Guard Statement trong Swift

func checkAge(_ age: Int) {
    guard age >= 0 else {
        print("Invalid age")
        return
    }
    print("Age is valid: \(age)")
}

func greet(_ name: String?) {
    guard let name = name else {
        print("No name provided")
        return
    }
    // name bây giờ là non-optional
    print("Hello, \(name)!")
}
 
greet("Alice")  // Hello, Alice!
greet(nil)      // No name provided

// Với if-let - phải lồng code
func processWithIf(value: Int?) {
    if let value = value {
        if value > 0 {
            print("Processing: \(value)")
        }
    }
}
 
// Với guard - code flat hơn
func processWithGuard(value: Int?) {
    guard let value = value else { return }
    guard value > 0 else { return }
    print("Processing: \(value)")
}

func validateUser(name: String?, age: Int?, email: String?) {
    guard let name = name, 
          let age = age, 
          let email = email else {
        print("Missing required fields")
        return
    }
    
    guard !name.isEmpty, 
          age >= 18, 
          email.contains("@") else {
        print("Invalid data")
        return
    }
    
    print("Valid user: \(name), \(age), \(email)")
}

func processNumber(_ input: String?) {
    guard let input = input,
          let number = Int(input),
          number > 0 else {
        print("Invalid input")
        return
    }
    print("Number: \(number)")
}

let data = ["1", "two", "3", "four", "5"]
 
for item in data {
    guard let number = Int(item) else {
        print("Skipping: \(item)")
        continue
    }
    print("Number: \(number)")
}

enum ValidationError: Error {
    case emptyName
    case invalidAge
}
 
func validatePerson(name: String?, age: Int?) throws -> (String, Int) {
    guard let name = name, !name.isEmpty else {
        throw ValidationError.emptyName
    }
    
    guard let age = age, age >= 0 else {
        throw ValidationError.invalidAge
    }
    
    return (name, age)
}

func processPayment(amount: Double?, cardNumber: String?, cvv: String?) -> Bool {
    guard let amount = amount, amount > 0 else {
        print("Invalid amount")
        return false
    }
    
    guard let cardNumber = cardNumber, cardNumber.count == 16 else {
        print("Invalid card number")
        return false
    }
    
    guard let cvv = cvv, cvv.count == 3 else {
        print("Invalid CVV")
        return false
    }
    
    print("Processing payment: \(amount)")
    return true
}

// ✅ Dùng guard cho early exit
func good(value: Int?) {
    guard let value = value, value > 0 else { return }
    // Happy path code here
}
 
// ❌ Không dùng guard khi cần else branch phức tạp
func notIdeal(value: Int?) {
    guard let value = value else {
        // Nhiều logic ở đây -> dùng if-let
        return
    }
}