16-Mar-2025

A priority queue is a special type of queue where each element has an associated priority. Elements are dequeued (removed) based on their priority, with the highest priority element being removed first. This contrasts with a standard queue which operates on a FIFO (First-In, First-Out) basis.

Think of it like a hospital emergency room. Patients with more severe conditions (higher priority) are seen before those with less urgent needs, even if they arrived later.

Implementation in Go

Go’s container/heap package provides tools for implementing a priority queue. Here’s an example of a min-priority queue (smallest element has the highest priority):

package main

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 a min-heap, so we use less 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)
}

func main() {
	// Some items and their priorities.
	items := map[string]int{
		"task1": 3,
		"task2": 1,
		"task3": 2,
	}

	// 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:    "task4",
		priority: 4,
	}
	heap.Push(&pq, item)
	pq.update(item, item.value, 0) // Now task4 has highest priority

	// 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:
	// 00:task4 01:task2 02:task3 03:task1
}

Key Properties and Use Cases:

• Priority-based retrieval: Elements are accessed based on priority, not insertion order.
• Efficient for managing tasks: Useful for scheduling tasks, prioritizing network traffic, and other scenarios where order of operations is priority-driven.
• Variations: Min-priority queues (smallest value first) and max-priority queues (largest value first) are common implementations.

This example demonstrates how to use Go’s container/heap package to create and manage a priority queue. Understanding and utilizing priority queues can significantly optimize algorithms and improve application performance when dealing with ordered data where priority is a crucial factor.