Add initial implementation of priority queue, uses container/heap

pkg/storage/pq/pq.go

@@ -0,0 +1,70 @@
+/*
+ * Minimalist Object Storage, (C) 2015 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package pq
+
+import "container/heap"
+
+// Item container for tasks in priority queue
+type Item struct {
+	task Task // task
+
+	// The index is needed by Fix and is maintained by the heap.Interface methods.
+	index int // The index of the item in the heap.
+}
+
+// A PriorityQueue implements heap.Interface and holds Items.
+type PriorityQueue []*Item
+
+// Len length of current priority queue
+func (pq PriorityQueue) Len() int { return len(pq) }
+
+// Less used internally by heap.Interface to arrange items in order
+func (pq PriorityQueue) Less(i, j int) bool {
+	// We want Pop to give us the highest, not lowest, priority so we use greater than here.
+	return pq[i].task.GetPriority() > pq[j].task.GetPriority()
+}
+
+// Swap used internally by heap.Interface to arrange incoming items
+func (pq PriorityQueue) Swap(i, j int) {
+	pq[i], pq[j] = pq[j], pq[i]
+	pq[i].index = i
+	pq[j].index = j
+}
+
+// Push push items onto priority queue
+func (pq *PriorityQueue) Push(x interface{}) {
+	n := len(*pq)
+	item := x.(*Item)
+	item.index = n
+	*pq = append(*pq, item)
+}
+
+// Pop pop items with highest priority
+func (pq *PriorityQueue) Pop() interface{} {
+	old := *pq
+	n := len(old)
+	item := old[n-1]
+	item.index = -1 // for safety
+	*pq = old[0 : n-1]
+	return item
+}
+
+// Fix modifies an item in-place on the queue
+func (pq *PriorityQueue) Fix(item *Item, task Task) {
+	item.task = task
+	heap.Fix(pq, item.index)
+}