package gohan

import (
	"fmt"
	"log"
	"os"
	"reflect"
	"strconv"
	"strings"
	"sync"
	"time"

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

var debug int

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

	i, err := strconv.Atoi(debugEnv)
	if err == nil {
		debug = i
		return
	}

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

var w = newWorld()

// world represents a collection of AllEntities, components and Systems.
type world struct {
	maxEntityID Entity

	maxComponentID componentID

	components [][]interface{} // components[Entity][componentID]Component

	allEntities []Entity

	modifiedEntities []Entity
	removedEntities  []Entity

	handledModifiedEntities map[Entity]bool

	// availableEntities is the set of EntityIDs available because they were
	// removed from the game.
	availableEntities []Entity

	systems               []System
	systemEntities        [][]Entity        // Slice of entities matching each system.
	systemNeeds           [][]componentID   // Slice of ComponentIDs needed by each system.
	systemUses            [][]componentID   // Slice of ComponentIDs used by each system.
	systemComponentIDs    [][]componentID   // Slice of ComponentIDs needed or used by each system.
	systemComponentFields [][]reflect.Value // Slice of component struct fields used by each system.

	systemReceivesUpdate []bool
	systemReceivesDraw   []bool

	systemUpdatedEntities  int
	systemUpdatedEntitiesV int
	systemUpdatedEntitiesT time.Time

	systemDrawnEntities  int
	systemDrawnEntitiesV int
	systemDrawnEntitiesT time.Time

	systemComponentNames    []string
	haveSystemComponentName map[string]bool

	cacheTime time.Duration

	entityMutex    sync.Mutex
	componentMutex sync.Mutex

	sync.Mutex
}

// NewWorld returns a new world.
func newWorld() *world {
	w := &world{
		cacheTime: time.Second,

		handledModifiedEntities: make(map[Entity]bool),
		haveSystemComponentName: make(map[string]bool),
	}

	// Pad slices to match IDs starting with 1.
	w.components = append(w.components, nil)
	w.systemComponentNames = append(w.systemComponentNames, "")

	return w
}

// AddSystem registers a system to start receiving Update and Draw calls.
func AddSystem(system System) {
	w.Lock()
	defer w.Unlock()

	systemIndex := len(w.systems)

	w.systems = append(w.systems, system)
	w.systemReceivesUpdate = append(w.systemReceivesUpdate, true)
	w.systemReceivesDraw = append(w.systemReceivesDraw, true)
	w.systemEntities = append(w.systemEntities, nil)
	w.systemComponentFields = append(w.systemComponentFields, nil)

	w.entityMutex.Lock()
	defer w.entityMutex.Unlock()
	w.modifiedEntities = append(w.modifiedEntities, w.allEntities...)

	sV := reflect.ValueOf(system)
	sT := reflect.TypeOf(system)
	if sV.Kind() == reflect.Ptr {
		sV = sV.Elem()
		sT = sT.Elem()
	}
	if sV.Kind() != reflect.Struct {
		panic("system must be a struct type")
	}

	var usedComponentIDs []componentID
	var neededComponentIDs []componentID
	w.systemComponentIDs = append(w.systemComponentIDs, nil)
	for i := 0; i < sT.NumField(); i++ {
		field := sV.Field(i)

		if !field.CanSet() {
			continue
		}

		tag := sT.Field(i).Tag.Get("gohan")
		if tag == "-" {
			continue
		}

		//log.Println("SET FIELD", systemIndex, field.String(), tag, field.CanSet())
		w.systemComponentFields[systemIndex] = append(w.systemComponentFields[systemIndex], field)

		id := componentIDByName(field.Type().String())
		if tag == "?" {
			usedComponentIDs = append(usedComponentIDs, id)
		} else {
			neededComponentIDs = append(neededComponentIDs, id)
		}

		w.systemComponentIDs[systemIndex] = append(w.systemComponentIDs[systemIndex], id)
	}

	w.systemNeeds = append(w.systemNeeds, neededComponentIDs)
	w.systemUses = append(w.systemUses, usedComponentIDs)
}

/*
// AddSystemAfter registers a system to start receiving Update and Draw calls
// after the specified system (or systems) are called first.
func AddSystemAfter(system System, after ...System) {
	gameSystems = append(gameSystems, system)
	gameSystemReceivesUpdate = append(gameSystemReceivesUpdate, true)
	gameSystemReceivesDraw = append(gameSystemReceivesDraw, true)
	gameSystemEntities = append(gameSystemEntities, nil)

	attachEntitiesToSystem(system)
}
*/

func (w *world) setSystemComponentFields(e Entity, i int) {
	//log.Println(len(w.systemComponentFields[i]))
	//log.Println(w.systemComponentFields[i])
	for j, field := range w.systemComponentFields[i] {
		//log.Println(j, field, field.String())
		id := w.systemComponentIDs[i][j]
		//log.Println("SYSTEM", i, "FIELD", j, "ID", id)
		if w.components[e][id] == nil {
			field.Set(reflect.Zero(field.Type()))
		} else {
			field.Set(reflect.ValueOf(w.components[e][id]))
		}
	}
}

func (w *world) updateSystem(i int) (int, error) {
	w.propagateEntityChanges()

	updated := 0
	for _, entity := range w.systemEntities[i] {
		w.setSystemComponentFields(entity, i)

		err := w.systems[i].Update(entity)
		if err != nil {
			if err == ErrUnregister {
				// Unregister system from Update events.
				w.systemReceivesUpdate[i] = false
				return 0, nil
			}
			return 0, fmt.Errorf("failed to update %s for entity %d: %+v", w.systemName(i), entity, err)
		}
		updated++
	}
	return updated, nil
}

func (w *world) _handleRemovedEntities() {
	for _, entity := range w.removedEntities {
		// Remove from attached systems.
	REMOVED:
		for i := range w.systemEntities {
			for j, e := range w.systemEntities[i] {
				if e == entity {
					w.systemEntities[i] = _removeAt(w.systemEntities[i], j)
					continue REMOVED
				}
			}
		}
	}

	// Mark EntityID as available.
	w.availableEntities = append(w.availableEntities, w.removedEntities...)

	w.removedEntities = w.removedEntities[:0]
}

// _handleModifiedEntities handles changes to entity components by attaching
// and detaching modified entities from affected systems.
func (w *world) _handleModifiedEntities() {
	if len(w.modifiedEntities) == 0 {
		return
	}

	for _, entity := range w.modifiedEntities {
		if w.handledModifiedEntities[entity] {
			continue
		}
		w.handledModifiedEntities[entity] = true

		for i := range w.systems {
			systemEntityIndex := -1
			for j, systemEntity := range w.systemEntities[i] {
				if systemEntity == entity {
					systemEntityIndex = j
					break
				}
			}

			var skip bool
			for _, componentID := range w.systemNeeds[i] {
				c := entity.getComponent(componentID)
				if c == nil {
					skip = true
					break
				}
			}
			if !skip {
				if systemEntityIndex != -1 {
					// Already attached.
					continue
				}

				w.systemEntities[i] = append(w.systemEntities[i], entity)

				if debug > 1 {
					log.Printf("Attached entity %d to %s.", entity, w.systemName(i))
				}
			} else if systemEntityIndex != -1 {
				// Detach from system.
				w.systemEntities[i] = _removeAt(w.systemEntities[i], systemEntityIndex)
			}
		}
	}

	for k := range w.handledModifiedEntities {
		delete(w.handledModifiedEntities, k)
	}

	w.modifiedEntities = w.modifiedEntities[:0]
}

func (w *world) propagateEntityChanges() {
	w.entityMutex.Lock()
	defer w.entityMutex.Unlock()

	w._handleRemovedEntities()
	w._handleModifiedEntities()
}

// Update updates the game state.
func Update() error {
	w.Lock()
	defer w.Unlock()

	var t time.Time
	if debug != 0 {
		t = time.Now()
	}

	var entitiesUpdated int
	for i, registered := range w.systemReceivesUpdate {
		if !registered {
			continue
		}

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

		entitiesUpdated += updated

		if debug != 0 {
			log.Printf("- %s: %d updated in %.2fms.", w.systemName(i), updated, float64(time.Since(t).Microseconds())/1000)
		}
	}
	w.systemUpdatedEntities = entitiesUpdated

	if debug != 0 {
		log.Printf("Handled %d entity updates in %.2fms.", entitiesUpdated, float64(time.Since(t).Microseconds())/1000)
	}
	return nil
}

// CurrentUpdates returns the number of System Update calls required to update
// the game state. Because entities may be handled by more than one System,
// this number may be higher than the number of active entities.
func CurrentUpdates() int {
	if time.Since(w.systemUpdatedEntitiesT) >= w.cacheTime {
		w.systemUpdatedEntitiesV = w.systemUpdatedEntities
		w.systemUpdatedEntitiesT = time.Now()
	}
	return w.systemUpdatedEntitiesV
}

func (w *world) drawSystem(i int, screen *ebiten.Image) (int, error) {
	w.propagateEntityChanges()

	var drawn int
	for _, entity := range w.systemEntities[i] {
		w.setSystemComponentFields(entity, i)

		err := w.systems[i].Draw(entity, screen)
		if err != nil {
			if err == ErrUnregister {
				// Unregister system from Draw events.
				w.systemReceivesDraw[i] = false
				return 0, nil
			}
			return 0, fmt.Errorf("failed to draw %s for entity %d: %+v", w.systemName(i), entity, err)
		}
		drawn++
	}
	return drawn, nil
}

// Draw draws the game on to the screen.
func Draw(screen *ebiten.Image) error {
	w.Lock()
	defer w.Unlock()

	var t time.Time
	if debug != 0 {
		t = time.Now()
	}

	var entitiesDrawn int
	for i, registered := range w.systemReceivesDraw {
		if !registered {
			continue
		}

		drawn, err := w.drawSystem(i, screen)
		if err != nil {
			return err
		}

		entitiesDrawn += drawn

		if debug != 0 {
			log.Printf("- %s: %d drawn in %.2fms.", w.systemName(i), drawn, float64(time.Since(t).Microseconds())/1000)
		}
	}
	w.systemDrawnEntities = entitiesDrawn

	if debug != 0 {
		log.Printf("Handled %d entity draws in %.2fms.", entitiesDrawn, float64(time.Since(t).Microseconds())/1000)
	}
	return nil
}

// CurrentDraws returns the number of System Draw calls required to draw the
// game on to the screen. Because entities may be handled by more than one
// System, this number may be higher than the number of active entities.
func CurrentDraws() int {
	if time.Since(w.systemDrawnEntitiesT) >= w.cacheTime {
		w.systemDrawnEntitiesV = w.systemDrawnEntities
		w.systemDrawnEntitiesT = time.Now()
	}
	return w.systemDrawnEntitiesV
}

// Preallocate creates and then immediately removes the specified number of entities.
// Because Gohan reuses removed entities, this has the effect of pre-allocating
// the memory used later to create entities normally. Pre-allocating enough
// entities to run your application after its systems has been added, but
// before any entities are created, will provide the greatest performance boost.
func Preallocate(entities int) {
	if len(w.availableEntities) >= entities {
		return
	}

	e := make([]Entity, entities)
	for i := 0; i < entities; i++ {
		e[i] = NewEntity()
	}
	for i := 0; i < entities; i++ {
		e[i].Remove()
	}
}

func uniqueComponentIDs(v []componentID) []componentID {
	var list []componentID
	keys := make(map[componentID]bool)
	for _, entry := range v {
		if _, value := keys[entry]; !value {
			keys[entry] = true
			list = append(list, entry)
		}
	}
	return list
}

func (w *world) componentName(id componentID) string {
	if int(id) < len(w.systemComponentNames) {
		return w.systemComponentNames[id]
	}
	return strconv.Itoa(int(id))
}

func (w *world) systemName(i int) string {
	if i < len(w.systems) {
		return getName(w.systems[i])
	}
	return strconv.Itoa(i)
}

func getName(v interface{}) string {
	t := reflect.TypeOf(v)
	if t.Kind() == reflect.Ptr {
		return strings.Title(t.Elem().Name())
	} else if t.Kind() == reflect.Struct {
		return strings.Title(t.Name())
	}
	return ""
}

// Reset removes all entities, components and systems.
func Reset() {
	old := w
	old.Lock()

	w = newWorld()

	old.Unlock()
}