var name = "홍길동"  // 변경 가능
let birthYear = 1990  // 변경 불가

let score: Int = 100
let pi: Double = 3.14159
let greeting: String = "안녕하세요"
let isActive: Bool = true

var optionalName: String? = "스위프트"
optionalName = nil  // 가능!

// 옵셔널 바인딩으로 안전하게 꺼내기
if let name = optionalName {
    print("이름: \(name)")
} else {
    print("이름이 없어요")
}

// 배열
var fruits = ["사과", "바나나", "딸기"]

// 딕셔너리
var scores = ["수학": 95, "영어": 88, "과학": 92]

// 셋 (중복 없는 집합)
var uniqueNumbers: Set = [1, 2, 3, 2, 1]  // {1, 2, 3}

// 기본 함수
func greet(name: String) -> String {
    return "안녕하세요, \(name)님!"
}

// 클로저
let multiply = { (a: Int, b: Int) -> Int in
    return a * b
}
print(multiply(3, 4))  // 12

struct Point {
    var x: Double
    var y: Double
    
    func distance(to other: Point) -> Double {
        let dx = x - other.x
        let dy = y - other.y
        return (dx*dx + dy*dy).squareRoot()
    }
}

protocol Greetable {
    var name: String { get }
    func greet() -> String
}

struct Person: Greetable {
    var name: String
    func greet() -> String {
        return "안녕하세요! 저는 \(name)입니다."
    }
}

struct ContentView: View {
    var body: some View {
        VStack {
            Text("안녕하세요!")
                .font(.largeTitle)
                .foregroundColor(.blue)
            Button("눌러봐요") {
                print("버튼 클릭!")
            }
        }
    }
}

struct CounterView: View {
    @State private var count = 0
    
    var body: some View {
        VStack {
            Text("카운트: \(count)")
                .font(.title)
            Button("증가") {
                count += 1
            }
        }
    }
}

DispatchQueue.global(qos: .background).async {
    // 백그라운드에서 무거운 작업
    let result = heavyTask()
    
    DispatchQueue.main.async {
        // 메인 스레드에서 UI 업데이트
        self.label.text = result
    }
}

func fetchUserData() async throws -> User {
    let url = URL(string: "https://api.example.com/user")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(User.self, from: data)
}

// 사용
Task {
    do {
        let user = try await fetchUserData()
        print("유저 이름: \(user.name)")
    } catch {
        print("에러: \(error)")
    }
}

func fetchData() async throws -> [Post] {
    let url = URL(string: "https://jsonplaceholder.typicode.com/posts")!
    let (data, response) = try await URLSession.shared.data(from: url)
    
    guard let httpResponse = response as? HTTPURLResponse,
          httpResponse.statusCode == 200 else {
        throw NetworkError.invalidResponse
    }
    
    return try JSONDecoder().decode([Post].self, from: data)
}

struct User: Codable {
    let id: Int
    let name: String
    let email: String
    let createdAt: Date
    
    enum CodingKeys: String, CodingKey {
        case id, name, email
        case createdAt = "created_at"  // 서버 키와 다를 때
    }
}