example_pq_test.go

     1  // Copyright 2012 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // This example demonstrates a priority queue built using the heap interface.
     6  package heap_test
     7  
     8  import (
     9  	"container/heap"
    10  	"fmt"
    11  )
    12  
    13  // An Item is something we manage in a priority queue.
    14  type Item struct {
    15  	value    string // The value of the item; arbitrary.
    16  	priority int    // The priority of the item in the queue.
    17  	// The index is needed by update and is maintained by the heap.Interface methods.
    18  	index int // The index of the item in the heap.
    19  }
    20  
    21  // A PriorityQueue implements heap.Interface and holds Items.
    22  type PriorityQueue []*Item
    23  
    24  func (pq PriorityQueue) Len() int { return len(pq) }
    25  
    26  func (pq PriorityQueue) Less(i, j int) bool {
    27  	// We want Pop to give us the highest, not lowest, priority so we use greater than here.
    28  	return pq[i].priority > pq[j].priority
    29  }
    30  
    31  func (pq PriorityQueue) Swap(i, j int) {
    32  	pq[i], pq[j] = pq[j], pq[i]
    33  	pq[i].index = i
    34  	pq[j].index = j
    35  }
    36  
    37  func (pq *PriorityQueue) Push(x any) {
    38  	n := len(*pq)
    39  	item := x.(*Item)
    40  	item.index = n
    41  	*pq = append(*pq, item)
    42  }
    43  
    44  func (pq *PriorityQueue) Pop() any {
    45  	old := *pq
    46  	n := len(old)
    47  	item := old[n-1]
    48  	old[n-1] = nil  // don't stop the GC from reclaiming the item eventually
    49  	item.index = -1 // for safety
    50  	*pq = old[0 : n-1]
    51  	return item
    52  }
    53  
    54  // update modifies the priority and value of an Item in the queue.
    55  func (pq *PriorityQueue) update(item *Item, value string, priority int) {
    56  	item.value = value
    57  	item.priority = priority
    58  	heap.Fix(pq, item.index)
    59  }
    60  
    61  // This example creates a PriorityQueue with some items, adds and manipulates an item,
    62  // and then removes the items in priority order.
    63  func Example_priorityQueue() {
    64  	// Some items and their priorities.
    65  	items := map[string]int{
    66  		"banana": 3, "apple": 2, "pear": 4,
    67  	}
    68  
    69  	// Create a priority queue, put the items in it, and
    70  	// establish the priority queue (heap) invariants.
    71  	pq := make(PriorityQueue, len(items))
    72  	i := 0
    73  	for value, priority := range items {
    74  		pq[i] = &Item{
    75  			value:    value,
    76  			priority: priority,
    77  			index:    i,
    78  		}
    79  		i++
    80  	}
    81  	heap.Init(&pq)
    82  
    83  	// Insert a new item and then modify its priority.
    84  	item := &Item{
    85  		value:    "orange",
    86  		priority: 1,
    87  	}
    88  	heap.Push(&pq, item)
    89  	pq.update(item, item.value, 5)
    90  
    91  	// Take the items out; they arrive in decreasing priority order.
    92  	for pq.Len() > 0 {
    93  		item := heap.Pop(&pq).(*Item)
    94  		fmt.Printf("%.2d:%s ", item.priority, item.value)
    95  	}
    96  	// Output:
    97  	// 05:orange 04:pear 03:banana 02:apple
    98  }
    99
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