package auth

import (
	"context"
	"encoding/json"
	"fmt"
	"sync"

	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"

	"github.com/gochat/gochat/internal/config"
	"github.com/gochat/gochat/internal/model"
	"gorm.io/gorm"
)

// Reference: P2E §1.3 — OAuth2 provider integration
// Replaces Chatwoot's OmniAuth-based Google/GitHub/Facebook login flow.

// OAuthProviderType defines supported OAuth2 provider types.
type OAuthProviderType string

const (
	OAuthProviderGoogle OAuthProviderType = "google"
	OAuthProviderGitHub OAuthProviderType = "github"
	OAuthProviderCustom OAuthProviderType = "custom"
)

// OAuthUserInfo represents user info extracted from OAuth2 provider.
type OAuthUserInfo struct {
	Provider  OAuthProviderType
	UID       string // provider-specific user ID
	Email     string
	Name      string
	AvatarURL string
}

// OAuthService manages OAuth2 authentication flows.
type OAuthService struct {
	db      *gorm.DB
	configs map[OAuthProviderType]*oauth2.Config
	mu      sync.RWMutex
}

// NewOAuthService creates an OAuth service with configured providers.
func NewOAuthService(db *gorm.DB, cfg *config.Config) *OAuthService {
	svc := &OAuthService{
		db:      db,
		configs: make(map[OAuthProviderType]*oauth2.Config),
	}
	svc.configureProviders(cfg)
	return svc
}

// configureProviders sets up OAuth2 configs from application config.
func (s *OAuthService) configureProviders(cfg *config.Config) {
	s.mu.Lock()
	defer s.mu.Unlock()

	// Google OAuth2 — ref: Chatwoot Google OmniAuth strategy
	if cfg.OAuth.Google.ClientID != "" {
		s.configs[OAuthProviderGoogle] = &oauth2.Config{
			ClientID:     cfg.OAuth.Google.ClientID,
			ClientSecret: cfg.OAuth.Google.ClientSecret,
			RedirectURL:  cfg.OAuth.Google.RedirectURL,
			Scopes:       []string{"openid", "email", "profile"},
			Endpoint:     google.Endpoint,
		}
	}

	// Generic/GitHub OAuth2 — ref: Chatwoot GitHub OmniAuth strategy
	if cfg.OAuth.GitHub.ClientID != "" {
		s.configs[OAuthProviderGitHub] = &oauth2.Config{
			ClientID:     cfg.OAuth.GitHub.ClientID,
			ClientSecret: cfg.OAuth.GitHub.ClientSecret,
			RedirectURL:  cfg.OAuth.GitHub.RedirectURL,
			Scopes:       []string{"user:email", "read:user"},
			Endpoint: oauth2.Endpoint{
				AuthURL:  "https://github.com/login/oauth/authorize",
				TokenURL: "https://github.com/login/oauth/access_token",
			},
		}
	}
}

// GetAuthURL generates the OAuth2 authorization URL for a provider.
// This is the first step of the OAuth2 flow: redirect user to provider login.
func (s *OAuthService) GetAuthURL(provider OAuthProviderType, state string) (string, error) {
	s.mu.RLock()
	config, ok := s.configs[provider]
	s.mu.RUnlock()

	if !ok {
		return "", fmt.Errorf("oauth provider %s not configured", provider)
	}

	return config.AuthCodeURL(state, oauth2.AccessTypeOffline), nil
}

// ExchangeCode exchanges an OAuth2 authorization code for user info.
// This is the callback step: provider redirects back with a code.
func (s *OAuthService) ExchangeCode(ctx context.Context, provider OAuthProviderType, code string) (*OAuthUserInfo, error) {
	s.mu.RLock()
	config, ok := s.configs[provider]
	s.mu.RUnlock()

	if !ok {
		return nil, fmt.Errorf("oauth provider %s not configured", provider)
	}

	token, err := config.Exchange(ctx, code)
	if err != nil {
		return nil, fmt.Errorf("oauth token exchange failed: %w", err)
	}

	// Extract user info based on provider
	switch provider {
	case OAuthProviderGoogle:
		return s.extractGoogleUserInfo(ctx, config, token)
	case OAuthProviderGitHub:
		return s.extractGitHubUserInfo(ctx, config, token)
	default:
		return nil, fmt.Errorf("unsupported oauth provider: %s", provider)
	}
}

// FindOrCreateUser finds an existing user by OAuth provider+UID, or creates a new one.
// Ref: Chatwoot's after_sign_in_path_for OmniAuth callback logic.
func (s *OAuthService) FindOrCreateUser(info *OAuthUserInfo) (*model.User, error) {
	var user model.User

	// Try to find existing user by provider + UID
	err := s.db.Where("provider = ? AND uid = ?", string(info.Provider), info.UID).First(&user).Error
	if err == nil {
		// Update last sign-in info
		user.SignInCount++
		s.db.Save(&user)
		return &user, nil
	}

	// If not found by provider+UID, try by email (link existing account)
	if info.Email != "" {
		err = s.db.Where("email = ?", info.Email).First(&user).Error
		if err == nil {
			// Link OAuth to existing account
			user.Provider = string(info.Provider)
			user.UID = info.UID
			if info.AvatarURL != "" && user.AvatarURL == "" {
				user.AvatarURL = info.AvatarURL
			}
			user.SignInCount++
			s.db.Save(&user)
			return &user, nil
		}
	}

	// Create new user from OAuth info
	user = model.User{
		Name:           info.Name,
		Email:          info.Email,
		Provider:       string(info.Provider),
		UID:            info.UID,
		AvatarURL:      info.AvatarURL,
		Password:       "oauth_no_password", // OAuth users don't need a password
	}

	if err := s.db.Create(&user).Error; err != nil {
		return nil, fmt.Errorf("failed to create oauth user: %w", err)
	}

	return &user, nil
}

// extractGoogleUserInfo extracts user info from Google OAuth2 token.
func (s *OAuthService) extractGoogleUserInfo(ctx context.Context, config *oauth2.Config, token *oauth2.Token) (*OAuthUserInfo, error) {
	client := config.Client(ctx, token)

	resp, err := client.Get("https://www.googleapis.com/oauth2/v2/userinfo")
	if err != nil {
		return nil, fmt.Errorf("failed to fetch google user info: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != 200 {
		return nil, fmt.Errorf("google userinfo endpoint returned status %d", resp.StatusCode)
	}

	var gUser struct {
		ID      string `json:"id"`
		Email   string `json:"email"`
		Name    string `json:"name"`
		Picture string `json:"picture"`
	}

	if err := json.NewDecoder(resp.Body).Decode(&gUser); err != nil {
		return nil, fmt.Errorf("failed to decode google user info: %w", err)
	}

	return &OAuthUserInfo{
		Provider:  OAuthProviderGoogle,
		UID:       gUser.ID,
		Email:     gUser.Email,
		Name:      gUser.Name,
		AvatarURL: gUser.Picture,
	}, nil
}

// extractGitHubUserInfo extracts user info from GitHub OAuth2 token.
func (s *OAuthService) extractGitHubUserInfo(ctx context.Context, config *oauth2.Config, token *oauth2.Token) (*OAuthUserInfo, error) {
	client := config.Client(ctx, token)

	resp, err := client.Get("https://api.github.com/user")
	if err != nil {
		return nil, fmt.Errorf("failed to fetch github user info: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != 200 {
		return nil, fmt.Errorf("github user endpoint returned status %d", resp.StatusCode)
	}

	var ghUser struct {
		ID        int    `json:"id"`
		Login     string `json:"login"`
		Email     string `json:"email"`
		Name      string `json:"name"`
		AvatarURL string `json:"avatar_url"`
	}

	if err := json.NewDecoder(resp.Body).Decode(&ghUser); err != nil {
		return nil, fmt.Errorf("failed to decode github user info: %w", err)
	}

	return &OAuthUserInfo{
		Provider:  OAuthProviderGitHub,
		UID:       fmt.Sprintf("%d", ghUser.ID),
		Email:     ghUser.Email,
		Name:      ghUser.Name,
		AvatarURL: ghUser.AvatarURL,
	}, nil
}

// RegisterGenericProvider allows registering a custom OAuth2 provider at runtime.
// Ref: Chatwoot's configurable OmniAuth strategies in config/initializers/omniauth.rb
func (s *OAuthService) RegisterGenericProvider(name OAuthProviderType, cfg *oauth2.Config) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.configs[name] = cfg
}

// IsProviderConfigured checks if a provider has been configured.
func (s *OAuthService) IsProviderConfigured(provider OAuthProviderType) bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	_, ok := s.configs[provider]
	return ok
}