package engine

import (
	"encoding/gob"
	"errors"
	"fmt"
	"io/fs"
	"reflect"
	"sync"

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

var _ interface {
	Disabler
	Hider
	Identifier
	Updater
	Scanner
} = &Game{}

var (
	errNilComponent = errors.New("nil component")
	errNilParent    = errors.New("nil parent")
)

func init() {
	gob.Register(&Game{})
}

// Game implements the ebiten methods using a collection of components. One
// component must be the designated root component - usually a scene of some
// kind.
type Game struct {
	Disabled
	Hidden
	ScreenWidth  int
	ScreenHeight int
	Root         DrawUpdater // typically a *Scene or SceneRef though

	dbmu sync.RWMutex
	byID map[string]Identifier              // Named components by ID
	byAB map[abKey]map[interface{}]struct{} // Ancestor/behaviour index
	par  map[interface{}]interface{}        // par[x] is parent of x
}

type abKey struct {
	ancestor  string
	behaviour reflect.Type
}

// Draw draws the entire thing, with default draw options.
func (g *Game) Draw(screen *ebiten.Image) {
	if g.Hidden {
		return
	}
	g.Root.Draw(screen, ebiten.DrawImageOptions{})
}

// Layout returns the configured screen width/height.
func (g *Game) Layout(outsideWidth, outsideHeight int) (w, h int) {
	return g.ScreenWidth, g.ScreenHeight
}

// Update updates the scene.
func (g *Game) Update() error {
	if g.Disabled {
		return nil
	}
	return g.Root.Update()
}

// Ident returns "__GAME__".
func (g *Game) Ident() string { return "__GAME__" }

// Component returns the component with a given ID, or nil if there is none.
// This only returns sensible values for registered components (e.g. after
// LoadAndPrepare).
func (g *Game) Component(id string) Identifier {
	g.dbmu.RLock()
	defer g.dbmu.RUnlock()
	return g.byID[id]
}

// Parent returns the parent of a given component, or nil if there is none.
// This only returns sensible values for registered components (e.g. after
// LoadAndPrepare).
func (g *Game) Parent(c interface{}) interface{} {
	g.dbmu.RLock()
	defer g.dbmu.RUnlock()
	return g.par[c]
}

// Query looks for components having both a given ancestor and implementing
// a given behaviour (see Behaviors in interface.go). This only returns sensible
// values after LoadAndPrepare. Note that every component is its own ancestor.
func (g *Game) Query(ancestorID string, behaviour reflect.Type) map[interface{}]struct{} {
	g.dbmu.RLock()
	defer g.dbmu.RUnlock()
	return g.byAB[abKey{ancestorID, behaviour}]
}

// Scan implements Scanner.
func (g *Game) Scan() []interface{} { return []interface{}{g.Root} }

// Walk calls visit with every component and its parent, reachable from the
// given component via Scan, for as long as visit returns nil. The parent of
// the first component (as passed to visit) will be nil.
func Walk(component interface{}, visit func(component, parent interface{}) error) error {
	return walk(component, nil, visit)
}

func walk(component, parent interface{}, visit func(component, parent interface{}) error) error {
	if err := visit(component, parent); err != nil {
		return err
	}
	sc, ok := component.(Scanner)
	if !ok {
		return nil
	}
	for _, c := range sc.Scan() {
		if err := walk(c, component, visit); err != nil {
			return err
		}
	}
	return nil
}

// LoadAndPrepare first calls Load on all Loaders. Once loading is complete, it
// builds the component databases and then calls Prepare on every Preparer.
// LoadAndPrepare must be called before any calls to Component or Query.
func (g *Game) LoadAndPrepare(assets fs.FS) error {
	// Load all the Loaders.
	if err := Walk(g, func(c, _ interface{}) error {
		l, ok := c.(Loader)
		if !ok {
			return nil
		}
		return l.Load(assets)
	}); err != nil {
		return err
	}

	// Build the component databases
	g.dbmu.Lock()
	g.byID = make(map[string]Identifier)
	g.byAB = make(map[abKey]map[interface{}]struct{})
	g.par = make(map[interface{}]interface{})
	if err := Walk(g, g.register); err != nil {
		return err
	}
	g.dbmu.Unlock()

	// Prepare all the Preppers
	for p := range g.Query(g.Ident(), PrepperType) {
		if err := p.(Prepper).Prepare(g); err != nil {
			return err
		}
	}
	return nil
}

// Register registers a component into the component database (as the
// child of a given parent). Passing a nil component or parent is an error.
// Registering multiple components with the same ID is also an error.
func (g *Game) Register(component, parent interface{}) error {
	if component == nil {
		return errNilComponent
	}
	if parent == nil && component != g {
		return errNilParent
	}
	g.dbmu.Lock()
	defer g.dbmu.Unlock()
	return g.register(component, parent)
}

func (g *Game) register(component, parent interface{}) error {
	// register in g.par
	if parent != nil {
		g.par[component] = parent
	}

	// register in g.byAB
	ct := reflect.TypeOf(component)
	for _, b := range Behaviours {
		if !ct.Implements(b) {
			continue
		}
		// TODO: better than O(len(path)^2) time and memory?
		for p := component; p != nil; p = g.par[p] {
			i, ok := p.(Identifier)
			if !ok {
				continue
			}
			k := abKey{i.Ident(), b}
			if g.byAB[k] == nil {
				g.byAB[k] = make(map[interface{}]struct{})
			}
			g.byAB[k][component] = struct{}{}
		}
	}

	// register in g.byID if needed
	i, ok := component.(Identifier)
	if !ok {
		return nil
	}
	id := i.Ident()
	if _, exists := g.byID[id]; exists {
		return fmt.Errorf("duplicate id %q", id)
	}
	g.byID[id] = i
	return nil
}

// Unregister removes the component from the component database.
// Passing a nil component has no effect.
func (g *Game) Unregister(component interface{}) {
	if component == nil {
		return
	}
	g.dbmu.Lock()
	g.unregister(component)
	g.dbmu.Unlock()
}

func (g *Game) unregister(component interface{}) {
	// unregister from g.byAB, using g.par to trace the path
	ct := reflect.TypeOf(component)
	for _, b := range Behaviours {
		if !ct.Implements(b) {
			continue
		}
		for p := component; p != nil; p = g.par[p] {
			i, ok := p.(Identifier)
			if !ok {
				continue
			}
			k := abKey{i.Ident(), b}
			if g.byAB[k] == nil {
				continue
			}
			delete(g.byAB[k], component)
		}
	}

	// unregister from g.par
	delete(g.par, component)

	// unregister from g.byID if needed
	i, ok := component.(Identifier)
	if !ok {
		return
	}
	delete(g.byID, i.Ident())
}