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Source file src/container/heap/example_pq_test.go

Documentation: container/heap

     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 interface{}) {
    38  	n := len(*pq)
    39  	item := x.(*Item)
    40  	item.index = n
    41  	*pq = append(*pq, item)
    42  }
    43  
    44  func (pq *PriorityQueue) Pop() interface{} {
    45  	old := *pq
    46  	n := len(old)
    47  	item := old[n-1]
    48  	item.index = -1 // for safety
    49  	*pq = old[0 : n-1]
    50  	return item
    51  }
    52  
    53  // update modifies the priority and value of an Item in the queue.
    54  func (pq *PriorityQueue) update(item *Item, value string, priority int) {
    55  	item.value = value
    56  	item.priority = priority
    57  	heap.Fix(pq, item.index)
    58  }
    59  
    60  // This example creates a PriorityQueue with some items, adds and manipulates an item,
    61  // and then removes the items in priority order.
    62  func Example_priorityQueue() {
    63  	// Some items and their priorities.
    64  	items := map[string]int{
    65  		"banana": 3, "apple": 2, "pear": 4,
    66  	}
    67  
    68  	// Create a priority queue, put the items in it, and
    69  	// establish the priority queue (heap) invariants.
    70  	pq := make(PriorityQueue, len(items))
    71  	i := 0
    72  	for value, priority := range items {
    73  		pq[i] = &Item{
    74  			value:    value,
    75  			priority: priority,
    76  			index:    i,
    77  		}
    78  		i++
    79  	}
    80  	heap.Init(&pq)
    81  
    82  	// Insert a new item and then modify its priority.
    83  	item := &Item{
    84  		value:    "orange",
    85  		priority: 1,
    86  	}
    87  	heap.Push(&pq, item)
    88  	pq.update(item, item.value, 5)
    89  
    90  	// Take the items out; they arrive in decreasing priority order.
    91  	for pq.Len() > 0 {
    92  		item := heap.Pop(&pq).(*Item)
    93  		fmt.Printf("%.2d:%s ", item.priority, item.value)
    94  	}
    95  	// Output:
    96  	// 05:orange 04:pear 03:banana 02:apple
    97  }
    98  

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