add Peek() method

pq.go

@@ -7,8 +7,10 @@ import "container/heap"
 type PQ interface {
 	// Push adds the ele
 	Push(Elem)
-	// Pop returns the highest priority Elem in PQ.
+	// Pop removes and returns the highest priority Elem in PQ.
 	Pop() Elem
+	// Peek returns the highest priority Elem in PQ (without removing it).
+	Peek() Elem
 	// Len returns the number of elements in the PQ.
 	Len() int
 	// Update `fixes` the PQ.
@@ -56,6 +58,13 @@ func (w *wrapper) Pop() Elem {
 	return heap.Pop(&w.heapinterface).(Elem)
 }
 
+func (w *wrapper) Peek() Elem {
+	if len(w.heapinterface.elems) == 0 {
+		return nil
+	}
+	return w.heapinterface.elems[0].(Elem)
+}
+
 func (w *wrapper) Update(index int) {
 	heap.Fix(&w.heapinterface, index)
 }

pq_test.go

@@ -51,14 +51,19 @@ func TestCorrectnessOfPop(t *testing.T) {
 		q.Push(&e)
 	}
 	var priorities []int
+	var peekPriorities []int
 	for q.Len() > 0 {
 		i := q.Pop().(*TestElem).Priority
 		t.Logf("popped %v", i)
 		priorities = append(priorities, i)
+		peekPriorities = append(peekPriorities, i)
 	}
-	if !sort.IntsAreSorted(priorities) {
+	if !sort.IntsAreSorted(peekPriorities) {
 		t.Fatal("the values were not returned in sorted order")
 	}
+	if !sort.IntsAreSorted(priorities) {
+		t.Fatal("the popped values were not returned in sorted order")
+	}
 }
 
 func TestUpdate(t *testing.T) {
@@ -73,12 +78,18 @@ func TestUpdate(t *testing.T) {
 	q.Push(middle)
 	q.Push(highest)
 	q.Push(lowest)
+	if q.Peek().(*TestElem).Key != highest.Key {
+		t.Fatal("head element doesn't have the highest priority")
+	}
 	if q.Pop().(*TestElem).Key != highest.Key {
 		t.Fatal("popped element doesn't have the highest priority")
 	}
 	q.Push(highest)           // re-add the popped element
 	highest.Priority = 0      // update the PQ
 	q.Update(highest.Index()) // fix the PQ
+	if q.Peek().(*TestElem).Key != middle.Key {
+		t.Fatal("middle element should now have the highest priority")
+	}
 	if q.Pop().(*TestElem).Key != middle.Key {
 		t.Fatal("middle element should now have the highest priority")
 	}