package realtime

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"log/slog"
	"net/http"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/coder/websocket"
	"github.com/coder/websocket/wsjson"
	"github.com/jackc/pgx/v5/pgxpool"

	"github.com/ultisuite/ulti-backend/internal/auth"
	"github.com/ultisuite/ulti-backend/internal/users"
)

type Event struct {
	Type    string `json:"type"`
	Payload any    `json:"payload"`
	Seq     uint64 `json:"seq,omitempty"`
}

type Hub struct {
	mu sync.RWMutex

	clients       map[string]map[*conn]struct{} // userID -> active connections
	sessionCounts map[string]map[string]int     // userID -> session fingerprint -> count
	history       map[string][]Event            // userID -> recent ordered events
	nextSeq       uint64

	maxConnectionsPerUser   int
	maxConnectionsPerSess   int
	replayBufferSizePerUser int

	logger   *slog.Logger
	verifier *auth.Holder
	db       *pgxpool.Pool
}

type conn struct {
	ws         *websocket.Conn
	userID     string
	sessionKey string
	writeMu    sync.Mutex
}

const (
	defaultMaxConnectionsPerUser   = 10
	defaultMaxConnectionsPerSess   = 3
	defaultReplayBufferSizePerUser = 500
	heartbeatInterval              = 25 * time.Second
	heartbeatTimeout               = 90 * time.Second
)

var (
	errVerifierUnavailable = errors.New("authentication unavailable")
	errUserStoreUnavailable = errors.New("user store unavailable")
	errMissingToken         = errors.New("missing token")
	errInvalidToken         = errors.New("invalid token")
	errUserProvisioning     = errors.New("failed to provision user")
)

func NewHub(verifier *auth.Holder, db *pgxpool.Pool) *Hub {
	return &Hub{
		clients:                 make(map[string]map[*conn]struct{}),
		sessionCounts:           make(map[string]map[string]int),
		history:                 make(map[string][]Event),
		maxConnectionsPerUser:   defaultMaxConnectionsPerUser,
		maxConnectionsPerSess:   defaultMaxConnectionsPerSess,
		replayBufferSizePerUser: defaultReplayBufferSizePerUser,
		logger:                  slog.Default().With("component", "ws-hub"),
		verifier:                verifier,
		db:                      db,
	}
}

func (h *Hub) HandleWS(w http.ResponseWriter, r *http.Request) {
	userID, token, err := h.authenticate(r)
	if err != nil {
		h.writeAuthError(w, err)
		return
	}
	sessionKey := tokenSessionKey(token)
	since := parseSinceCursor(r)

	ws, err := websocket.Accept(w, r, &websocket.AcceptOptions{
		OriginPatterns: []string{"*"},
	})
	if err != nil {
		h.logger.Error("websocket accept", "error", err)
		return
	}

	c := &conn{ws: ws, userID: userID, sessionKey: sessionKey}
	if err := h.register(c); err != nil {
		h.logger.Warn("ws rejected", "user_id", userID, "session", sessionKey, "error", err)
		_ = ws.Close(websocket.StatusPolicyViolation, err.Error())
		return
	}
	defer h.unregister(c)

	if err := h.replaySince(c, since); err != nil {
		h.logger.Warn("ws replay failed", "user_id", userID, "error", err)
		return
	}

	ctx := r.Context()
	lastPong := newLastPongTracker()
	go h.runHeartbeat(ctx, c, lastPong)

	for {
		var incoming Event
		if err := wsjson.Read(ctx, ws, &incoming); err != nil {
			break
		}

		switch incoming.Type {
		case TypeWSPong:
			lastPong.touch()
		case TypeWSPing:
			if err := h.writeConn(ctx, c, Event{Type: TypeWSPong, Payload: map[string]any{}}); err != nil {
				return
			}
			lastPong.touch()
		}

		if isHeartbeatTimedOut(lastPong) {
			_ = ws.Close(websocket.StatusPolicyViolation, "heartbeat timeout")
			break
		}
	}
}

func (h *Hub) authenticate(r *http.Request) (string, string, error) {
	if h.verifier == nil || !h.verifier.Ready() {
		return "", "", errVerifierUnavailable
	}
	if h.db == nil {
		return "", "", errUserStoreUnavailable
	}

	token, ok := extractToken(r)
	if !ok {
		return "", "", errMissingToken
	}
	claims, err := h.verifier.Verify(r.Context(), token)
	if err != nil {
		return "", "", errInvalidToken
	}
	userID, err := users.EnsureUser(r.Context(), h.db, claims)
	if err != nil {
		return "", "", errUserProvisioning
	}
	return userID, token, nil
}

func (h *Hub) writeAuthError(w http.ResponseWriter, err error) {
	switch err {
	case errVerifierUnavailable, errUserStoreUnavailable:
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
	case errMissingToken, errInvalidToken:
		http.Error(w, err.Error(), http.StatusUnauthorized)
	default:
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
}

func extractToken(r *http.Request) (string, bool) {
	if authz := strings.TrimSpace(r.Header.Get("Authorization")); authz != "" {
		if token, ok := strings.CutPrefix(authz, "Bearer "); ok && strings.TrimSpace(token) != "" {
			return strings.TrimSpace(token), true
		}
		return "", false
	}
	for _, key := range []string{"token", "access_token"} {
		if token := strings.TrimSpace(r.URL.Query().Get(key)); token != "" {
			return token, true
		}
	}
	return "", false
}

func tokenSessionKey(token string) string {
	sum := sha256.Sum256([]byte(token))
	return hex.EncodeToString(sum[:8])
}

func parseSinceCursor(r *http.Request) uint64 {
	raw := strings.TrimSpace(r.URL.Query().Get("since"))
	if raw == "" {
		return 0
	}
	v, err := strconv.ParseUint(raw, 10, 64)
	if err != nil {
		return 0
	}
	return v
}

func (h *Hub) runHeartbeat(ctx context.Context, c *conn, tracker *lastPongTracker) {
	ticker := time.NewTicker(heartbeatInterval)
	defer ticker.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-ticker.C:
			if isHeartbeatTimedOut(tracker) {
				_ = c.ws.Close(websocket.StatusPolicyViolation, "heartbeat timeout")
				return
			}
			if err := h.writeConn(ctx, c, Event{Type: TypeWSPing, Payload: map[string]any{}}); err != nil {
				return
			}
		}
	}
}

type lastPongTracker struct {
	mu sync.RWMutex
	ts time.Time
}

func newLastPongTracker() *lastPongTracker {
	return &lastPongTracker{ts: time.Now()}
}

func (t *lastPongTracker) touch() {
	t.mu.Lock()
	defer t.mu.Unlock()
	t.ts = time.Now()
}

func (t *lastPongTracker) value() time.Time {
	t.mu.RLock()
	defer t.mu.RUnlock()
	return t.ts
}

func isHeartbeatTimedOut(t *lastPongTracker) bool {
	return time.Since(t.value()) > heartbeatTimeout
}

func (h *Hub) replaySince(c *conn, since uint64) error {
	if since == 0 {
		return nil
	}

	h.mu.RLock()
	history := append([]Event(nil), h.history[c.userID]...)
	h.mu.RUnlock()

	for _, event := range history {
		if event.Seq <= since {
			continue
		}
		if err := h.writeConn(context.Background(), c, event); err != nil {
			return err
		}
	}
	return nil
}

func (h *Hub) writeConn(ctx context.Context, c *conn, event Event) error {
	c.writeMu.Lock()
	defer c.writeMu.Unlock()
	return wsjson.Write(ctx, c.ws, event)
}

func (h *Hub) nextSequenceLocked() uint64 {
	h.nextSeq++
	return h.nextSeq
}

func (h *Hub) appendHistoryLocked(userID string, event Event) {
	history := append(h.history[userID], event)
	if len(history) > h.replayBufferSizePerUser {
		history = history[len(history)-h.replayBufferSizePerUser:]
	}
	h.history[userID] = history
}

func (h *Hub) snapshotConnectionsLocked(userID string) []*conn {
	conns := h.clients[userID]
	if len(conns) == 0 {
		return nil
	}
	out := make([]*conn, 0, len(conns))
	for c := range conns {
		out = append(out, c)
	}
	return out
}

func (h *Hub) canRegisterLocked(c *conn) error {
	if len(h.clients[c.userID]) >= h.maxConnectionsPerUser {
		return errors.New("too many websocket connections for user")
	}
	if h.sessionCounts[c.userID] == nil {
		return nil
	}
	if h.sessionCounts[c.userID][c.sessionKey] >= h.maxConnectionsPerSess {
		return errors.New("too many websocket connections for session")
	}
	return nil
}

func (h *Hub) register(c *conn) error {
	h.mu.Lock()
	defer h.mu.Unlock()

	if h.clients[c.userID] == nil {
		h.clients[c.userID] = make(map[*conn]struct{})
	}
	if err := h.canRegisterLocked(c); err != nil {
		return err
	}

	h.clients[c.userID][c] = struct{}{}
	if h.sessionCounts[c.userID] == nil {
		h.sessionCounts[c.userID] = make(map[string]int)
	}
	h.sessionCounts[c.userID][c.sessionKey]++

	h.logger.Info("ws connected",
		"user_id", c.userID,
		"session", c.sessionKey,
		"user_conns", len(h.clients[c.userID]),
		"session_conns", h.sessionCounts[c.userID][c.sessionKey],
	)
	return nil
}

func (h *Hub) unregister(c *conn) {
	h.mu.Lock()
	defer h.mu.Unlock()

	if conns, ok := h.clients[c.userID]; ok {
		delete(conns, c)
		if len(conns) == 0 {
			delete(h.clients, c.userID)
		}
	}

	if sessions := h.sessionCounts[c.userID]; sessions != nil {
		next := sessions[c.sessionKey] - 1
		if next <= 0 {
			delete(sessions, c.sessionKey)
		} else {
			sessions[c.sessionKey] = next
		}
		if len(sessions) == 0 {
			delete(h.sessionCounts, c.userID)
		}
	}

	_ = c.ws.Close(websocket.StatusNormalClosure, "")
}

func (h *Hub) Broadcast(userID string, event Event) {
	h.mu.Lock()
	event.Seq = h.nextSequenceLocked()
	h.appendHistoryLocked(userID, event)
	conns := h.snapshotConnectionsLocked(userID)
	h.mu.Unlock()

	for _, c := range conns {
		if err := h.writeConn(context.Background(), c, event); err != nil {
			h.logger.Error("ws write", "error", err, "user_id", userID)
			go h.unregister(c)
		}
	}
}

func (h *Hub) HistoryHead(userID string) uint64 {
	h.mu.RLock()
	defer h.mu.RUnlock()
	history := h.history[userID]
	if len(history) == 0 {
		return 0
	}
	return history[len(history)-1].Seq
}

func (h *Hub) SetLimits(maxUser, maxSession, replaySize int) {
	h.mu.Lock()
	defer h.mu.Unlock()
	if maxUser > 0 {
		h.maxConnectionsPerUser = maxUser
	}
	if maxSession > 0 {
		h.maxConnectionsPerSess = maxSession
	}
	if replaySize > 0 {
		h.replayBufferSizePerUser = replaySize
	}
}