SwiftPriorityQueue

SwiftPriorityQueue is a pure Swift (no Cocoa) implementation of a generic priority queue data structure, appropriate for use on all platforms (macOS, iOS, Linux, etc.) where Swift is supported. It features a straightforward interface and can be used with any type that implements Comparable. It utilizes comparisons between elements rather than separate numeric priorities to determine order.

Internally, SwiftPriorityQueue uses a classic binary heap, resulting in O(lg n) pushes and pops. It includes in-source documentation, an A* based example maze solving program (for macOS), and unit tests (pull requests are welcome for additional unit tests in particular).

Features

• Easy-to-use method interface
• Small, self contained, pure Swift code base
• Classic binary heap implementation with O(lg n) pushes and pops
• Iterable through standard Swift for…in loops (implements Sequence and IteratorProtocol)
• In-source documentation
• A fun maze solving A* based example program

Installation

Release 1.3.0 and above supports Swift 5. Use release 1.2.1 for Swift 4. Use release 1.1.2 for Swift 3 and Swift 2 support. Use release 1.0 for Swift 1.2 support.

CocoaPods

Use the CocoaPod SwiftPriorityQueue.

Swift Package Manager (SPM)

Add this repository as a dependency.

Manual

Copy SwiftPriorityQueue.swift into your project.

Documentation

There is a large amount of documentation in the source code using the standard Swift documentation technique (compatible with Jazzy). Essentially though, SwiftPriorityQueue has the three critical methods you’d expect - push(), pop(), and peek().

Initialization

When you create a new PriorityQueue you can optionally specify whether the priority queue is ascending or descending. What does this mean? If the priority queue is ascending, its smallest values (as determined by their implementation of Comparable aka <) will be popped first, and if it’s descending, its largest values will be popped first.

var pq: PriorityQueue<String> = PriorityQueue<String>(ascending: true)

You can also provide an array of starting values to be pushed sequentially immediately into the priority queue.

var pq: PriorityQueue<Int> = PriorityQueue<Int>(startingValues: [6, 2, 3, 235, 4, 500])

Or you can specify both.

var pq: PriorityQueue<Int> = PriorityQueue<Int>(ascending: false, startingValues: [6, 2, 3, 235, 4, 500])

Or you can specify neither. By default a PriorityQueue is descending and empty. As you’ve probably noticed, a PriorityQueue takes a generic type. This type must be Comparable, as its comparison will be used for determining priority. This means that your custom types must implement Comparable and utilize the overridden < to determine priority.

Methods

PriorityQueue has all of the standard methods you’d expect a priority queue data structure to have.

• push(element: T) - Puts an element into the priority queue. O(lg n)
• push(element: T, maxCount: Int) -> T? - Adds an element while limiting the size of the priority queue to maxCount. If more than maxCount elements are in the priority queue after the addition, the lowest priority element will be discarded and returned. Note this is inefficient because this is a binary heap, so only the highet priority item is efficient to retrieve. O(n)
• pop() -> T? - Returns and removes the element with the highest (or lowest if ascending) priority or nil if the priority queue is empty. O(lg n)
• peek() -> T? - Returns the element with the highest (or lowest if ascending) priority or nil if the priority queue is empty. O(1)
• clear() - Removes all elements from the priority queue.
• remove(item: T) - Removes the first found instance of item in the priority queue. Silently returns if not found. O(n)
• removeAll(item: T) - Removes all instances of item in the priority queue through repeated calls to remove(). Silently returns if not found.

Properties

• count: Int - The number of elements in the priority queue.
• isEmpty: Bool - true if the priority queue has zero elements, and false otherwise.

Standard Swift Protocols

PriorityQueue implements IteratorProtocol, Sequence and Collection so you can treat PriorityQueue like any other Swift sequence/collection. This means you can use Swift standard library fucntions on a PriorityQueue and iterate through a PriorityQueue like this:

for item in pq {  // pq is a PriorityQueue<String>
    print(item)
}

When you do this, every item from the PriorityQueue is popped in order. PriorityQueue also implements CustomStringConvertible and CustomDebugStringConvertible.

print(pq)

Note: PriorityQueue is not thread-safe (do not manipulate it from multiple threads at once).

Limited Heap Size Example

Suppose you want to only keep the maxCount highest priority items in the priority queue.

For example, say you only want the priority queue to only ever have 4 elements:

var pq: PriorityQueue<Int> = PriorityQueue<Int>()
let maxCount = 4         

pq.push(4, maxCount: maxCount)
pq.push(5, maxCount: maxCount)
pq.push(0, maxCount: maxCount)
pq.push(3, maxCount: maxCount)  
pq.push(6, maxCount: maxCount)     
pq.push(1, maxCount: maxCount)     

In this case, after 4 elements were pushed, only the smallest elements were kept (because the order was ascending). So, the final priority queue has the elements 0, 1, 3, 4 in it.

Just for Fun - A* (astar.swift)

A* is a pathfinding algorithm that uses a priority queue. The sample program that comes with SwiftPriorityQueue is a maze solver that uses A*. You can find some in-source documentation if you want to reuse this algorithm inside astar.swift.

Authorship & License

SwiftPriorityQueue is written by David Kopec (@davecom on GitHub) and released under the MIT License (see LICENSE). You can find my contact information on my GitHub profile page. I encourage you to submit pull requests and open issues here on GitHub. Thank you to all of the contributors over the years.