Computed Properties trong Swift

struct Rectangle {
    var width: Double
    var height: Double
    
    // Computed property (read-only)
    var area: Double {
        width * height
    }
    
    var perimeter: Double {
        2 * (width + height)
    }
}
 
let rect = Rectangle(width: 10, height: 5)
print(rect.area)       // 50.0
print(rect.perimeter)  // 30.0

struct Temperature {
    var celsius: Double
    
    var fahrenheit: Double {
        get {
            celsius * 9 / 5 + 32
        }
        set {
            celsius = (newValue - 32) * 5 / 9
        }
    }
    
    var kelvin: Double {
        get { celsius + 273.15 }
        set { celsius = newValue - 273.15 }
    }
}
 
var temp = Temperature(celsius: 25)
print(temp.fahrenheit)  // 77.0
 
temp.fahrenheit = 100
print(temp.celsius)     // 37.77...

struct Point {
    var x: Double
    var y: Double
    
    // newValue là tên mặc định
    var magnitude: Double {
        get {
            sqrt(x * x + y * y)
        }
        set {
            let ratio = newValue / magnitude
            x *= ratio
            y *= ratio
        }
    }
}

struct Circle {
    var radius: Double
    
    // Có thể bỏ get khi read-only
    var diameter: Double { radius * 2 }
    var circumference: Double { 2 * .pi * radius }
    var area: Double { .pi * radius * radius }
}
 
let circle = Circle(radius: 5)
print(circle.area)  // 78.53...

class Person {
    var firstName: String
    var lastName: String
    
    var fullName: String {
        get { "\(firstName) \(lastName)" }
        set {
            let parts = newValue.split(separator: " ")
            firstName = String(parts.first ?? "")
            lastName = String(parts.last ?? "")
        }
    }
    
    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }
}
 
enum Direction {
    case north, south, east, west
    
    var opposite: Direction {
        switch self {
        case .north: return .south
        case .south: return .north
        case .east: return .west
        case .west: return .east
        }
    }
}

struct DataManager {
    // Lazy: tính một lần, lưu kết quả
    lazy var expensiveData: [Int] = {
        print("Loading data...")
        return Array(1...1000000)
    }()
    
    // Computed: tính mỗi lần truy cập
    var currentTime: Date {
        Date()
    }
}

struct User {
    var firstName: String
    var lastName: String
    var email: String
    
    var isValidEmail: Bool {
        email.contains("@") && email.contains(".")
    }
    
    var displayName: String {
        "\(firstName) \(lastName.prefix(1))."
    }
}
 
struct Cart {
    var items: [CartItem]
    
    var subtotal: Double {
        items.reduce(0) { $0 + $1.price * Double($1.quantity) }
    }
    
    var tax: Double { subtotal * 0.1 }
    var total: Double { subtotal + tax }
    var itemCount: Int { items.reduce(0) { $0 + $1.quantity } }
}