systemDesigns.swift

// Toaster
class Bread {
	var cookedTime: Double = 0.0
}

enum TrayLevel { 
	case down 
	case up 
} 

class Status {
	var slots: (Bread?, Bread?)
	var toastTime: Double
	
	init(slots: (Bread?, Bread?), toastTime: Double) {
		self.slots = slots
		sell.toastTime = toastTime
	}
}

protocol StatusDelegate {
	func didUpdateStatus()
}

class Toaster {
	var status: Status {
		didSet {
			statusDelegate.didUpdateStatus()
		}
	}
	var statusDelegate: StatusDelegate?
	var didEject: Bool
	var toastLevel: Double
	var tray: TrayLevel
	
	init() {
		status = Status(slots: (0, 0), toastTime: 0)
		didEject = true
		toastLevel = 0
		tray = .down
	}
	
	// assuming: 
	// 1. only add one slice at a time
	// 2. the slots are indexed 0 and 1
	func addBread(_ bread: Bread, toSlot slot: String) {
		if slots.0 != nil && slots.1 != nil {
			print("Both slots are in use. Try again later")
			return
		}
		
		if slot == 0 && slot.0 != nil {
			print("Slot 0 is in use. Adding bread to slot 1...")
			slots.1 = bread
		} else if slot == 1 && slot.1 != nil {
			print("Slot 1 is in use. Adding bread to slot 0...")
			slots.0 = bread
		} else {
			slots.slot = bread
		}
	}
	
	func changeLevel(level: Double) {
		toastLevel = level
		// if the tray is down, change level to 0 will force eject it
		if tray == .down & level == 0 {
			eject()
		}
	}
	
	func run() {
		if slots.0 == nil && slots.1 == nil {
			print("Please add bread")
			return
		}
	
		didEject = false
		tray = .down
		toast()
	}
	
	// recursive call
	func toast() {
		if didEject == false && toastLevel > 0 {
			DispatchQueue.main.asyncAfter(deadline: now() + 0.5) {
				// keep the heat up every 0.5 seconds, and turn the level down by 0.05 (1 level = 10 seconds)
				if slots.0 != nil { slots.0.cookedTime = slots.0.cookedTime + 0.5 } 
				if slots.1 != nil { slots.1.cookedTime = slots.1.cookedTime + 0.5 } 
				toastLevel -= 0.05
				
				toast()
			}
		} else if toastLevel == 0 {
			eject()
		}
	}
	
	func takeBread(atSlot slot: Int) -> Bread? {
		if slots.slot == nil {
			print("The slot is empty")
			return nil
		} else {
			let bread = slots.slot 
			// empty the slot
			slots.slot = nil
			return bread
		}
	}
	
	func eject() -> (Bread?, Bread?) {
		didEject = true
		tray = .up
		return status.slots
	}
}




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