Readers and writers problem solved in swift using semaphores

read-write.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
    }
}

func randomString(length: Int) -> String {
    let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    return String((0...length-1).map{ _ in letters.randomElement()! })
}

var wrt: Semaphore = Semaphore(n: 1)
var mutex: Semaphore = Semaphore(n: 1)
var readers: Int = 0
var ran = 0

var book: String = ""

func reader(read: (String) -> Void) {
    while ran < 100 {
        mutex.P()
        readers = readers + 1
        
        if readers == 1 {
            wrt.P()
        }
        
        mutex.V()
        
        read(book)
        
        mutex.P()
        readers = readers - 1
        
        if readers == 0 {
            wrt.V()
        }
        
        mutex.V()
    }
}

func writer() {
    while ran < 100 {
        wrt.P()
        book = book + randomString(length: 2)
        wrt.V()
    }
}

DispatchQueue.global(qos: .userInitiated).async {
    writer()
}

DispatchQueue.global(qos: .userInitiated).async {
    writer()
}

DispatchQueue.global(qos: .userInitiated).async {
    reader { (b: String) in
        print("Reader 1 just read \(b)")
        ran = ran + 1
    }
}

DispatchQueue.global(qos: .userInitiated).async {
    reader { (b: String) in
        print("Reader 2 just read \(b)")
        ran = ran + 1
    }
}

DispatchQueue.global(qos: .userInitiated).async {
    reader { (b: String) in
        print("Reader 3 just read \(b)")
        ran = ran + 1
    }
}