Semaphores for solving the mutual exclusion problem

semaphore.swift

import UIKit

class Semaphore {
    var n: Int
    
    init(n: Int) {
        self.n = n
    }
 
    func P() {
        while self.n == 0 {}
        self.n = self.n - 1
    }
    
    func V() {
        self.n = self.n + 1
    }
}

var s: Semaphore = Semaphore(n: 1)
var ran: Int = 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
    }
}