# 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