//
//  DispatchQueue+Once.swift
//  VDS
//
//  Created by Matt Bruce on 7/27/22.
//

import Foundation

extension DispatchQueue {
    private static var _onceTracker = Set<String>()
    
    public class func once(
        file: String = #file,
        function: String = #function,
        line: Int = #line,
        block: () -> Void
    ) {
        let token = "\(file):\(function):\(line)"
        once(token: token, block: block)
    }
    
    /**
     Executes a block of code, associated with a unique token, only once.  The code is thread safe and will
     only execute the code once even in the presence of multithreaded calls.
     
     - parameter token: A unique reverse DNS style name such as com.vectorform.<name> or a GUID
     - parameter block: Block to execute once
     */
    public class func once(
        token: String,
        block: () -> Void
    ) {
        // Peek ahead to avoid the intersection.
        guard !_onceTracker.contains(token) else { return }
        
        objc_sync_enter(self)
        defer { objc_sync_exit(self) }
        
        // Double check we are the first in the critical section.
        guard !_onceTracker.contains(token) else { return }
        
        // Execute.
        _onceTracker.insert(token)
        block()
    }
}