package bot

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"sync"
	"time"

	"github.com/rs/zerolog"
	"maunium.net/go/mautrix"
	"maunium.net/go/mautrix/appservice"
	"maunium.net/go/mautrix/crypto/cryptohelper"
	"maunium.net/go/mautrix/event"
	"maunium.net/go/mautrix/id"
)

// presenceHeartbeatInterval is how often the appservice re-asserts its online
// presence. In masdevice mode the /sync long-poll refreshed presence implicitly;
// appservice mode has no /sync, so we push presence explicitly. Synapse ties
// "online" to recent activity and force-offlines a non-syncing user ~30s after its
// last update (SYNC_ONLINE_TIMEOUT). A slower tick makes the bot FLAP (green right
// after each PUT, then offline until the next) — empirically observed at 60s. So we
// tick well under 30s to hold it continuously green. A hard crash stops the
// heartbeat and Synapse offlines the bot within ~30s on its own.
const presenceHeartbeatInterval = 20 * time.Second

// Waits applied when an inbound event arrives before its megolm session does.
// The short wait covers the common race (keys moments behind the event); only
// after it lapses do we spend an m.room_key_request and wait the long one. The
// budget bounds the whole recovery so a permanently-unreadable event can't pin a
// goroutine forever. Mirrors cryptohelper's own 3s/22s ladder.
const (
	initialSessionWait    = 3 * time.Second
	extendedSessionWait   = 22 * time.Second
	sessionRecoveryBudget = initialSessionWait + extendedSessionWait + 30*time.Second
)

// cryptoToDeviceTypes are the to-device event types the crypto machine must see
// to establish Olm/Megolm sessions and share/receive room keys. In /sync mode
// the cryptohelper gets these automatically; under the appservice transaction
// model we route each one to mach.HandleToDeviceEvent ourselves.
var cryptoToDeviceTypes = []event.Type{
	event.ToDeviceEncrypted,
	event.ToDeviceRoomKey,
	event.ToDeviceForwardedRoomKey,
	event.ToDeviceRoomKeyRequest,
	event.ToDeviceRoomKeyWithheld,
	event.ToDeviceOrgMatrixRoomKeyWithheld,
	event.ToDeviceSecretRequest,
	event.ToDeviceSecretSend,
	event.ToDeviceDummy,
}

// newAppserviceSession builds the appservice-mode Session: as_token auth, a
// cryptohelper that mints the bot's device via MSC4190, and an EventProcessor
// fed by Synapse's transaction pushes (in place of /sync). The bot is an
// appservice user — Synapse forbids AS users from /sync, so all events, plus the
// E2EE extensions (to-device / device lists / OTK counts), arrive over the
// transaction API instead.
func newAppserviceSession(cfg Config) (*Session, error) {
	if err := os.MkdirAll(cfg.DataDir, 0o755); err != nil {
		return nil, fmt.Errorf("create data dir: %w", err)
	}
	if cfg.UserID == "" {
		return nil, fmt.Errorf("BOT_USER_ID is required")
	}
	if cfg.RegistrationPath == "" {
		return nil, fmt.Errorf("AS_REGISTRATION is required in appservice mode")
	}
	if cfg.HomeserverDomain == "" {
		return nil, fmt.Errorf("HOMESERVER_DOMAIN is required in appservice mode (server_name, e.g. parodia.dev)")
	}
	if !((&appservice.HostConfig{Hostname: cfg.ListenHost, Port: cfg.ListenPort}).IsConfigured()) {
		return nil, fmt.Errorf("AS_LISTEN_HOST/AS_LISTEN_PORT must be set in appservice mode")
	}

	ctx := context.Background()

	reg, err := appservice.LoadRegistration(cfg.RegistrationPath)
	if err != nil {
		return nil, fmt.Errorf("load appservice registration %q: %w", cfg.RegistrationPath, err)
	}
	// Gate for to-device delivery: handleTransaction only pumps to-device events
	// when this is set (appservice/http.go). Force it on regardless of the yaml so
	// E2EE key exchange can't silently break on a missing field.
	reg.EphemeralEvents = true

	as, err := appservice.CreateFull(appservice.CreateOpts{
		Registration:     reg,
		HomeserverDomain: cfg.HomeserverDomain,
		HomeserverURL:    cfg.Homeserver,
		HostConfig:       appservice.HostConfig{Hostname: cfg.ListenHost, Port: cfg.ListenPort},
	})
	if err != nil {
		return nil, fmt.Errorf("create appservice: %w", err)
	}
	// Surface the HTTP listener + transaction logs (default is a silent Nop).
	as.Log = zerolog.New(os.Stderr).With().Timestamp().Str("component", "appservice").Logger().
		Level(zerolog.InfoLevel)

	userID := id.UserID(cfg.UserID)
	if as.BotMXID() != userID {
		return nil, fmt.Errorf("registration sender_localpart resolves to %s but BOT_USER_ID is %s", as.BotMXID(), userID)
	}

	client := as.BotClient() // as_token auth + SetAppServiceUserID (?user_id=) assertion
	// Assert the device via ?org.matrix.msc3202.device_id= on E2EE requests, and
	// satisfy cryptohelper.Init's syncer check: with this set, Init permits a nil
	// Syncer (we drive the crypto machine from transactions, not /sync). The param
	// is only emitted once DeviceID is non-empty (url.go), so setting it now is a
	// no-op for the whoami/device-create calls below; CreateDeviceMSC4190 re-sets it.
	client.SetAppServiceDeviceID = true

	// Validate the token + identity before we start listening.
	whoami, err := client.Whoami(ctx)
	if err != nil {
		return nil, fmt.Errorf("appservice token validation failed (whoami): %w", err)
	}
	if whoami.UserID != userID {
		return nil, fmt.Errorf("appservice identity mismatch: token resolves to %s but BOT_USER_ID is %s", whoami.UserID, userID)
	}
	slog.Info("appservice token valid", "user_id", whoami.UserID)

	// ---- E2EE via cryptohelper (MSC4190 device creation, no /login) ----
	cryptoDBPath := filepath.Join(cfg.DataDir, "crypto.db")
	ch, err := cryptohelper.NewCryptoHelper(client, []byte("gogobee_pickle_key"), cryptoDBPath)
	if err != nil {
		return nil, fmt.Errorf("init crypto helper: %w", err)
	}
	// MSC4190: the appservice creates/refreshes its own device via PUT /devices
	// instead of the UIA-gated login. The crypto store persists the device ID, so
	// restarts reuse it. LoginAs carries only the display name (never calls /login
	// in MSC4190 mode). client.Syncer is nil here, so Init does NOT wire /sync
	// handlers — we drive the crypto machine from transactions below instead.
	ch.MSC4190 = true
	ch.LoginAs = &mautrix.ReqLogin{InitialDeviceDisplayName: cfg.DisplayName}
	if err := ch.Init(ctx); err != nil {
		return nil, fmt.Errorf("crypto helper init (MSC4190 device create): %w", err)
	}
	client.Crypto = ch

	// ---- Event processor: replicate the cryptohelper's /sync wiring against
	// the appservice transaction channels ----
	ep := appservice.NewEventProcessor(as)
	mach := ch.Machine()

	// Best-effort cross-signing bootstrap so the bot's device shows as verified to
	// users who trust its master key. Best-effort: under MAS the key upload may be
	// refused, which we log and ignore — E2EE still functions without it.
	bootstrapCrossSigning(ctx, mach, cfg.DataDir)

	// Crypto plumbing that /sync would otherwise carry:
	ep.OnOTK(mach.HandleOTKCounts)
	ep.OnDeviceList(mach.HandleDeviceLists)
	for _, t := range cryptoToDeviceTypes {
		ep.On(t, mach.HandleToDeviceEvent)
	}

	// Keep the client's StateStore current so outgoing sends know which rooms are
	// encrypted and who to share keys with (no /sync backfill here), and let the
	// crypto machine track membership for key sharing. PrependHandler so state is
	// updated before app handlers (auto-join/moderation) run for the same event.
	ep.PrependHandler(event.StateEncryption, func(ctx context.Context, evt *event.Event) {
		mautrix.UpdateStateStore(ctx, as.StateStore, evt)
	})
	ep.PrependHandler(event.StateMember, func(ctx context.Context, evt *event.Event) {
		mautrix.UpdateStateStore(ctx, as.StateStore, evt)
		mach.HandleMemberEvent(ctx, evt)
	})

	// Without /sync there is no state backfill, so the client's StateStore starts
	// empty. Before we hand off a decrypted event (whose reply may need to be
	// encrypted), resolve the room once: mark it encrypted and populate its member
	// list. Otherwise outbound sends go plaintext (IsEncrypted=false) and, worse,
	// the group session is shared to nobody (GetRoomJoinedOrInvitedMembers empty)
	// so recipients can't decrypt the bot's replies.
	var resolved sync.Map // roomID -> struct{}, resolved once
	resolveRoom := func(ctx context.Context, roomID id.RoomID) {
		if _, done := resolved.LoadOrStore(roomID, struct{}{}); done {
			return
		}
		var enc event.EncryptionEventContent
		if err := client.StateEvent(ctx, roomID, event.StateEncryption, "", &enc); err == nil && enc.Algorithm != "" {
			_ = as.StateStore.SetEncryptionEvent(ctx, roomID, &enc)
		}
		if members, err := client.Members(ctx, roomID); err == nil {
			_ = as.StateStore.ReplaceCachedMembers(ctx, roomID, members.Chunk)
		} else {
			slog.Warn("appservice: failed to fetch room members; will retry", "room", roomID, "err", err)
			resolved.Delete(roomID) // allow a later event to retry
		}
	}

	// recoverSession handles an event whose megolm session we don't have yet. The
	// keys are often merely in flight (a to-device m.room_key racing the room
	// event), so wait briefly; if they never land, ask the sender to re-share and
	// wait longer. Only once that fails is the event genuinely unreadable.
	//
	// This duplicates cryptohelper.HandleEncrypted's wait/request ladder on
	// purpose: that path is gated on a /sync token being present in the context
	// (mautrix.SyncTokenContextKey), which appservice transactions never carry, so
	// wiring ch.ASEventProcessor would still leave us dropping these events.
	//
	// Runs detached from the transaction context, which is cancelled as soon as we
	// ACK the transaction — long before the keys could arrive.
	recoverSession := func(evt *event.Event) {
		ctx, cancel := context.WithTimeout(context.Background(), sessionRecoveryBudget)
		defer cancel()

		content := evt.Content.AsEncrypted()
		got := ch.WaitForSession(ctx, evt.RoomID, content.SenderKey, content.SessionID, initialSessionWait)
		if !got {
			ch.RequestSession(ctx, evt.RoomID, content.SenderKey, content.SessionID, evt.Sender, content.DeviceID)
			got = ch.WaitForSession(ctx, evt.RoomID, content.SenderKey, content.SessionID, extendedSessionWait)
		}
		if !got {
			slog.Warn("appservice: gave up decrypting event, no room key",
				"room", evt.RoomID, "event", evt.ID, "session", content.SessionID)
			return
		}

		decrypted, err := ch.Decrypt(ctx, evt)
		if err != nil {
			slog.Warn("appservice: failed to decrypt event after key request",
				"room", evt.RoomID, "event", evt.ID, "err", err)
			return
		}
		slog.Debug("appservice: recovered event after key request", "room", evt.RoomID, "event", evt.ID)
		ep.Dispatch(ctx, decrypted)
	}

	// Decrypt inbound encrypted room events and re-dispatch the plaintext so the
	// normal message/reaction handlers fire (mirrors cryptohelper.HandleEncrypted).
	ep.On(event.EventEncrypted, func(ctx context.Context, evt *event.Event) {
		resolveRoom(ctx, evt.RoomID)
		decrypted, err := ch.Decrypt(ctx, evt)
		if err != nil {
			if errors.Is(err, cryptohelper.NoSessionFound) {
				go recoverSession(evt)
				return
			}
			slog.Warn("appservice: failed to decrypt event", "room", evt.RoomID, "event", evt.ID, "err", err)
			return
		}
		ep.Dispatch(ctx, decrypted)
	})

	return &Session{
		Client: client,
		mode:   "appservice",
		as:     as,
		ep:     ep,
	}, nil
}

// runAppservice starts the transaction dispatchers and the HTTP listener, then
// blocks until ctx is cancelled.
func (s *Session) runAppservice(ctx context.Context) error {
	s.ep.Start(ctx)
	s.as.Ready = true

	errCh := make(chan struct{})
	go func() {
		s.as.Start() // blocks in ListenAndServe until Stop()
		close(errCh)
	}()

	slog.Info("appservice listener started", "address", s.as.Host.Address())

	select {
	case <-ctx.Done():
		return nil
	case <-errCh:
		return fmt.Errorf("appservice HTTP listener exited unexpectedly")
	}
}

// runPresenceHeartbeat keeps the bot's Matrix presence "online" while the
// appservice runs. Without /sync, nothing else refreshes presence, so it would
// otherwise freeze at its last value. On graceful shutdown it best-effort sets
// presence "offline"; on a hard crash the heartbeat simply stops and Synapse
// decays the stale "online" state on its own.
func (s *Session) runPresenceHeartbeat(ctx context.Context) {
	setPresence := func(ctx context.Context, presence event.Presence) {
		if err := s.Client.SetPresence(ctx, mautrix.ReqPresence{Presence: presence}); err != nil {
			slog.Warn("appservice: set presence failed", "presence", presence, "err", err)
		}
	}

	setPresence(ctx, event.PresenceOnline)

	ticker := time.NewTicker(presenceHeartbeatInterval)
	defer ticker.Stop()
	for {
		select {
		case <-ctx.Done():
			// Detach from the cancelled ctx so the final PUT still goes out.
			offCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
			setPresence(offCtx, event.PresenceOffline)
			cancel()
			return
		case <-ticker.C:
			setPresence(ctx, event.PresenceOnline)
		}
	}
}