Initial commit

2 years ago · 33ec0b1d86
commit 33ec0b1d86
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--- a/21
+++ b/21
@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2021 Trevor Slocum <trevor@rocketnine.space>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
--- a/README.md
+++ b/README.md
@ -0,0 +1,16 @@
+# gohan
+[![GoDoc](https://code.rocketnine.space/tslocum/godoc-static/raw/branch/master/badge.svg)](https://docs.rocketnine.space/code.rocketnine.space/tslocum/gohan)
+[![Donate](https://img.shields.io/liberapay/receives/rocketnine.space.svg?logo=liberapay)](https://liberapay.com/rocketnine.space)
+
+[Entity component system](https://en.wikipedia.org/wiki/Entity_component_system) framework for [Ebiten](https://ebiten.org)
+
+**Note:** This framework is still in development.  Breaking changes may be
+made until v1.0 is released.
+
+## Documentation
+
+Documentation is available via [godoc](https://docs.rocketnine.space/code.rocketnine.space/tslocum/gohan).
+
+## Support
+
+Please share issues and suggestions [here](https://code.rocketnine.space/tslocum/gohan/issues).
--- a/component.go
+++ b/component.go
@ -0,0 +1,67 @@
+package gohan
+
+import "fmt"
+
+// ComponentID is a component identifier. Each Component is assigned a unique ID
+// via NextComponentID, and implements a ComponentID method which returns the ID.
+type ComponentID int
+
+// Component represents data for an entity, and how it interacts with the world.
+type Component interface {
+	ComponentID() ComponentID
+}
+
+var nextComponentID ComponentID
+
+// NextComponentID returns the next available ComponentID.
+func NextComponentID() ComponentID {
+	id := nextComponentID
+	nextComponentID++
+	return id
+}
+
+func (entity EntityID) propagateChanges() {
+	for i, system := range gameSystems {
+		systemEntityIndex := -1
+		for j, systemEntity := range gameSystemEntities[i] {
+			if systemEntity == entity {
+				systemEntityIndex = j
+				break
+			}
+		}
+
+		if system.Matches(entity) {
+			if systemEntityIndex != -1 {
+				// Already attached.
+				continue
+			}
+
+			gameSystemEntities[i] = append(gameSystemEntities[i], entity)
+			print(fmt.Sprintf("Attached entity %d to system %d.", entity, i))
+		} else if systemEntityIndex != -1 {
+			// Detach from system.
+			gameSystemEntities[i] = append(gameSystemEntities[i][:systemEntityIndex], gameSystemEntities[i][systemEntityIndex+1:]...)
+		}
+	}
+}
+
+// AddComponent adds a Component to an Entity.
+func (entity EntityID) AddComponent(component Component) {
+	componentID := component.ComponentID()
+
+	if gameComponents[entity] == nil {
+		gameComponents[entity] = make(map[ComponentID]interface{})
+	}
+	gameComponents[entity][componentID] = component
+
+	entity.propagateChanges()
+}
+
+// Component gets a Component of an Entity.
+func (entity EntityID) Component(componentID ComponentID) interface{} {
+	components := gameComponents[entity]
+	if components == nil {
+		return nil
+	}
+	return components[componentID]
+}
--- a/doc.go
+++ b/doc.go
@ -0,0 +1,4 @@
+/*
+Package gohan provides an Entity Component System framework for Ebiten.
+*/
+package gohan
--- a/entity.go
+++ b/entity.go
@ -0,0 +1,13 @@
+package gohan
+
+// EntityID is an entity identifier.
+type EntityID int
+
+var nextEntityID EntityID
+
+// NextEntityID returns the next available EntityID.
+func NextEntityID() EntityID {
+	entityID := nextEntityID
+	nextEntityID++
+	return entityID
+}
--- a/examples/twinstick/asset/asset.go
+++ b/examples/twinstick/asset/asset.go
@ -0,0 +1,8 @@
+//go:build example
+// +build example
+
+package asset
+
+import "github.com/hajimehoshi/ebiten/v2"
+
+var ImgWhiteSquare = ebiten.NewImage(32, 32)
--- a/examples/twinstick/component/bullet.go
+++ b/examples/twinstick/component/bullet.go
@ -0,0 +1,17 @@
+//go:build example
+// +build example
+
+package component
+
+import (
+	"code.rocketnine.space/tslocum/gohan"
+)
+
+type BulletComponent struct {
+}
+
+var BulletComponentID = gohan.NextComponentID()
+
+func (p *BulletComponent) ComponentID() gohan.ComponentID {
+	return BulletComponentID
+}
--- a/examples/twinstick/component/position.go
+++ b/examples/twinstick/component/position.go
@ -0,0 +1,18 @@
+//go:build example
+// +build example
+
+package component
+
+import (
+	"code.rocketnine.space/tslocum/gohan"
+)
+
+type PositionComponent struct {
+	X, Y float64
+}
+
+var PositionComponentID = gohan.NextComponentID()
+
+func (p *PositionComponent) ComponentID() gohan.ComponentID {
+	return PositionComponentID
+}
--- a/examples/twinstick/component/velocity.go
+++ b/examples/twinstick/component/velocity.go
@ -0,0 +1,18 @@
+//go:build example
+// +build example
+
+package component
+
+import (
+	"code.rocketnine.space/tslocum/gohan"
+)
+
+type VelocityComponent struct {
+	X, Y float64
+}
+
+var VelocityComponentID = gohan.NextComponentID()
+
+func (c *VelocityComponent) ComponentID() gohan.ComponentID {
+	return VelocityComponentID
+}
--- a/examples/twinstick/component/weapon.go
+++ b/examples/twinstick/component/weapon.go
@ -0,0 +1,27 @@
+//go:build example
+// +build example
+
+package component
+
+import (
+	"time"
+
+	"code.rocketnine.space/tslocum/gohan"
+)
+
+type WeaponComponent struct {
+	Ammo int
+
+	Damage int
+
+	FireRate time.Duration
+	LastFire time.Time
+
+	BulletSpeed float64
+}
+
+var WeaponComponentID = gohan.NextComponentID()
+
+func (p *WeaponComponent) ComponentID() gohan.ComponentID {
+	return WeaponComponentID
+}
--- a/examples/twinstick/entity/bullet.go
+++ b/examples/twinstick/entity/bullet.go
@ -0,0 +1,27 @@
+//go:build example
+// +build example
+
+package entity
+
+import (
+	"code.rocketnine.space/tslocum/gohan"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/component"
+)
+
+func NewBullet(x, y, xSpeed, ySpeed float64) gohan.EntityID {
+	bullet := gohan.NextEntityID()
+
+	bullet.AddComponent(&component.PositionComponent{
+		X: x,
+		Y: y,
+	})
+
+	bullet.AddComponent(&component.VelocityComponent{
+		X: xSpeed,
+		Y: ySpeed,
+	})
+
+	bullet.AddComponent(&component.BulletComponent{})
+
+	return bullet
+}
--- a/examples/twinstick/entity/player.go
+++ b/examples/twinstick/entity/player.go
@ -0,0 +1,36 @@
+//go:build example
+// +build example
+
+package entity
+
+import (
+	"math"
+	"time"
+
+	"code.rocketnine.space/tslocum/gohan"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/component"
+)
+
+func NewPlayer() gohan.EntityID {
+	player := gohan.NextEntityID()
+
+	// Set position to -1,-1 to indicate the player has not been assigned a
+	// position yet. We will place the player in the center of the screen when
+	// we receive the screen dimensions for the first time.
+	player.AddComponent(&component.PositionComponent{
+		X: -1,
+		Y: -1,
+	})
+
+	player.AddComponent(&component.VelocityComponent{})
+
+	weapon := &component.WeaponComponent{
+		Ammo:        math.MaxInt64,
+		Damage:      1,
+		FireRate:    100 * time.Millisecond,
+		BulletSpeed: 15,
+	}
+	player.AddComponent(weapon)
+
+	return player
+}
--- a/examples/twinstick/game/game.go
+++ b/examples/twinstick/game/game.go
@ -0,0 +1,113 @@
+//go:build example
+// +build example
+
+package game
+
+import (
+	"image/color"
+	"os"
+	"sync"
+
+	"code.rocketnine.space/tslocum/gohan"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/asset"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/component"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/entity"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/system"
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+// game is an isometric demo game.
+type game struct {
+	w, h int
+
+	player gohan.EntityID
+
+	op *ebiten.DrawImageOptions
+
+	disableEsc bool
+
+	debugMode  bool
+	cpuProfile *os.File
+
+	movementSystem *system.MovementSystem
+
+	sync.Mutex
+}
+
+// NewGame returns a new isometric demo game.
+func NewGame() (*game, error) {
+	g := &game{
+		op: &ebiten.DrawImageOptions{},
+	}
+
+	g.addSystems()
+
+	err := g.loadAssets()
+	if err != nil {
+		return nil, err
+	}
+
+	g.player = entity.NewPlayer()
+
+	asset.ImgWhiteSquare.Fill(color.White)
+
+	return g, nil
+}
+
+// Layout is called when the game's layout changes.
+func (g *game) Layout(outsideWidth, outsideHeight int) (int, int) {
+	s := ebiten.DeviceScaleFactor()
+	w, h := int(s*float64(outsideWidth)), int(s*float64(outsideHeight))
+	if w != g.w || h != g.h {
+		g.w, g.h = w, h
+		g.movementSystem.ScreenW, g.movementSystem.ScreenH = float64(w), float64(h)
+
+		position := g.player.Component(component.PositionComponentID).(*component.PositionComponent)
+		if position.X == -1 && position.Y == -1 {
+			position.X, position.Y = float64(g.w)/2-16, float64(g.h)/2-16
+		}
+	}
+	return g.w, g.h
+}
+
+func (g *game) Update() error {
+	if ebiten.IsWindowBeingClosed() {
+		g.Exit()
+		return nil
+	}
+
+	return gohan.Update()
+}
+
+func (g *game) Draw(screen *ebiten.Image) {
+	err := gohan.Draw(screen)
+	if err != nil {
+		panic(err)
+	}
+}
+
+func (g *game) addSystems() {
+	gohan.AddSystem(system.NewMovementInputSystem(g.player))
+
+	g.movementSystem = &system.MovementSystem{}
+	gohan.AddSystem(g.movementSystem)
+
+	gohan.AddSystem(system.NewFireInputSystem(g.player))
+
+	renderBullet := system.NewDrawBulletsSystem()
+	gohan.AddSystem(renderBullet)
+
+	renderPlayer := system.NewDrawPlayerSystem(g.player)
+	gohan.AddSystem(renderPlayer)
+
+	printInfo := system.NewPrintInfoSystem(g.player)
+	gohan.AddSystem(printInfo)
+}
+
+func (g *game) loadAssets() error {
+	return nil
+}
+
+func (g *game) Exit() {
+	os.Exit(0)
+}
--- a/examples/twinstick/main.go
+++ b/examples/twinstick/main.go
@ -0,0 +1,45 @@
+//go:build example
+// +build example
+
+package main
+
+import (
+	"log"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/game"
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+func main() {
+	ebiten.SetWindowTitle("Twin-Stick Shooter Example - Gohan")
+	ebiten.SetWindowResizable(true)
+	ebiten.SetFullscreen(true)
+	ebiten.SetMaxTPS(144)
+	ebiten.SetRunnableOnUnfocused(true)
+	ebiten.SetWindowClosingHandled(true)
+	ebiten.SetFPSMode(ebiten.FPSModeVsyncOn)
+	ebiten.SetCursorShape(ebiten.CursorShapeCrosshair)
+
+	g, err := game.NewGame()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	sigc := make(chan os.Signal, 1)
+	signal.Notify(sigc,
+		syscall.SIGINT,
+		syscall.SIGTERM)
+	go func() {
+		<-sigc
+
+		g.Exit()
+	}()
+
+	err = ebiten.RunGame(g)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
--- a/examples/twinstick/system/draw_bullets.go
+++ b/examples/twinstick/system/draw_bullets.go
@ -0,0 +1,43 @@
+//go:build example
+// +build example
+
+package system
+
+import (
+	"code.rocketnine.space/tslocum/gohan"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/asset"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/component"
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+type DrawBulletsSystem struct {
+	op *ebiten.DrawImageOptions
+}
+
+func NewDrawBulletsSystem() *DrawBulletsSystem {
+	return &DrawBulletsSystem{
+		op: &ebiten.DrawImageOptions{},
+	}
+}
+
+func (s *DrawBulletsSystem) Matches(entity gohan.EntityID) bool {
+	position := entity.Component(component.PositionComponentID)
+	bullet := entity.Component(component.BulletComponentID)
+
+	return position != nil && bullet != nil
+}
+
+func (s *DrawBulletsSystem) Update(_ gohan.EntityID) error {
+	return gohan.ErrSystemWithoutUpdate
+}
+
+func (s *DrawBulletsSystem) Draw(entity gohan.EntityID, screen *ebiten.Image) error {
+	position := entity.Component(component.PositionComponentID).(*component.PositionComponent)
+
+	s.op.GeoM.Reset()
+	s.op.GeoM.Translate(-16, -16)
+	s.op.GeoM.Scale(0.5, 0.5)
+	s.op.GeoM.Translate(position.X, position.Y)
+	screen.DrawImage(asset.ImgWhiteSquare, s.op)
+	return nil
+}
--- a/examples/twinstick/system/draw_player.go
+++ b/examples/twinstick/system/draw_player.go
@ -0,0 +1,40 @@
+//go:build example
+// +build example
+
+package system
+
+import (
+	"code.rocketnine.space/tslocum/gohan"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/asset"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/component"
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+type drawPlayerSystem struct {
+	player gohan.EntityID
+	op     *ebiten.DrawImageOptions
+}
+
+func NewDrawPlayerSystem(player gohan.EntityID) *drawPlayerSystem {
+	return &drawPlayerSystem{
+		player: player,
+		op:     &ebiten.DrawImageOptions{},
+	}
+}
+
+func (s *drawPlayerSystem) Matches(entity gohan.EntityID) bool {
+	return entity == s.player
+}
+
+func (s *drawPlayerSystem) Update(_ gohan.EntityID) error {
+	return gohan.ErrSystemWithoutUpdate
+}
+
+func (s *drawPlayerSystem) Draw(entity gohan.EntityID, screen *ebiten.Image) error {
+	position := entity.Component(component.PositionComponentID).(*component.PositionComponent)
+
+	s.op.GeoM.Reset()
+	s.op.GeoM.Translate(position.X-16, position.Y-16)
+	screen.DrawImage(asset.ImgWhiteSquare, s.op)
+	return nil
+}
--- a/examples/twinstick/system/input_fire.go
+++ b/examples/twinstick/system/input_fire.go
@ -0,0 +1,90 @@
+//go:build example
+// +build example
+
+package system
+
+import (
+	"math"
+	"time"
+
+	"code.rocketnine.space/tslocum/gohan"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/component"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/entity"
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+func angle(x1, y1, x2, y2 float64) float64 {
+	return math.Atan2(y1-y2, x1-x2)
+}
+
+type fireInputSystem struct {
+	player gohan.EntityID
+}
+
+func NewFireInputSystem(player gohan.EntityID) *fireInputSystem {
+	return &fireInputSystem{
+		player: player,
+	}
+}
+
+func (_ *fireInputSystem) Matches(e gohan.EntityID) bool {
+	weapon := e.Component(component.WeaponComponentID)
+
+	return weapon != nil
+}
+
+func (s *fireInputSystem) fire(weapon *component.WeaponComponent, position *component.PositionComponent, fireAngle float64) {
+	if time.Since(weapon.LastFire) < weapon.FireRate {
+		return
+	}
+
+	weapon.Ammo--
+	weapon.LastFire = time.Now()
+
+	speedX := math.Cos(fireAngle) * -weapon.BulletSpeed
+	speedY := math.Sin(fireAngle) * -weapon.BulletSpeed
+
+	bullet := entity.NewBullet(position.X, position.Y, speedX, speedY)
+	_ = bullet
+}
+
+func (s *fireInputSystem) Update(_ gohan.EntityID) error {
+	weapon := s.player.Component(component.WeaponComponentID).(*component.WeaponComponent)
+
+	if weapon.Ammo <= 0 {
+		return nil
+	}
+
+	position := s.player.Component(component.PositionComponentID).(*component.PositionComponent)
+
+	if ebiten.IsMouseButtonPressed(ebiten.MouseButtonLeft) {
+		cursorX, cursorY := ebiten.CursorPosition()
+		fireAngle := angle(position.X, position.Y, float64(cursorX), float64(cursorY))
+		s.fire(weapon, position, fireAngle)
+	}
+
+	switch {
+	case ebiten.IsKeyPressed(ebiten.KeyLeft) && ebiten.IsKeyPressed(ebiten.KeyUp):
+		s.fire(weapon, position, math.Pi/4)
+	case ebiten.IsKeyPressed(ebiten.KeyLeft) && ebiten.IsKeyPressed(ebiten.KeyDown):
+		s.fire(weapon, position, -math.Pi/4)
+	case ebiten.IsKeyPressed(ebiten.KeyRight) && ebiten.IsKeyPressed(ebiten.KeyUp):
+		s.fire(weapon, position, math.Pi*.75)
+	case ebiten.IsKeyPressed(ebiten.KeyRight) && ebiten.IsKeyPressed(ebiten.KeyDown):
+		s.fire(weapon, position, -math.Pi*.75)
+	case ebiten.IsKeyPressed(ebiten.KeyLeft):
+		s.fire(weapon, position, 0)
+	case ebiten.IsKeyPressed(ebiten.KeyRight):
+		s.fire(weapon, position, math.Pi)
+	case ebiten.IsKeyPressed(ebiten.KeyUp):
+		s.fire(weapon, position, math.Pi/2)
+	case ebiten.IsKeyPressed(ebiten.KeyDown):
+		s.fire(weapon, position, -math.Pi/2)
+	}
+
+	return nil
+}
+
+func (_ *fireInputSystem) Draw(_ gohan.EntityID, _ *ebiten.Image) error {
+	return gohan.ErrSystemWithoutDraw
+}
--- a/examples/twinstick/system/input_move.go
+++ b/examples/twinstick/system/input_move.go
@ -0,0 +1,57 @@
+//go:build example
+// +build example
+
+package system
+
+import (
+	"code.rocketnine.space/tslocum/gohan"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/component"
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+type movementInputSystem struct {
+	player gohan.EntityID
+}
+
+func NewMovementInputSystem(player gohan.EntityID) *movementInputSystem {
+	return &movementInputSystem{
+		player: player,
+	}
+}
+
+func (s *movementInputSystem) Matches(e gohan.EntityID) bool {
+	return e == s.player
+}
+
+func (s *movementInputSystem) Update(e gohan.EntityID) error {
+	velocity := s.player.Component(component.VelocityComponentID).(*component.VelocityComponent)
+	if ebiten.IsKeyPressed(ebiten.KeyA) {
+		velocity.X -= 0.5
+		if velocity.X < -5 {
+			velocity.X = -5
+		}
+	}
+	if ebiten.IsKeyPressed(ebiten.KeyD) {
+		velocity.X += 0.5
+		if velocity.X > 5 {
+			velocity.X = 5
+		}
+	}
+	if ebiten.IsKeyPressed(ebiten.KeyW) {
+		velocity.Y -= 0.5
+		if velocity.Y < -5 {
+			velocity.Y = -5
+		}
+	}
+	if ebiten.IsKeyPressed(ebiten.KeyS) {
+		velocity.Y += 0.5
+		if velocity.Y > 5 {
+			velocity.Y = 5
+		}
+	}
+	return nil
+}
+
+func (s *movementInputSystem) Draw(_ gohan.EntityID, _ *ebiten.Image) error {
+	return gohan.ErrSystemWithoutDraw
+}
--- a/examples/twinstick/system/movement.go
+++ b/examples/twinstick/system/movement.go
@ -0,0 +1,59 @@
+//go:build example
+// +build example
+
+package system
+
+import (
+	"code.rocketnine.space/tslocum/gohan"
+	"code.rocketnine.space/tslocum/gohan/_examples/twinstick/component"
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+type MovementSystem struct {
+	ScreenW, ScreenH float64
+}
+
+func (_ *MovementSystem) Matches(entity gohan.EntityID) bool {
+	position := entity.Component(component.PositionComponentID)
+	velocity := entity.Component(component.VelocityComponentID)
+
+	return position != nil && velocity != nil
+}
+
+func (s *MovementSystem) Update(entity gohan.EntityID) error {
+	position := entity.Component(component.PositionComponentID).(*component.PositionComponent)
+	velocity := entity.Component(component.VelocityComponentID).(*component.VelocityComponent)
+
+	bullet := entity.Component(component.BulletComponentID)
+
+	// Check for collision.
+	if bullet == nil {
+		if position.X+velocity.X < 16 {
+			position.X = 16
+			velocity.X = 0
+		} else if position.X+velocity.X > s.ScreenW-16 {
+			position.X = s.ScreenW - 16
+			velocity.X = 0
+		}
+		if position.Y+velocity.Y < 16 {
+			position.Y = 16
+			velocity.Y = 0
+		} else if position.Y+velocity.Y > s.ScreenH-16 {
+			position.Y = s.ScreenH - 16
+			velocity.Y = 0
+		}
+	}
+
+	position.X, position.Y = position.X+velocity.X, position.Y+velocity.Y
+
+	if bullet == nil {
+		velocity.X *= 0.95
+		velocity.Y *= 0.95
+	}
+
+	return nil
+}
+
+func (_ *MovementSystem) Draw(entity gohan.EntityID, screen *ebiten.Image) error {
+	return gohan.ErrSystemWithoutDraw
+}
--- a/examples/twinstick/system/printinfo.go
+++ b/examples/twinstick/system/printinfo.go
@ -0,0 +1,43 @@
+//go:build example
+// +build example
+
+package system
+
+import (
+	"fmt"
+
+	"code.rocketnine.space/tslocum/gohan"
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+)
+
+type printInfoSystem struct {
+	img    *ebiten.Image
+	op     *ebiten.DrawImageOptions
+	player gohan.EntityID
+}
+
+func NewPrintInfoSystem(player gohan.EntityID) *printInfoSystem {
+	p := &printInfoSystem{
+		img:    ebiten.NewImage(200, 100),
+		op:     &ebiten.DrawImageOptions{},
+		player: player,
+	}
+	p.op.GeoM.Scale(2, 2)
+	return p
+}
+
+func (s *printInfoSystem) Matches(e gohan.EntityID) bool {
+	return e == s.player
+}
+
+func (s *printInfoSystem) Update(_ gohan.EntityID) error {
+	return gohan.ErrSystemWithoutUpdate
+}
+
+func (s *printInfoSystem) Draw(entity gohan.EntityID, screen *ebiten.Image) error {
+	s.img.Clear()
+	ebitenutil.DebugPrint(s.img, fmt.Sprintf("KEY  WASD+MOUSE\nTPS  %0.0f\nFPS  %0.0f", ebiten.CurrentTPS(), ebiten.CurrentFPS()))
+	screen.DrawImage(s.img, s.op)
+	return nil
+}
--- a/go.mod
+++ b/go.mod
@ -0,0 +1,15 @@
+module code.rocketnine.space/tslocum/gohan
+
+go 1.17
+
+require github.com/hajimehoshi/ebiten/v2 v2.2.2
+
+require (
+	github.com/go-gl/glfw/v3.3/glfw v0.0.0-20210727001814-0db043d8d5be // indirect
+	github.com/jezek/xgb v0.0.0-20210312150743-0e0f116e1240 // indirect
+	golang.org/x/exp v0.0.0-20190731235908-ec7cb31e5a56 // indirect
+	golang.org/x/image v0.0.0-20210628002857-a66eb6448b8d // indirect
+	golang.org/x/mobile v0.0.0-20210902104108-5d9a33257ab5 // indirect
+	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c // indirect
+	golang.org/x/sys v0.0.0-20210917161153-d61c044b1678 // indirect
+)
--- a/go.sum
+++ b/go.sum
@ -0,0 +1,66 @@
+github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802/go.mod h1:IVnqGOEym/WlBOVXweHU+Q+/VP0lqqI8lqeDx9IjBqo=
+github.com/go-gl/glfw/v3.3/glfw v0.0.0-20210727001814-0db043d8d5be h1:vEIVIuBApEBQTEJt19GfhoU+zFSV+sNTa9E9FdnRYfk=
+github.com/go-gl/glfw/v3.3/glfw v0.0.0-20210727001814-0db043d8d5be/go.mod h1:tQ2UAYgL5IevRw8kRxooKSPJfGvJ9fJQFa0TUsXzTg8=
+github.com/hajimehoshi/bitmapfont/v2 v2.1.3/go.mod h1:2BnYrkTQGThpr/CY6LorYtt/zEPNzvE/ND69CRTaHMs=
+github.com/hajimehoshi/ebiten/v2 v2.2.2 h1:92E+ogdNyH1P/LlvMQ7vonbFDh6bl+O7Ak+H1HX0RX8=
+github.com/hajimehoshi/ebiten/v2 v2.2.2/go.mod h1:olKl/qqhMBBAm2oI7Zy292nCtE+nitlmYKNF3UpbFn0=
+github.com/hajimehoshi/file2byteslice v0.0.0-20210813153925-5340248a8f41/go.mod h1:CqqAHp7Dk/AqQiwuhV1yT2334qbA/tFWQW0MD2dGqUE=
+github.com/hajimehoshi/go-mp3 v0.3.2/go.mod h1:qMJj/CSDxx6CGHiZeCgbiq2DSUkbK0UbtXShQcnfyMM=
+github.com/hajimehoshi/oto v0.6.1/go.mod h1:0QXGEkbuJRohbJaxr7ZQSxnju7hEhseiPx2hrh6raOI=
+github.com/hajimehoshi/oto/v2 v2.1.0-alpha.2/go.mod h1:rUKQmwMkqmRxe+IAof9+tuYA2ofm8cAWXFmSfzDN8vQ=
+github.com/jakecoffman/cp v1.1.0/go.mod h1:JjY/Fp6d8E1CHnu74gWNnU0+b9VzEdUVPoJxg2PsTQg=
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+github.com/jfreymuth/oggvorbis v1.0.3/go.mod h1:1U4pqWmghcoVsCJJ4fRBKv9peUJMBHixthRlBeD6uII=
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--- a/gohan.go
+++ b/gohan.go
@ -0,0 +1,129 @@
+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
+}
--- a/system.go
+++ b/system.go
@ -0,0 +1,33 @@
+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 EntityID) bool
+
+	// Update is called once for each matching entity each time the game state is updated.
+	Update(entity EntityID) error
+
+	// Draw is called once for each matching entity each time the game is drawn to the screen.
+	Draw(entity EntityID, 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")
+)