// Here's a basic interface for geometric shapes.
interface Geometry {
    area(): number;
    perim(): number;
}

// For our example we'll implement this interface on
// `Rect` and `Circle` classes.
class Rect implements Geometry {
    constructor(private width: number, private height: number) {}

    area(): number {
        return this.width * this.height;
    }

    perim(): number {
        return 2 * this.width + 2 * this.height;
    }
}

class Circle implements Geometry {
    constructor(private radius: number) {}

    area(): number {
        return Math.PI * this.radius * this.radius;
    }

    perim(): number {
        return 2 * Math.PI * this.radius;
    }
}

// If a variable has an interface type, then we can call
// methods that are in the named interface. Here's a
// generic `measure` function taking advantage of this
// to work on any `Geometry`.
function measure(g: Geometry): void {
    console.log(g);
    console.log(g.area());
    console.log(g.perim());
}

function main() {
    const r = new Rect(3, 4);
    const c = new Circle(5);

    // The `Circle` and `Rect` classes both
    // implement the `Geometry` interface so we can use
    // instances of these classes as arguments to `measure`.
    measure(r);
    measure(c);
}

main();