Reverse iterator

README.md

@@ -28,6 +28,8 @@ Implementation of various data structures and algorithms in Go.
     - [Iterator](#iterator)
       - [IteratorWithIndex](#iteratorwithindex)
       - [IteratorWithKey](#iteratorwithkey)
+      - [ReverseIteratorWithIndex](#reverseiteratorwithindex)
+      - [ReverseIteratorWithKey](#reverseiteratorwithkey)
     - [Enumerable](#enumerable)
       - [EnumerableWithIndex](#enumerablewithindex)
       - [EnumerableWithKey](#enumerablewithkey)
@@ -53,17 +55,18 @@ Containers are either ordered or unordered. All ordered containers provide [stat
 
 | Container | Ordered | [Iterator](#iterator) | [Enumerable](#enumerable) | Ordered by |
 | :--- | :---: | :---: | :---: | :---: |
-| [ArrayList](#arraylist) | yes | yes | yes | index |
+| [ArrayList](#arraylist) | yes | yes* | yes | index |
 | [SinglyLinkedList](#singlylinkedlist) | yes | yes | yes | index |
-| [DoublyLinkedList](#doublylinkedlist) | yes | yes | yes | index |
+| [DoublyLinkedList](#doublylinkedlist) | yes | yes* | yes | index |
 | [HashSet](#hashset) | no | no | no | index |
-| [TreeSet](#treeset) | yes | yes | yes | index |
+| [TreeSet](#treeset) | yes | yes* | yes | index |
 | [LinkedListStack](#linkedliststack) | yes | yes | no | index |
-| [ArrayStack](#arraystack) | yes | yes | no | index |
+| [ArrayStack](#arraystack) | yes | yes* | no | index |
 | [HashMap](#hashmap) | no | no | no | key |
-| [TreeMap](#treemap) | yes | yes | yes | key |
-| [RedBlackTree](#redblacktree) | yes | yes | no | key |
-| [BinaryHeap](#binaryheap) | yes | yes | no | index |
+| [TreeMap](#treemap) | yes | yes* | yes | key |
+| [RedBlackTree](#redblacktree) | yes | yes* | no | key |
+| [BinaryHeap](#binaryheap) | yes | yes* | no | index |
+|  |  | <sub><sup>*reversible</sup></sub> |  |  |
 
 ### Lists
 
@@ -572,12 +575,12 @@ Some data structures (e.g. TreeMap, TreeSet) require a comparator function to au
 
 Comparator is defined as:
 
-Return values:
+Return values (int):
 
 ```go
--1, if a < b
- 0, if a == b
- 1, if a > b
+negative , if a < b
+zero     , if a == b
+positive , if a > b
 ```
 
 Comparator signature:
@@ -642,7 +645,7 @@ func main() {
 
 ### Iterator
 
-All ordered containers have stateful iterators. Typically an iterator is obtained by _Iterator()_ function of an ordered container. Once obtained, iterator's _Next()_ function moves the iterator to the next element and returns true if there was a next element. If there was an element, then element's can be obtained by iterator's _Value()_ function. Depending on the ordering type, it's position can be obtained by iterator's _Index()_ or _Key()_ functions.
+All ordered containers have stateful iterators. Typically an iterator is obtained by _Iterator()_ function of an ordered container. Once obtained, iterator's _Next()_ function moves the iterator to the next element and returns true if there was a next element. If there was an element, then element's can be obtained by iterator's _Value()_ function. Depending on the ordering type, it's position can be obtained by iterator's _Index()_ or _Key()_ functions. Some containers even provide reversible iterators, essentially the same, but provide another extra _Prev()_ function that moves the iterator to the previous element and returns true if there was a previous element.
 
 #### IteratorWithIndex
 
@@ -668,6 +671,32 @@ for it.Next() {
 }
 ```
 
+#### ReverseIteratorWithIndex
+
+A [iterator](#iterator) whose elements are referenced by an index. Typical usage:
+
+```go
+it := list.Iterator()
+for it.Next() { /* Move to end */ }
+for it.Prev() {
+	index, value := it.Index(), it.Value()
+    ...
+}
+```
+
+#### ReverseIteratorWithKey
+
+A [iterator](#iterator) whose elements are referenced by a key. Typical usage:
+
+```go
+it := map.Iterator()
+for it.Next() { /* Move to end */ }
+for it.Prev() {
+	key, value := it.Key(), it.Value()
+    ...
+}
+```
+
 ### Enumerable
 
 Enumerable functions for ordered containers that implement [EnumerableWithIndex](#enumerablewithindex) or [EnumerableWithKey](#enumerablewithkey) interfaces.

containers/containers.go

@@ -24,11 +24,18 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
+// Package containers provides core interfaces and functions for data structures.
+//
+// Container is the base interface for all data structures to implement.
+//
+// Iterators provide stateful iterators.
+//
+// Enumerable provides Ruby inspired (each, select, map, find, any?, etc.) container functions.
 package containers
 
 import "github.com/emirpasic/gods/utils"
 
-// Container is base interface that all data structures implement
+// Container is base interface that all data structures implement.
 type Container interface {
 	Empty() bool
 	Size() int

containers/enumerable.go

@@ -24,9 +24,6 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Enumerable functions for ordered containers.
-// Ruby's enumerable inspired package.
-
 package containers
 
 // EnumerableWithIndex provides functions for ordered containers whose values can be fetched by an index.

containers/iterator.go

@@ -24,8 +24,6 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Stateful iterator pattern for ordered containers.
-
 package containers
 
 // IteratorWithIndex is stateful iterator for ordered containers whose values can be fetched by an index.
@@ -55,3 +53,33 @@ type IteratorWithKey interface {
 	// Does not modify the state of the iterator.
 	Key() interface{}
 }
+
+// ReverseIteratorWithIndex is stateful iterator for ordered containers whose values can be fetched by an index.
+//
+// Essentially it is the same as IteratorWithIndex, but provides additional Prev() function to enable traversal in reverse.
+type ReverseIteratorWithIndex interface {
+	// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+	// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+	// Modifies the state of the iterator.
+	Prev() bool
+
+	IteratorWithIndex
+	// Next() bool
+	// Value() interface{}
+	// Index() int
+}
+
+// ReverseIteratorWithKey is a stateful iterator for ordered containers whose elements are key value pairs.
+//
+// Essentially it is the same as IteratorWithKey, but provides additional Prev() function to enable traversal in reverse.
+type ReverseIteratorWithKey interface {
+	// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+	// If Prev() returns true, then previous element's index and value can be retrieved by Key() and Value().
+	// Modifies the state of the iterator.
+	Prev() bool
+
+	IteratorWithKey
+	// Next() bool
+	// Value() interface{}
+	// Key() interface{}
+}

lists/arraylist/arraylist.go

@@ -24,10 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of list using a slice.
+// Package arraylist implements the array list.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/List_%28abstract_data_type%29
-
+//
+// Reference: https://en.wikipedia.org/wiki/List_%28abstract_data_type%29
 package arraylist
 
 import (
@@ -41,7 +42,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ lists.List = (*List)(nil)
 	var _ containers.EnumerableWithIndex = (*List)(nil)
-	var _ containers.IteratorWithIndex = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithIndex = (*Iterator)(nil)
 }
 
 // List holds the elements in a slice
@@ -194,7 +195,19 @@ func (list *List) Iterator() Iterator {
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.list.size {
+		iterator.index++
+	}
+	return iterator.list.withinRange(iterator.index)
+}
+
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	if iterator.index >= 0 {
+		iterator.index--
+	}
 	return iterator.list.withinRange(iterator.index)
 }
 

lists/arraylist/arraylist_test.go

@@ -299,11 +299,21 @@ func TestListChaining(t *testing.T) {
 	}
 }
 
-func TestListIterator(t *testing.T) {
+func TestListIteratorNextOnEmpty(t *testing.T) {
+	list := New()
+	it := list.Iterator()
+	for it.Next() {
+		t.Errorf("Shouldn't iterate on empty list")
+	}
+}
+
+func TestListIteratorNext(t *testing.T) {
 	list := New()
 	list.Add("a", "b", "c")
 	it := list.Iterator()
+	count := 0
 	for it.Next() {
+		count++
 		index := it.Index()
 		value := it.Value()
 		switch index {
@@ -323,13 +333,52 @@ func TestListIterator(t *testing.T) {
 			t.Errorf("Too many")
 		}
 	}
-	list.Clear()
-	it = list.Iterator()
-	for it.Next() {
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
+	}
+}
+
+func TestListIteratorPrevOnEmpty(t *testing.T) {
+	list := New()
+	it := list.Iterator()
+	for it.Prev() {
 		t.Errorf("Shouldn't iterate on empty list")
 	}
 }
 
+func TestListIteratorPrev(t *testing.T) {
+	list := New()
+	list.Add("a", "b", "c")
+	it := list.Iterator()
+	for it.Next() {
+	}
+	count := 0
+	for it.Prev() {
+		count++
+		index := it.Index()
+		value := it.Value()
+		switch index {
+		case 0:
+			if actualValue, expectedValue := value, "a"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 1:
+			if actualValue, expectedValue := value, "b"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		case 2:
+			if actualValue, expectedValue := value, "c"; actualValue != expectedValue {
+				t.Errorf("Got %v expected %v", actualValue, expectedValue)
+			}
+		default:
+			t.Errorf("Too many")
+		}
+	}
+	if actualValue, expectedValue := count, 3; actualValue != expectedValue {
+		t.Errorf("Got %v expected %v", actualValue, expectedValue)
+	}
+}
+
 func BenchmarkList(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		list := New()

lists/doublylinkedlist/doublylinkedlist.go

@@ -24,10 +24,11 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-// Implementation of doubly linked list.
+// Package doublylinkedlist implements the doubly-linked list.
+//
 // Structure is not thread safe.
-// References: http://en.wikipedia.org/wiki/Doubly_linked_list
-
+//
+// Reference: https://en.wikipedia.org/wiki/List_%28abstract_data_type%29
 package doublylinkedlist
 
 import (
@@ -41,7 +42,7 @@ import (
 func assertInterfaceImplementation() {
 	var _ lists.List = (*List)(nil)
 	var _ containers.EnumerableWithIndex = (*List)(nil)
-	var _ containers.IteratorWithIndex = (*Iterator)(nil)
+	var _ containers.ReverseIteratorWithIndex = (*Iterator)(nil)
 }
 
 // List holds the elements, where each element points to the next and previous element
@@ -315,19 +316,40 @@ func (list *List) Iterator() Iterator {
 // If Next() returns true, then next element's index and value can be retrieved by Index() and Value().
 // Modifies the state of the iterator.
 func (iterator *Iterator) Next() bool {
-	iterator.index++
+	if iterator.index < iterator.list.size {
+		iterator.index++
+	}
 	if !iterator.list.withinRange(iterator.index) {
 		iterator.element = nil
 		return false
 	}
-	if iterator.element != nil {
+	if iterator.index != 0 {
 		iterator.element = iterator.element.next
 	} else {
 		iterator.element = iterator.list.first
 	}
 	return true
 }
 
+// Prev moves the iterator to the previous element and returns true if there was a previous element in the container.
+// If Prev() returns true, then previous element's index and value can be retrieved by Index() and Value().
+// Modifies the state of the iterator.
+func (iterator *Iterator) Prev() bool {
+	if iterator.index >= 0 {
+		iterator.index--
+	}
+	if !iterator.list.withinRange(iterator.index) {
+		iterator.element = nil
+		return false
+	}
+	if iterator.index == iterator.list.size-1 {
+		iterator.element = iterator.list.last
+	} else {
+		iterator.element = iterator.element.prev
+	}
+	return iterator.list.withinRange(iterator.index)
+}
+
 // Value returns the current element's value.
 // Does not modify the state of the iterator.
 func (iterator *Iterator) Value() interface{} {