Semaphores implemented using monitors and condition variables

monitors.swift

import UIKit

class Condition {
    var waiting: Bool
    
    init() {
        waiting = false
    }
    
    func wait() {
        waiting = true
        while waiting {}
    }
    
    func signal() {
        waiting = false
    }
}

class SemaphoreMonitor {
    var busy: Bool
    var free: Condition
    
    init() {
        busy = false
        free = Condition()
    }
    
    func P() {
        if busy {
            free.wait()
        }
        
        busy = true
    }
    
    func V() {
        busy = false
        free.signal()
    }
}

var s = SemaphoreMonitor()
var ran = 0

func processOne(task: () -> Void) {
    while ran < 50 {
        s.P()
        task()
        s.V()
    }
}

func processTwo(task: () -> Void) {
    while (ran < 50) {
        s.P()
        task()
        s.V()
    }
}

DispatchQueue.global(qos: .userInitiated).async {
    processOne {
        let date = NSDate().timeIntervalSince1970
        print("Process one ran at \(date)")
        ran = ran + 1
    }
}

DispatchQueue.global(qos: .userInitiated).async {
    processTwo {
        let date = NSDate().timeIntervalSince1970
        print("Process two ran at \(date)")
        ran = ran + 1
    }
}