package gamemanager

import (
	"encoding/base64"
	"errors"
	"fmt"

	"google.golang.org/protobuf/encoding/protojson"
	"google.golang.org/protobuf/proto"
)

// GameManager contains configuration info about stories and managed GameState objects.
type GameManager struct {
	Book         StoryBook
	EngineConfig EngineConfig
}

// SpeculativeState is used to provide "what if" GameStates for next available actions.
type SpeculativeState struct {
	Description string
	StateToken  string
}

var (
	// ErrRoomNotFound occurs when the room doesn't exist in the given story.
	ErrRoomNotFound = errors.New("room not found")
	// ErrExitNotFound occurs when the exit doesn't exit in the given room.
	ErrExitNotFound = errors.New("exit not found")
)

// NewGameManager creates a new GameManager, using Stories in the Config.
func NewGameManager(book StoryBook, config EngineConfig) (*GameManager, error) {
	return &GameManager{
		Book:         book,
		EngineConfig: config,
	}, nil
}

// StartGame creates a new GameState representing the start of a given storyID.
func (gm *GameManager) StartGame(storyID string) (GameState, error) {
	startingScene, _ := gm.GetRoomByID(storyID, gm.Book.Stories[storyID].Metadata.StartingScene)

	return GameState{
		StoryID:     storyID,
		CurrentRoom: startingScene.ID,
		Conditions:  []string{},
	}, nil
}

// GetStories lists all StoryMetadata for stories tracked by GameManager.
func (gm *GameManager) GetStories() []StoryMetadata {
	metadata := make([]StoryMetadata, len(gm.Book.Stories))

	i := 0

	for _, e := range gm.Book.Stories {
		metadata[i] = e.Metadata
		i++
	}

	return metadata
}

// GetRoomByID finds Room information based on room and story IDs.
func (gm *GameManager) GetRoomByID(storyID string, roomID string) (Room, error) {
	for _, r := range gm.Book.Stories[storyID].Rooms {
		if r.ID == roomID {
			return r, nil
		}
	}

	return Room{}, fmt.Errorf("%w: %s", ErrRoomNotFound, roomID)
}

// DeserializeState converts a stateToken into a GameState.
func (gm *GameManager) DeserializeState(stateToken string) (GameState, error) {
	decodedString, decodeErr := base64.StdEncoding.DecodeString(stateToken)
	if decodeErr != nil {
		return GameState{}, fmt.Errorf("couldn't decode state token: %w", decodeErr)
	}

	var st StateToken

	switch gm.EngineConfig.StateTokenFormat {
	case JSON:
		if unmarshallErr := protojson.Unmarshal(decodedString, &st); unmarshallErr != nil {
			return GameState{}, fmt.Errorf("failed to load state: %w", unmarshallErr)
		}
	case Protobuf:
		if unmarshallErr := proto.Unmarshal(decodedString, &st); unmarshallErr != nil {
			return GameState{}, fmt.Errorf("failed to load state: %w", unmarshallErr)
		}
	default:
		return GameState{}, fmt.Errorf("%w: %d", ErrInvalidSerializationFormat, gm.EngineConfig.StateTokenFormat)
	}

	return GameState{
		StoryID:     st.StoryID,
		CurrentRoom: st.CurrentRoom,
		Conditions:  st.Conditions,
	}, nil
}

// SerializeState generates a stateToken based on the given GameState.
func (gm *GameManager) SerializeState(gameState GameState) (string, error) {
	st := StateToken{
		StoryID:     gameState.StoryID,
		CurrentRoom: gameState.CurrentRoom,
		Conditions:  gameState.Conditions,
	}

	switch gm.EngineConfig.StateTokenFormat {
	case Protobuf:
		stateBytes, err := proto.Marshal(&st)
		if err != nil {
			return "", fmt.Errorf("failed to save state: %w", err)
		}

		return base64.StdEncoding.EncodeToString(stateBytes), nil
	case JSON:
		stateBytes, err := protojson.Marshal(&st)
		if err != nil {
			return "", fmt.Errorf("failed to save state: %w", err)
		}

		return base64.StdEncoding.EncodeToString(stateBytes), nil
	default:
		return "", fmt.Errorf("%w: %d", ErrInvalidSerializationFormat, gm.EngineConfig.StateTokenFormat)
	}
}

// ConstructSpeculativeStates makes a SpeculativeState from the active GameState and actions available in currentRoom.
func (gm *GameManager) ConstructSpeculativeStates(gameState GameState, currentRoom Room) ([]SpeculativeState, error) {
	speculativeStates := make([]SpeculativeState, len(currentRoom.Exits))

	usableExits := currentRoom.filterExits(gameState)
	for i, e := range usableExits {
		newState := gameState.UseExit(e)

		token, serializeErr := gm.SerializeState(newState)
		if serializeErr != nil {
			return speculativeStates, fmt.Errorf("couldn't serialize constructed state: %w", serializeErr)
		}

		speculativeStates[i] = SpeculativeState{Description: e.Description, StateToken: token}
	}

	return speculativeStates, nil
}

// ConstructStartingState generates a starting stateToken for each loaded story.
func (gm *GameManager) ConstructStartingState(story StoryMetadata) (SpeculativeState, error) {
	startState, startErr := gm.StartGame(story.ID)
	if startErr != nil {
		return SpeculativeState{}, fmt.Errorf("could not generate starting state: %w", startErr)
	}

	startToken, serializeError := gm.SerializeState(startState)
	if serializeError != nil {
		return SpeculativeState{}, fmt.Errorf("could not serialize state: %w", serializeError)
	}

	return SpeculativeState{
		Description: fmt.Sprintf("%s by %s - %s", story.Name, story.Author, story.Description),
		StateToken:  startToken,
	}, nil
}