package gohan

import (
	"errors"

	"github.com/hajimehoshi/ebiten/v2"
)

// System represents a system that runs continuously. While the system must
// implement the Update and Draw methods, a special error value may be returned
// indicating that the system does not utilize one of the methods. System
// methods which return one of these special error values will not be called again.
//
// See ErrSystemWithoutUpdate and ErrSystemWithoutDraw.
type System interface {
	// Matches returns whether the provided entity is handled by this system.
	Matches(entity Entity) bool

	// Update is called once for each matching entity each time the game state is updated.
	Update(entity Entity) error

	// Draw is called once for each matching entity each time the game is drawn to the screen.
	Draw(entity Entity, screen *ebiten.Image) error
}

// Special error values.
var (
	// ErrSystemWithoutUpdate is the error returned when a System does not implement Update.
	ErrSystemWithoutUpdate = errors.New("system does not implement update")

	// ErrSystemWithoutDraw is the error returned when a System does not implement Draw.
	ErrSystemWithoutDraw = errors.New("system does not implement draw")
)