container/heap: add example

godoc doesn't have the fu to present the example well, but this gives
us an example of an example to develop example fu.

Fixes #2840.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5645063

src/pkg/container/heap/example_test.go

+// 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
+}
+
+// 99:seven 88:five 77:zero 66:nine 55:three 44:two 33:six 22:one 11:four 00:eight
+func ExampleInterface() {
+	// The full code of this example, including the methods that implement
+	// heap.Interface, is in the file src/pkg/container/heap/example_test.go.
+
+	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)
+	}
+}
+
+// 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)
+}

src/pkg/container/heap/heap.go

@@ -6,10 +6,11 @@
 // heap.Interface. A heap is a tree with the property that each node is the
 // highest-valued node in its subtree.
 //
-// A heap is a common way to impement a priority queue. To build a priority
+// A heap is a common way to implement a priority queue. To build a priority
 // queue, implement the Heap interface with the (negative) priority as the
 // ordering for the Less method, so Push adds items while Pop removes the
-// highest-priority item from the queue.
+// highest-priority item from the queue. The Examples include such an
+// implementation; the file example_test.go has the complete source.
 //
 package heap