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use std::marker::PhantomData;

// --- Const generics ---
fn sum_array<const N: usize>(arr: [i32; N]) -> i32 {
    arr.iter().sum()
}

#[derive(Debug)]
struct Matrix<const ROWS: usize, const COLS: usize> {
    data: [[f64; COLS]; ROWS],
}

impl<const ROWS: usize, const COLS: usize> Matrix<ROWS, COLS> {
    fn new() -> Self {
        Matrix { data: [[0.0; COLS]; ROWS] }
    }
    fn shape(&self) -> (usize, usize) { (ROWS, COLS) }
}

// --- Phantom types ---
struct User;
struct Post;

#[derive(Debug, Clone, Copy)]
struct Id<T> {
    value: u64,
    _phantom: PhantomData<T>,
}

impl<T> Id<T> {
    fn new(value: u64) -> Self {
        Id { value, _phantom: PhantomData }
    }
}

fn get_user_name(_id: Id<User>) -> &'static str { "Alice" }
// fn get_user_name(_id: Id<Post>) — would be a type error at call site

fn main() {
    println!("sum [1,2,3] = {}", sum_array([1, 2, 3]));
    println!("sum [10,20] = {}", sum_array([10, 20]));

let m: Matrix<3, 4> = Matrix::new();
    println!("matrix shape: {:?}", m.shape());

let user_id: Id<User> = Id::new(1);
    let post_id: Id<Post> = Id::new(1);
    println!("user_id = {:?}", user_id);
    println!("post_id = {:?}", post_id);
    println!("user name = {}", get_user_name(user_id));
    // get_user_name(post_id); // compile error: expected Id<User>, got Id<Post>
}