package oauth

import (
	"context"
	"crypto/rand"
	"encoding/json"
	"errors"
	"fmt"
	"time"

	"github.com/redis/go-redis/v9"
	"golang.org/x/oauth2"
)

const pendingKeyPrefix = "mail_oauth_pending:"
const pendingTTL = 10 * time.Minute

var ErrUnknownState = errors.New("oauth state expired or unknown")
var ErrProviderDisabled = errors.New("oauth provider not configured")

type Provider string

const (
	ProviderGoogle    Provider = "google"
	ProviderMicrosoft Provider = "microsoft"
)

type PendingAccount struct {
	UserExternalID string `json:"user_external_id"`
	Provider       string `json:"provider"`
	Email          string `json:"email"`
	Name           string `json:"name"`
	ProviderID     string `json:"provider_id"`
	IMAPHost       string `json:"imap_host"`
	IMAPPort       int    `json:"imap_port"`
	IMAPTLS        bool   `json:"imap_tls"`
	SMTPHost       string `json:"smtp_host"`
	SMTPPort       int    `json:"smtp_port"`
	SMTPTLS        bool   `json:"smtp_tls"`
	PKCEVerifier   string `json:"pkce_verifier"`
}

type Config struct {
	GoogleClientID     string
	GoogleClientSecret string
	MicrosoftClientID  string
	MicrosoftSecret    string
	MicrosoftTenant    string
	RedirectURL        string
}

type Service struct {
	cfg Config
	rdb *redis.Client
}

func NewService(cfg Config, rdb *redis.Client) *Service {
	return &Service{cfg: cfg, rdb: rdb}
}

func (s *Service) EnabledProviders() []string {
	var out []string
	if s.providerConfig(ProviderGoogle) != nil {
		out = append(out, string(ProviderGoogle))
	}
	if s.providerConfig(ProviderMicrosoft) != nil {
		out = append(out, string(ProviderMicrosoft))
	}
	return out
}

func (s *Service) Start(ctx context.Context, userExternalID string, provider Provider, pending PendingAccount) (authURL, state string, err error) {
	oauthCfg := s.providerConfig(provider)
	if oauthCfg == nil {
		return "", "", ErrProviderDisabled
	}
	verifier, challenge, err := newPKCE()
	if err != nil {
		return "", "", err
	}
	state, err = randomToken(24)
	if err != nil {
		return "", "", err
	}
	pending.UserExternalID = userExternalID
	pending.Provider = string(provider)
	pending.PKCEVerifier = verifier

	if err := s.savePending(ctx, state, pending); err != nil {
		return "", "", err
	}

	authURL = oauthCfg.AuthCodeURL(state,
		oauth2.AccessTypeOffline,
		oauth2.SetAuthURLParam("code_challenge", challenge),
		oauth2.SetAuthURLParam("code_challenge_method", "S256"),
		oauth2.SetAuthURLParam("prompt", "consent"),
	)
	return authURL, state, nil
}

func (s *Service) Exchange(ctx context.Context, state, code string) (PendingAccount, *oauth2.Token, error) {
	pending, err := s.loadPending(ctx, state)
	if err != nil {
		return PendingAccount{}, nil, err
	}
	oauthCfg := s.providerConfig(Provider(pending.Provider))
	if oauthCfg == nil {
		return PendingAccount{}, nil, ErrProviderDisabled
	}
	token, err := oauthCfg.Exchange(ctx, code, oauth2.SetAuthURLParam("code_verifier", pending.PKCEVerifier))
	if err != nil {
		return PendingAccount{}, nil, fmt.Errorf("token exchange: %w", err)
	}
	_ = s.rdb.Del(ctx, pendingKeyPrefix+state).Err()
	return pending, token, nil
}

func (s *Service) Refresh(ctx context.Context, provider, refreshToken string) (*oauth2.Token, error) {
	oauthCfg := s.providerConfig(Provider(provider))
	if oauthCfg == nil {
		return nil, ErrProviderDisabled
	}
	src := oauthCfg.TokenSource(ctx, &oauth2.Token{RefreshToken: refreshToken})
	token, err := src.Token()
	if err != nil {
		return nil, err
	}
	return token, nil
}

func (s *Service) providerConfig(provider Provider) *oauth2.Config {
	switch provider {
	case ProviderGoogle:
		if s.cfg.GoogleClientID == "" || s.cfg.GoogleClientSecret == "" || s.cfg.RedirectURL == "" {
			return nil
		}
		return &oauth2.Config{
			ClientID:     s.cfg.GoogleClientID,
			ClientSecret: s.cfg.GoogleClientSecret,
			RedirectURL:  s.cfg.RedirectURL,
			Scopes:       []string{"https://mail.google.com/"},
			Endpoint: oauth2.Endpoint{
				AuthURL:  "https://accounts.google.com/o/oauth2/v2/auth",
				TokenURL: "https://oauth2.googleapis.com/token",
			},
		}
	case ProviderMicrosoft:
		if s.cfg.MicrosoftClientID == "" || s.cfg.MicrosoftSecret == "" || s.cfg.RedirectURL == "" {
			return nil
		}
		tenant := s.cfg.MicrosoftTenant
		if tenant == "" {
			tenant = "common"
		}
		return &oauth2.Config{
			ClientID:     s.cfg.MicrosoftClientID,
			ClientSecret: s.cfg.MicrosoftSecret,
			RedirectURL:  s.cfg.RedirectURL,
			Scopes: []string{
				"offline_access",
				"https://outlook.office.com/IMAP.AccessAsUser.All",
				"https://outlook.office.com/SMTP.Send",
			},
			Endpoint: oauth2.Endpoint{
				AuthURL:  fmt.Sprintf("https://login.microsoftonline.com/%s/oauth2/v2.0/authorize", tenant),
				TokenURL: fmt.Sprintf("https://login.microsoftonline.com/%s/oauth2/v2.0/token", tenant),
			},
		}
	default:
		return nil
	}
}

func (s *Service) savePending(ctx context.Context, state string, pending PendingAccount) error {
	if s.rdb == nil {
		return errors.New("oauth state store unavailable")
	}
	raw, err := json.Marshal(pending)
	if err != nil {
		return err
	}
	return s.rdb.Set(ctx, pendingKeyPrefix+state, raw, pendingTTL).Err()
}

func (s *Service) loadPending(ctx context.Context, state string) (PendingAccount, error) {
	if s.rdb == nil {
		return PendingAccount{}, errors.New("oauth state store unavailable")
	}
	raw, err := s.rdb.Get(ctx, pendingKeyPrefix+state).Bytes()
	if err != nil {
		if errors.Is(err, redis.Nil) {
			return PendingAccount{}, ErrUnknownState
		}
		return PendingAccount{}, err
	}
	var pending PendingAccount
	if err := json.Unmarshal(raw, &pending); err != nil {
		return PendingAccount{}, err
	}
	return pending, nil
}

func newPKCE() (verifier, challenge string, err error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", "", err
	}
	verifier = base64URLEncode(b)
	sum, err := sha256Sum(verifier)
	if err != nil {
		return "", "", err
	}
	challenge = base64URLEncode(sum)
	return verifier, challenge, nil
}

func randomToken(n int) (string, error) {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64URLEncode(b), nil
}