gohan/gohan.go

package gohan

import (
	"fmt"
	"log"
	"os"
	"strings"
	"time"

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

var (
	gameComponents = make(map[EntityID]map[ComponentID]interface{})

	gameSystems        []System
	gameSystemEntities [][]EntityID // Slice of entities matching each system.

	gameSystemReceivesUpdate []bool
	gameSystemReceivesDraw   []bool

	debug bool
)

func init() {
	debugEnv := os.Getenv("GOHAN_DEBUG")
	debugEnv = strings.TrimSpace(debugEnv)
	debugEnv = strings.ToLower(debugEnv)

	debug = debugEnv == "1" || debugEnv == "t" || debugEnv == "y" || debugEnv == "on" || debugEnv == "yes" || debugEnv == "true"
}

// print prints debug information (when enabled).
func print(s string) {
	if !debug {
		return
	}

	log.Println(s)
}

func attachEntitiesToSystem(system System) {
	// This function is always called on a newly added system.
	systemID := len(gameSystemEntities) - 1

	for entity := EntityID(0); entity < nextEntityID; entity++ {
		if system.Matches(entity) {
			gameSystemEntities[systemID] = append(gameSystemEntities[systemID], entity)

			print(fmt.Sprintf("Attached entity %d to system %d.", entity, systemID))
		}
	}
}

// RegisterSystem registers a system to start receiving Update and Draw calls.
func RegisterSystem(system System) {
	gameSystems = append(gameSystems, system)
	gameSystemReceivesUpdate = append(gameSystemReceivesUpdate, true)
	gameSystemReceivesDraw = append(gameSystemReceivesDraw, true)
	gameSystemEntities = append(gameSystemEntities, nil)

	attachEntitiesToSystem(system)
}

func updateSystem(i int) (int, error) {
	updated := 0
	for _, entity := range gameSystemEntities[i] {
		err := gameSystems[i].Update(entity)
		if err != nil {
			if err == ErrSystemWithoutUpdate {
				// Unregister system from Update events.
				gameSystemReceivesUpdate[i] = false
				return 0, nil
			}
			return 0, fmt.Errorf("failed to update system %d for entity %d: %s", i, entity, err)
		}
		updated++
	}
	return updated, nil
}

// Update updates the game state.
func Update() error {
	var t time.Time
	if debug {
		t = time.Now()
	}
	var systems int
	for i, registered := range gameSystemReceivesUpdate {
		if !registered {
			continue
		}

		updated, err := updateSystem(i)
		if err != nil {
			return err
		}

		print(fmt.Sprintf("System %d: updated %d entities.", i, updated))
		systems++
	}
	if debug {
		print(fmt.Sprintf("Finished updating %d systems in %.2fms.", systems, float64(time.Since(t).Microseconds())/1000))
	}
	return nil
}

// Draw draws the game on to the screen.
func Draw(screen *ebiten.Image) error {
DRAWSYSTEMS:
	for i, registered := range gameSystemReceivesDraw {
		if !registered {
			continue
		}

		for _, entity := range gameSystemEntities[i] {
			err := gameSystems[i].Draw(entity, screen)
			if err != nil {
				if err == ErrSystemWithoutDraw {
					// Unregister system from Draw events.
					gameSystemReceivesDraw[i] = false
					continue DRAWSYSTEMS
				}
				return fmt.Errorf("failed to draw system %d for entity %d: %s", i, entity, err)
			}
		}
	}
	return nil
}