 # A collection that contains only one item.
 #
 # Used to pass arguments by reference.