Day 7：第五週總結練習

// 1. 定義一個 trait，任何圖形都能算面積與週長
trait Shape {
    fn area(&self) -> f64;
    fn perimeter(&self) -> f64;
}

struct Circle { r: f64 }
struct Rect { w: f64, h: f64 }

impl Shape for Circle {
    fn area(&self) -> f64 { std::f64::consts::PI * self.r * self.r }
    fn perimeter(&self) -> f64 { 2.0 * std::f64::consts::PI * self.r }
}
impl Shape for Rect {
    fn area(&self) -> f64 { self.w * self.h }
    fn perimeter(&self) -> f64 { 2.0 * (self.w + self.h) }
}

// 2. 泛型函式：不靠 Box，直接用泛型 + bound
fn report<T: Shape>(shape: &T) {
    println!("area={:.2}, perimeter={:.2}", shape.area(), shape.perimeter());
}

// 3. 回傳 impl Shape
fn describe(shape: impl Shape) -> String {
    format!("area {:.2}", shape.area())
}

fn main() {
    let c = Circle { r: 2.0 };
    let r = Rect { w: 3.0, h: 4.0 };
    report(&c);
    report(&r);
    println!("{}", describe(c));
    println!("{}", describe(r));
}