Go Interface for Polymorphism

interface.go

// You can edit this code!
// Click here and start typing.
package main

import (
	"fmt"
	"math"
)

type Shape interface {
	area() float64
}

func totalArea(shapes ...Shape) float64 {
	var area float64
	for _, s := range shapes {
		area += s.area()
	}

	return area
}

type Circle struct {
	x, y, r float64
}

func NewCircle(x, y, r float64) *Circle {
	return &Circle{
		x: x,
		y: y,
		r: r,
	}
}

func (c *Circle) area() float64 {
	return math.Pi * c.r * c.r
}

type Rectangle struct {
	x1, y1, x2, y2 float64
}

func NewRectangle(x1, y1, x2, y2 float64) *Rectangle {
	return &Rectangle{
		x1: x1,
		y1: y1,
		x2: x2,
		y2: y2,
	}
}

func distance(x1, y1, x2, y2 float64) float64 {
	a := x2 - x1
	b := y2 - y1
	return math.Sqrt(a*a + b*b)
}

func (r *Rectangle) area() float64 {
	l := distance(r.x1, r.y1, r.x1, r.y2)
	w := distance(r.x1, r.y1, r.x2, r.y1)
	return l * w
}

func main() {
	c := NewCircle(0, 0, 2)
	r := NewRectangle(0, 0, 2, 2)

	fmt.Println(totalArea(c, r))
}