1 files changed, 115 insertions, 0 deletions
diff --git a/deque.go b/deque.go
new file mode 100644
index 0000000..0001f68
--- /dev/null
+++ b/deque.go
@@ -0,0 +1,115 @@
+package share
+
+// Deque is a double-ended queue with an unlimited capacity.
+type Deque[T any] struct {
+	// Sending to PutTail adds an element to the back of the deque
+	// and never blocks.
+	PutTail chan<- T
+
+	// Sending to PutHead adds an element to the front of the deque
+	// and never blocks.
+	PutHead chan<- T
+
+	// Receiving from TakeHead removes an element from the front
+	// of the deque, or, if the queue is empty, blocks until an element
+	// is enqueued.
+	TakeHead <-chan T
+
+	// Receiving from TakeTail removes an element from the back
+	// of the deque, or, if the deque is empty, blocks until an element
+	// is enqueued.
+	TakeTail <-chan T
+}
+
+func NewDeque[T any]() Deque[T] {
+	putTail, putHead := make(chan T), make(chan T)
+	takeHead, takeTail := make(chan T), make(chan T)
+
+	go run(putTail, putHead, takeHead, takeTail)
+
+	return Deque[T]{
+		PutTail:  putTail,
+		PutHead:  putHead,
+		TakeHead: takeHead,
+		TakeTail: takeTail,
+	}
+}
+
+func run[T any](putTail, putHead <-chan T, takeHead, takeTail chan<- T) {
+	defer close(takeTail)
+	defer close(takeHead)
+
+	var buf []T
+
+	// While the Put channels are open,
+	for ok := true; ok; {
+		if len(buf) > 0 {
+			buf, ok = putOrTake(putTail, putHead, takeHead, takeTail, buf)
+		} else {
+			buf, ok = put(putTail, putHead, buf)
+		}
+	}
+
+	flush(takeHead, takeTail, buf)
+}
+
+func flush[T any](takeHead, takeTail chan<- T, buf []T) {
+	for len(buf) > 0 {
+		select {
+		case takeHead <- buf[0]:
+			buf = buf[1:]
+		case takeTail <- buf[len(buf)-1]:
+			buf = buf[:len(buf)-1]
+		}
+	}
+}
+
+func putOrTake[T any](putTail, putHead <-chan T, takeHead, takeTail chan<- T, buf []T) ([]T, bool) {
+	select {
+	case takeHead <- buf[0]:
+		buf = buf[1:]
+
+	case takeTail <- buf[len(buf)-1]:
+		buf = buf[:len(buf)-1]
+
+	case v, ok := <-putTail:
+		if !ok {
+			return buf, false
+		}
+		buf = append(buf, v)
+
+	case v, ok := <-putHead:
+		if !ok {
+			return buf, false
+		}
+		buf = append([]T{v}, buf...)
+	}
+
+	return buf, true
+}
+
+func put[T any](putTail, putHead <-chan T, buf []T) ([]T, bool) {
+	var v T
+	var ok bool
+
+	select {
+	case v, ok = <-putTail:
+		if ok {
+			buf = append(buf, v)
+		}
+	case v, ok = <-putHead:
+		if ok {
+			buf = append([]T{v}, buf...)
+		}
+	}
+
+	return buf, ok
+}
+
+// Close the Put channels of the deque.
+// The deque will wait until all elements have been drained through
+// either of the Take channels before closing them.
+func (dq Deque[T]) Close() {
+	close(dq.PutTail)
+	close(dq.PutHead)
+}