From 56282db30afb831635fa3b6b08eafc9f02a8d30c Mon Sep 17 00:00:00 2001
From: Andras Banki-Horvath <bhandras@gmail.com>
Date: Fri, 22 Nov 2019 16:08:56 +0100
Subject: [PATCH] queue: Introducing a general purpose priority queue.

This commit introduces PriorityQueue, which is a general, heap
based priority queue, and PriorityQueueItem which is an interface
that concrete priority queue items must implement.
This implementation is encapsulated, users do not need to use any
other package for full functionality.
PriorityQueue exports the usual public methids: Push, Pop, Top,
Empty and Len. For full documentaton consult the priority_queue.go,
for usage: priority_queue_test.go
---
 queue/priority_queue.go      | 74 ++++++++++++++++++++++++++++++++++++
 queue/priority_queue_test.go | 67 ++++++++++++++++++++++++++++++++
 2 files changed, 141 insertions(+)
 create mode 100644 queue/priority_queue.go
 create mode 100644 queue/priority_queue_test.go

diff --git a/queue/priority_queue.go b/queue/priority_queue.go
new file mode 100644
index 00000000..aae7b423
--- /dev/null
+++ b/queue/priority_queue.go
@@ -0,0 +1,74 @@
+package queue
+
+import (
+	"container/heap"
+)
+
+// PriorityQueueItem is an interface that represents items in a PriorityQueue.
+// Users of PriorityQueue will need to define a Less function such that
+// PriorityQueue will be able to use that to build and restore an underlying
+// heap.
+type PriorityQueueItem interface {
+	Less(other PriorityQueueItem) bool
+}
+
+type priorityQueue []PriorityQueueItem
+
+// Len returns the length of the priorityQueue.
+func (pq priorityQueue) Len() int { return len(pq) }
+
+// Less is used to order PriorityQueueItem items in the queue.
+func (pq priorityQueue) Less(i, j int) bool {
+	return pq[i].Less(pq[j])
+}
+
+// Swap swaps two items in the priorityQueue. Swap is used by heap.Interface.
+func (pq priorityQueue) Swap(i, j int) {
+	pq[i], pq[j] = pq[j], pq[i]
+}
+
+// Push adds a new item the the priorityQueue.
+func (pq *priorityQueue) Push(x interface{}) {
+	item := x.(PriorityQueueItem)
+	*pq = append(*pq, item)
+}
+
+// Pop removes the top item from the priorityQueue.
+func (pq *priorityQueue) Pop() interface{} {
+	old := *pq
+	n := len(old)
+	item := old[n-1]
+	old[n-1] = nil
+	*pq = old[0 : n-1]
+	return item
+}
+
+// Priority wrap a standard heap in a more object-oriented structure.
+type PriorityQueue struct {
+	queue priorityQueue
+}
+
+// Len returns the length of the queue.
+func (pq *PriorityQueue) Len() int {
+	return len(pq.queue)
+}
+
+// Empty returns true if the queue is empty.
+func (pq *PriorityQueue) Empty() bool {
+	return len(pq.queue) == 0
+}
+
+// Push adds an item to the priority queue.
+func (pq *PriorityQueue) Push(item PriorityQueueItem) {
+	heap.Push(&pq.queue, item)
+}
+
+// Pop removes the top most item from the queue.
+func (pq *PriorityQueue) Pop() PriorityQueueItem {
+	return heap.Pop(&pq.queue).(PriorityQueueItem)
+}
+
+// Top returns the top most item from the queue without removing it.
+func (pq *PriorityQueue) Top() PriorityQueueItem {
+	return pq.queue[0]
+}
diff --git a/queue/priority_queue_test.go b/queue/priority_queue_test.go
new file mode 100644
index 00000000..be1209e0
--- /dev/null
+++ b/queue/priority_queue_test.go
@@ -0,0 +1,67 @@
+package queue
+
+import (
+	"math/rand"
+	"testing"
+	"time"
+)
+
+type testQueueItem struct {
+	Value  int
+	Expiry time.Time
+}
+
+func (e testQueueItem) Less(other PriorityQueueItem) bool {
+	return e.Expiry.Before(other.(*testQueueItem).Expiry)
+}
+
+func TestExpiryQueue(t *testing.T) {
+	// The number of elements we push to the queue.
+	count := 100
+	// Generate a random permutation of a range [0, count)
+	array := rand.Perm(count)
+	// t0 holds a reference time point.
+	t0 := time.Date(1975, time.April, 5, 12, 0, 0, 0, time.UTC)
+
+	var testQueue PriorityQueue
+
+	if testQueue.Len() != 0 && !testQueue.Empty() {
+		t.Fatal("Expected the queue to be empty")
+	}
+
+	// Create elements with expiry of t0 + value * second.
+	for _, value := range array {
+		testQueue.Push(&testQueueItem{
+			Value:  value,
+			Expiry: t0.Add(time.Duration(value) * time.Second),
+		})
+	}
+
+	// Now expect that we can retrieve elements in order of their expiry.
+	for i := 0; i < count; i++ {
+		expectedQueueLen := count - i
+		if testQueue.Len() != expectedQueueLen {
+			t.Fatalf("Expected the queue len %v, got %v",
+				expectedQueueLen, testQueue.Len())
+		}
+
+		if testQueue.Empty() {
+			t.Fatalf("Did not expect the queue to be empty")
+		}
+
+		top := testQueue.Top().(*testQueueItem)
+		if top.Value != i {
+			t.Fatalf("Expected queue top %v, got %v", i, top.Value)
+		}
+
+		popped := testQueue.Pop().(*testQueueItem)
+		if popped != top {
+			t.Fatalf("Expected queue top %v equal to popped: %v",
+				top, popped)
+		}
+	}
+
+	if testQueue.Len() != 0 || !testQueue.Empty() {
+		t.Fatalf("Expected the queue to be empty")
+	}
+}