Swift LIFO Queue

LIFOOperationQueue.swift

//
//  LIFOOperationQueue.swift
//  
//
//  Created by Antoine Marandon on 27/07/2018.
//

import Foundation

/// Last in First Out Operation queue.
class LIFOOperationQueue {
    enum Priority {
        case high
        case low
    }
    private let queue: DispatchQueue
    private var highPriorityOperations = [Operation]()
    private var lowPriorityOperations = [Operation]()

    init(queue: DispatchQueue) {
        self.queue = queue
    }

    private func next() {
        if let operation = highPriorityOperations.first {
            highPriorityOperations.remove(at: 0)
            queue.async {
                operation.start()
            }
        } else if let operation = lowPriorityOperations.first {
            lowPriorityOperations.remove(at: 0)
            queue.async {
                operation.start()
            }
        }
    }

    /// Add an operation to the queue. Adding a duplicate operation cause it to be bumped to the top of the queue.
    func addOperation(operation: Operation, priority: Priority) {
        queue.async {
            if let index = self.highPriorityOperations.index(of: operation) {
                self.highPriorityOperations.remove(at: index)
            }
            if let index = self.lowPriorityOperations.index(of: operation) {
                self.lowPriorityOperations.remove(at: index)
            }
            switch priority {
            case .high:
                self.highPriorityOperations.insert(operation, at: 0)
            case .low:

                self.lowPriorityOperations.insert(operation, at: 0)
            }
            self.queue.async {
                self.next()
            }
        }
    }

    /// De prioritize an operation.
    func pushBack(operation: Operation) {
        queue.async {
            if let index = self.highPriorityOperations.index(of: operation) {
                self.highPriorityOperations.remove(at: index)
            }
            if let index = self.lowPriorityOperations.index(of: operation) {
                self.lowPriorityOperations.remove(at: index)
            }
            self.lowPriorityOperations.append(operation)
            self.queue.async {
                self.next()
            }
        }
    }
}