package api

import (
	"context"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"net"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"

	"golang.org/x/crypto/bcrypt"

	"github.com/cdnutter/playlink/internal/store"
)

const (
	cookieName = "playlink_session"
	sessionTTL = 30 * 24 * time.Hour
	csrfHeader = "X-Playlink-Request"
)

type ctxKey int

const userKey ctxKey = 1

func hashToken(t string) string {
	h := sha256.Sum256([]byte(t))
	return hex.EncodeToString(h[:])
}

func newToken() (string, error) {
	var b [32]byte
	if _, err := rand.Read(b[:]); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b[:]), nil
}

// limiter is a small per-IP fixed-window limiter for login attempts.
type limiter struct {
	mu     sync.Mutex
	hits   map[string][]time.Time
	max    int
	window time.Duration
}

func newLimiter(max int, window time.Duration) *limiter {
	return &limiter{hits: map[string][]time.Time{}, max: max, window: window}
}

func (l *limiter) allow(key string) bool {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := time.Now()
	kept := l.hits[key][:0]
	for _, t := range l.hits[key] {
		if now.Sub(t) < l.window {
			kept = append(kept, t)
		}
	}
	if len(kept) >= l.max {
		l.hits[key] = kept
		return false
	}
	l.hits[key] = append(kept, now)
	return true
}

func clientIP(r *http.Request) string {
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return r.RemoteAddr
	}
	return host
}

func (s *Server) startSession(w http.ResponseWriter, r *http.Request, uid int64) error {
	tok, err := newToken()
	if err != nil {
		return err
	}
	exp := time.Now().Add(sessionTTL)
	if err := s.st.CreateSession(r.Context(), hashToken(tok), uid, exp.Unix()); err != nil {
		return err
	}
	http.SetCookie(w, &http.Cookie{Name: cookieName, Value: tok, Path: "/", Expires: exp, HttpOnly: true,
		Secure: s.cfg.SecureCookies, SameSite: http.SameSiteStrictMode})
	return nil
}

func (s *Server) currentUser(r *http.Request) (store.User, string, error) {
	c, err := r.Cookie(cookieName)
	if err != nil || c.Value == "" {
		return store.User{}, "", store.ErrNotFound
	}
	th := hashToken(c.Value)
	uid, err := s.st.SessionUser(r.Context(), th)
	if err != nil {
		return store.User{}, "", err
	}
	u, err := s.st.UserByID(r.Context(), uid)
	return u, th, err
}

// requireAuth rejects unauthenticated requests and, for state-changing
// methods, requests that are not same-origin API calls (CSRF defence in
// addition to the SameSite=Strict cookie).
func (s *Server) requireAuth(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		u, _, err := s.currentUser(r)
		if err != nil {
			writeErr(w, http.StatusUnauthorized, errors.New("not signed in"))
			return
		}
		next.ServeHTTP(w, r.WithContext(context.WithValue(r.Context(), userKey, u)))
	})
}

func (s *Server) csrf(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		switch r.Method {
		case http.MethodGet, http.MethodHead, http.MethodOptions:
		default:
			if r.Header.Get(csrfHeader) != "1" {
				writeErr(w, http.StatusForbidden, errors.New("missing request header"))
				return
			}
			if o := r.Header.Get("Origin"); o != "" {
				u, err := url.Parse(o)
				if err != nil || !strings.EqualFold(u.Host, r.Host) {
					writeErr(w, http.StatusForbidden, errors.New("cross-origin request refused"))
					return
				}
			}
		}
		next.ServeHTTP(w, r)
	})
}

type credentials struct {
	Username string `json:"username"`
	Password string `json:"password"`
}

func validPassword(p string) error {
	if len(p) < 10 {
		return errors.New("password must be at least 10 characters")
	}
	if len(p) > 72 {
		return errors.New("password must be at most 72 bytes")
	}
	return nil
}

func (s *Server) handleAuthState(w http.ResponseWriter, r *http.Request) {
	n, err := s.st.UserCount(r.Context())
	if err != nil {
		writeErr(w, 500, err)
		return
	}
	resp := map[string]any{"setup_required": n == 0, "authenticated": false}
	if u, _, err := s.currentUser(r); err == nil {
		resp["authenticated"] = true
		resp["username"] = u.Username
	}
	writeJSON(w, 200, resp)
}

func (s *Server) handleSetup(w http.ResponseWriter, r *http.Request) {
	var c credentials
	if !readJSON(w, r, &c) {
		return
	}
	c.Username = strings.TrimSpace(c.Username)
	if c.Username == "" {
		writeErr(w, 400, errors.New("username is required"))
		return
	}
	if err := validPassword(c.Password); err != nil {
		writeErr(w, 400, err)
		return
	}
	hash, err := bcrypt.GenerateFromPassword([]byte(c.Password), 12)
	if err != nil {
		writeErr(w, 500, err)
		return
	}
	if err := s.st.CreateFirstUser(r.Context(), c.Username, string(hash)); err != nil {
		writeErr(w, 409, err)
		return
	}
	u, err := s.st.UserByName(r.Context(), c.Username)
	if err != nil {
		writeErr(w, 500, err)
		return
	}
	if err := s.startSession(w, r, u.ID); err != nil {
		writeErr(w, 500, err)
		return
	}
	s.log.Info("initial user created", "user", u.Username)
	writeJSON(w, 200, map[string]any{"ok": true})
}

// dummyHash keeps login timing similar for unknown usernames.
var dummyHash, _ = bcrypt.GenerateFromPassword([]byte("playlink-timing-equalizer"), 12)

func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
	if !s.loginLimit.allow(clientIP(r)) {
		writeErr(w, http.StatusTooManyRequests, errors.New("too many attempts; try again in a few minutes"))
		return
	}
	var c credentials
	if !readJSON(w, r, &c) {
		return
	}
	u, err := s.st.UserByName(r.Context(), strings.TrimSpace(c.Username))
	hash := dummyHash
	if err == nil {
		hash = []byte(u.PasswordHash)
	}
	if bcrypt.CompareHashAndPassword(hash, []byte(c.Password)) != nil || err != nil {
		s.log.Warn("failed login", "ip", clientIP(r))
		writeErr(w, http.StatusUnauthorized, errors.New("invalid username or password"))
		return
	}
	if err := s.startSession(w, r, u.ID); err != nil {
		writeErr(w, 500, err)
		return
	}
	writeJSON(w, 200, map[string]any{"ok": true})
}

func (s *Server) handleLogout(w http.ResponseWriter, r *http.Request) {
	if c, err := r.Cookie(cookieName); err == nil {
		_ = s.st.DeleteSession(r.Context(), hashToken(c.Value))
	}
	http.SetCookie(w, &http.Cookie{Name: cookieName, Value: "", Path: "/", MaxAge: -1, HttpOnly: true,
		Secure: s.cfg.SecureCookies, SameSite: http.SameSiteStrictMode})
	writeJSON(w, 200, map[string]any{"ok": true})
}

func (s *Server) handleChangePassword(w http.ResponseWriter, r *http.Request) {
	var body struct {
		Current string `json:"current"`
		New     string `json:"new"`
	}
	if !readJSON(w, r, &body) {
		return
	}
	u, th, err := s.currentUser(r)
	if err != nil {
		writeErr(w, 401, err)
		return
	}
	if bcrypt.CompareHashAndPassword([]byte(u.PasswordHash), []byte(body.Current)) != nil {
		writeErr(w, 400, errors.New("current password is incorrect"))
		return
	}
	if err := validPassword(body.New); err != nil {
		writeErr(w, 400, err)
		return
	}
	hash, err := bcrypt.GenerateFromPassword([]byte(body.New), 12)
	if err != nil {
		writeErr(w, 500, err)
		return
	}
	if err := s.st.SetPassword(r.Context(), u.ID, string(hash)); err != nil {
		writeErr(w, 500, err)
		return
	}
	_ = s.st.DeleteUserSessions(r.Context(), u.ID, th) // sign out other devices
	writeJSON(w, 200, map[string]any{"ok": true})
}