Package heap

This example inserts several ints into an IntHeap, checks the minimum, and removes them in order of priority.

Code:

// This example demonstrates an integer heap built using the heap interface.
package heap_test

import (
		"container/heap"
		"fmt"
)

// An IntHeap is a min-heap of ints.
type IntHeap []int

func (h IntHeap) Len() int					 { return len(h) }
func (h IntHeap) Less(i, j int) bool { return h[i] < h[j] }
func (h IntHeap) Swap(i, j int)			{ h[i], h[j] = h[j], h[i] }

func (h *IntHeap) Push(x interface{}) {
		// Push and Pop use pointer receivers because they modify the slice's length,
		// not just its contents.
		*h = append(*h, x.(int))
}

func (h *IntHeap) Pop() interface{} {
		old := *h
		n := len(old)
		x := old[n-1]
		*h = old[0 : n-1]
		return x
}

// This example inserts several ints into an IntHeap, checks the minimum,
// and removes them in order of priority.
func Example_intHeap() {
		h := &IntHeap{2, 1, 5}
		heap.Init(h)
		heap.Push(h, 3)
		fmt.Printf("minimum: %d\n", (*h)[0])
		for h.Len() > 0 {
				fmt.Printf("%d ", heap.Pop(h))
		}
		// Output:
		// minimum: 1
		// 1 2 3 5
}

This example creates a PriorityQueue with some items, adds and manipulates an item, and then removes the items in priority order.

Code:

// 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 update 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{}) {
		n := len(*pq)
		item := x.(*Item)
		item.index = n
		*pq = append(*pq, item)
}

func (pq *PriorityQueue) Pop() interface{} {
		old := *pq
		n := len(old)
		item := old[n-1]
		old[n-1] = nil	// avoid memory leak
		item.index = -1 // for safety
		*pq = old[0 : n-1]
		return item
}

// update modifies the priority and value of an Item in the queue.
func (pq *PriorityQueue) update(item *Item, value string, priority int) {
		item.value = value
		item.priority = priority
		heap.Fix(pq, item.index)
}

// This example creates a PriorityQueue with some items, adds and manipulates an item,
// and then removes the items in priority order.
func Example_priorityQueue() {
		// Some items and their priorities.
		items := map[string]int{
				"banana": 3, "apple": 2, "pear": 4,
		}

		// Create a priority queue, put the items in it, and
		// establish the priority queue (heap) invariants.
		pq := make(PriorityQueue, len(items))
		i := 0
		for value, priority := range items {
				pq[i] = &Item{
						value:		value,
						priority: priority,
						index:		i,
				}
				i++
		}
		heap.Init(&pq)

		// Insert a new item and then modify its priority.
		item := &Item{
				value:		"orange",
				priority: 1,
		}
		heap.Push(&pq, item)
		pq.update(item, item.value, 5)

		// Take the items out; they arrive in decreasing priority order.
		for pq.Len() > 0 {
				item := heap.Pop(&pq).(*Item)
				fmt.Printf("%.2d:%s ", item.priority, item.value)
		}
		// Output:
		// 05:orange 04:pear 03:banana 02:apple
}