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// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This example demonstrates a priority queue built using the heap interface.
package heap_test
import (
"container/heap"
"fmt"
)
// An Item is something we manage in a priority queue.
type Item struct {
value string // The value of the item; arbitrary.
priority int // The priority of the item in the queue.
// The index is needed by changePriority 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
func (pq PriorityQueue) Len() int { return len(pq) }
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].priority > pq[j].priority
}
func (pq PriorityQueue) Swap(i, j int) {
pq[i], pq[j] = pq[j], pq[i]
pq[i].index = i
pq[j].index = j
}
func (pq *PriorityQueue) Push(x interface{}) {
// Push and Pop use pointer receivers because they modify the slice's length,
// not just its contents.
// To simplify indexing expressions in these methods, we save a copy of the
// slice object. We could instead write (*pq)[i].
a := *pq
n := len(a)
a = a[0 : n+1]
item := x.(*Item)
item.index = n
a[n] = item
*pq = a
}
func (pq *PriorityQueue) Pop() interface{} {
a := *pq
n := len(a)
item := a[n-1]
item.index = -1 // for safety
*pq = a[0 : n-1]
return item
}
// update is not used by the example but shows how to take the top item from
// the queue, update its priority and value, and put it back.
func (pq *PriorityQueue) update(value string, priority int) {
item := heap.Pop(pq).(*Item)
item.value = value
item.priority = priority
heap.Push(pq, item)
}
// changePriority is not used by the example but shows how to change the
// priority of an arbitrary item.
func (pq *PriorityQueue) changePriority(item *Item, priority int) {
heap.Remove(pq, item.index)
item.priority = priority
heap.Push(pq, item)
}
// This example pushes 10 items into a PriorityQueue and takes them out in
// order of priority.
func Example() {
const nItem = 10
// Random priorities for the items (a permutation of 0..9, times 11)).
priorities := [nItem]int{
77, 22, 44, 55, 11, 88, 33, 99, 00, 66,
}
values := [nItem]string{
"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine",
}
// Create a priority queue and put some items in it.
pq := make(PriorityQueue, 0, nItem)
for i := 0; i < cap(pq); i++ {
item := &Item{
value: values[i],
priority: priorities[i],
}
heap.Push(&pq, item)
}
// Take the items out; should arrive in decreasing priority order.
// For example, the highest priority (99) is the seventh item, so output starts with 99:"seven".
for i := 0; i < nItem; i++ {
item := heap.Pop(&pq).(*Item)
fmt.Printf("%.2d:%s ", item.priority, item.value)
}
// Output:
// 99:seven 88:five 77:zero 66:nine 55:three 44:two 33:six 22:one 11:four 00:eight
}
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