queue: allow Peek to handle negative offsets

queue/queue.go

@@ -99,10 +99,12 @@ func (q *Queue[T]) Front() T {
 
 // Peek reports whether q has a value at offset n from the front of the queue,
 // and if so returns its value. Peek(0) returns the same value as Front.
+// Negative offsets count forward from the end of the queue.
 func (q *Queue[T]) Peek(n int) (T, bool) {
 	if n < 0 {
-		panic("index out of range")
-	} else if n >= q.n {
+		n += q.n
+	}
+	if n < 0 || n >= q.n {
 		var zero T
 		return zero, false
 	}

queue/queue_test.go

@@ -51,16 +51,24 @@ func TestQueue(t *testing.T) {
 		t.Errorf("Front: got %v, want 1", front)
 	}
 
-	// Make sure we can peek all the locations.
+	// Make sure we can peek all the locations, both positive and negative.
 	for i, want := range []int{1, 2, 3} {
 		if v, ok := q.Peek(i); !ok || v != want {
-			t.Errorf("Peek(0): got (%v, %v), want (%v, true)", v, ok, want)
+			t.Errorf("Peek(%d): got (%v, %v), want (%v, true)", i, v, ok, want)
+		}
+	}
+	for i, want := range []int{3, 2, 1} {
+		pos := -(i + 1)
+		if v, ok := q.Peek(pos); !ok || v != want {
+			t.Errorf("Peek(%d): got (%v, %v), want (%v, true)", pos, v, ok, want)
 		}
 	}
 
-	// Peek off the end should return 0, false.
-	if v, ok := q.Peek(10); ok || v != 0 {
-		t.Errorf("Peek(10): got (%v, %v), want (0, false)", v, ok)
+	// Peek off the end (in either direction) should return 0, false.
+	for _, pos := range []int{-10, -4, 5, 9} {
+		if v, ok := q.Peek(pos); ok || v != 0 {
+			t.Errorf("Peek(%d): got (%v, %v), want (0, false)", pos, v, ok)
+		}
 	}
 
 	// Pop should work in order.
@@ -82,6 +90,7 @@ func TestQueue(t *testing.T) {
 	q.Push(0)
 	check(0, 1, 2, 3, 4)
 
+	// PopLast should work in reverse order.
 	for _, want := range []int{4, 3, 2} {
 		if v, ok := q.PopLast(); !ok || v != want {
 			t.Errorf("PopLast: got (%v, %v), want (%v, true)", v, ok, want)