datatypes: add a forward iterator for LinkedList<T>, add forward and backward iterators for DoublyLinkedList<T>. Add tests for both.

vlib/datatypes/doubly_linked_list.v

@@ -7,6 +7,7 @@ mut:
 	prev &DoublyListNode<T> = unsafe { 0 }
 }
 
+// DoublyLinkedList<T> represents a generic doubly linked list of elements, each of type T.
 pub struct DoublyLinkedList<T> {
 mut:
 	head &DoublyListNode<T> = unsafe { 0 }
@@ -254,7 +255,7 @@ pub fn (list DoublyLinkedList<T>) str() string {
 
 // array returns a array representation of the linked list
 pub fn (list DoublyLinkedList<T>) array() []T {
-	mut result_array := []T{}
+	mut result_array := []T{cap: list.len}
 	mut node := list.head
 	for unsafe { node != 0 } {
 		result_array << node.data
@@ -264,7 +265,7 @@ pub fn (list DoublyLinkedList<T>) array() []T {
 }
 
 // next implements the iter interface to use DoublyLinkedList with
-// V's for loop syntax.
+// V's `for x in list {` loop syntax.
 pub fn (mut list DoublyLinkedList<T>) next() ?T {
 	if list.iter == unsafe { nil } {
 		// initialize new iter object
@@ -283,7 +284,58 @@ pub fn (mut list DoublyLinkedList<T>) next() ?T {
 	return list.iter.node.data
 }
 
-struct DoublyListIter<T> {
+// iterator returns a new iterator instance for the `list`.
+pub fn (mut list DoublyLinkedList<T>) iterator() DoublyListIter<T> {
+	return DoublyListIter<T>{
+		node: list.head
+	}
+}
+
+// back_iterator returns a new backwards iterator instance for the `list`.
+pub fn (mut list DoublyLinkedList<T>) back_iterator() DoublyListIterBack<T> {
+	return DoublyListIterBack<T>{
+		node: list.tail
+	}
+}
+
+// DoublyListIter<T> is an iterator for DoublyLinkedList.
+// It starts from *the start* and moves forwards to *the end* of the list.
+// It can be used with V's `for x in iter {` construct.
+// One list can have multiple independent iterators, pointing to different positions/places in the list.
+// A DoublyListIter iterator instance always traverses the list from *start to finish*.
+pub struct DoublyListIter<T> {
 mut:
 	node &DoublyListNode<T> = unsafe { 0 }
 }
+
+// next returns *the next* element of the list, or `none` when the end of the list is reached.
+// It is called by V's `for x in iter{` on each iteration.
+pub fn (mut iter DoublyListIter<T>) next() ?T {
+	if iter.node == unsafe { nil } {
+		return none
+	}
+	res := iter.node.data
+	iter.node = iter.node.next
+	return res
+}
+
+// DoublyListIterBack<T> is an iterator for DoublyLinkedList.
+// It starts from *the end* and moves backwards to *the start* of the list.
+// It can be used with V's `for x in iter {` construct.
+// One list can have multiple independent iterators, pointing to different positions/places in the list.
+// A DoublyListIterBack iterator instance always traverses the list from *finish to start*.
+pub struct DoublyListIterBack<T> {
+mut:
+	node &DoublyListNode<T> = unsafe { 0 }
+}
+
+// next returns *the previous* element of the list, or `none` when the start of the list is reached.
+// It is called by V's `for x in iter{` on each iteration.
+pub fn (mut iter DoublyListIterBack<T>) next() ?T {
+	if iter.node == unsafe { nil } {
+		return none
+	}
+	res := iter.node.data
+	iter.node = iter.node.prev
+	return res
+}

vlib/datatypes/doubly_linked_list_test.v

@@ -166,3 +166,49 @@ fn test_array() ? {
 	list.push_back(3)
 	assert list.array() == [1, 2, 3]
 }
+
+fn test_string_array() ? {
+	mut list := DoublyLinkedList<[]string>{}
+	list.push_back(['a'])
+	list.push_back(['b'])
+	list.push_back(['c'])
+	assert list.array() == [['a'], ['b'], ['c']]
+}
+
+fn test_iteration_with_for() ? {
+	mut list := DoublyLinkedList<int>{}
+	list.push_back(1)
+	list.push_back(2)
+	list.push_back(3)
+	mut res := []int{}
+	for x in list {
+		res << x
+	}
+	assert res == [1, 2, 3]
+}
+
+fn test_iterator() ? {
+	mut list := DoublyLinkedList<int>{}
+	list.push_back(1)
+	list.push_back(2)
+	list.push_back(3)
+	mut iter := list.iterator()
+	mut res := []int{}
+	for x in iter {
+		res << x
+	}
+	assert res == [1, 2, 3]
+}
+
+fn test_back_iterator() ? {
+	mut list := DoublyLinkedList<int>{}
+	list.push_back(1)
+	list.push_back(2)
+	list.push_back(3)
+	mut iter := list.back_iterator()
+	mut res := []int{}
+	for x in iter {
+		res << x
+	}
+	assert res == [3, 2, 1]
+}

vlib/datatypes/linked_list.v

@@ -10,6 +10,10 @@ pub struct LinkedList<T> {
 mut:
 	head &ListNode<T> = unsafe { 0 }
 	len  int
+	// Internal iter pointer for allowing safe modification
+	// of the list while iterating. TODO: use an option
+	// instead of a pointer to determine if it is initialized.
+	iter &ListIter<T> = unsafe { 0 }
 }
 
 // is_empty checks if the linked list is empty
@@ -152,7 +156,7 @@ pub fn (list LinkedList<T>) str() string {
 
 // array returns a array representation of the linked list
 pub fn (list LinkedList<T>) array() []T {
-	mut result_array := []T{}
+	mut result_array := []T{cap: list.len}
 	mut node := list.head
 	for unsafe { node != 0 } {
 		result_array << node.data
@@ -160,3 +164,50 @@ pub fn (list LinkedList<T>) array() []T {
 	}
 	return result_array
 }
+
+// next implements the iteration interface to use LinkedList
+// with V's `for` loop syntax.
+pub fn (mut list LinkedList<T>) next() ?T {
+	if list.iter == unsafe { nil } {
+		// initialize new iter object
+		list.iter = &ListIter<T>{
+			node: list.head
+		}
+		return list.next()
+	}
+	if list.iter.node == unsafe { nil } {
+		list.iter = unsafe { nil }
+		return none
+	}
+	defer {
+		list.iter.node = list.iter.node.next
+	}
+	return list.iter.node.data
+}
+
+// iterator returns a new iterator instance for the `list`.
+pub fn (mut list LinkedList<T>) iterator() ListIter<T> {
+	return ListIter<T>{
+		node: list.head
+	}
+}
+
+// ListIter<T> is an iterator for LinkedList.
+// It can be used with V's `for x in iter {` construct.
+// One list can have multiple independent iterators, pointing to different positions/places in the list.
+// An iterator instance always traverses the list from start to finish.
+pub struct ListIter<T> {
+mut:
+	node &ListNode<T> = unsafe { 0 }
+}
+
+// next returns the next element of the list, or `none` when the end of the list is reached.
+// It is called by V's `for x in iter{` on each iteration.
+pub fn (mut iter ListIter<T>) next() ?T {
+	if iter.node == unsafe { nil } {
+		return none
+	}
+	res := iter.node.data
+	iter.node = iter.node.next
+	return res
+}

vlib/datatypes/linked_list_test.v

@@ -120,3 +120,51 @@ fn test_array() ? {
 	list.push(3)
 	assert list.array() == [1, 2, 3]
 }
+
+fn test_linked_list_iterating_with_for() ? {
+	mut list := LinkedList<int>{}
+	list.push(1)
+	list.push(2)
+	list.push(3)
+	mut res := []int{}
+	for x in list {
+		res << x
+	}
+	assert res == [1, 2, 3]
+}
+
+fn test_linked_list_separate_iterators() ? {
+	mut list := LinkedList<int>{}
+	list.push(1)
+	list.push(2)
+	list.push(3)
+	mut it1 := list.iterator()
+	mut it2 := list.iterator()
+	mut it3 := list.iterator()
+	assert it1.next()? == 1
+	assert it1.next()? == 2
+	assert it1.next()? == 3
+	assert it2.next()? == 1
+	if _ := it1.next() {
+		assert false
+	} else {
+		assert true
+	}
+	if _ := it1.next() {
+		assert false
+	} else {
+		assert true
+	}
+	assert it2.next()? == 2
+	assert it2.next()? == 3
+	if _ := it2.next() {
+		assert false
+	} else {
+		assert true
+	}
+	mut res := []int{}
+	for x in it3 {
+		res << x
+	}
+	assert res == [1, 2, 3]
+}