// Package server is the "slow path": a local HTTP server hosting the preset
// editor, opened in the user's default browser.
//
// It exists because the fast path must not involve a browser and the editor
// must not involve a MessageBox. Switching a preset is two clicks in the tray;
// managing fifty of them needs a real UI, and an embedded web app is the only
// way to get one without linking a GUI toolkit and giving up cross-compilation.
//
// The server is not running most of the time. The tray starts it on demand and
// it shuts itself down once the browser stops sending heartbeats.
package server

import (
	"context"
	"crypto/rand"
	"crypto/subtle"
	"embed"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"io/fs"
	"log"
	"net"
	"net/http"
	"strings"
	"sync"
	"time"

	"gitea.apointless.space/bsncubed/ipswap/internal/config"
	"gitea.apointless.space/bsncubed/ipswap/internal/netcfg"
	"gitea.apointless.space/bsncubed/ipswap/internal/preset"
)

//go:embed web
var webFS embed.FS

const (
	// idleTimeout is how long the server survives without a heartbeat. The
	// browser beats every 30s, so this tolerates a few missed beats before
	// concluding the tab is gone.
	idleTimeout = 5 * time.Minute
	// tokenHeader carries the session token on mutating requests.
	tokenHeader = "X-Ipswap-Token"
)

// Server hosts the editor.
type Server struct {
	store *preset.Store
	mgr   netcfg.Manager
	paths config.Paths

	// onSettingsChange lets the tray react to a settings save, e.g. to
	// rewrite the Run key when start-with-Windows is toggled.
	onSettingsChange func(config.Settings)
	// onPresetsChange asks the tray to rebuild its menu.
	onPresetsChange func()

	mu       sync.Mutex
	settings config.Settings
	token    string
	srv      *http.Server
	ln       net.Listener
	lastBeat time.Time
	stop     chan struct{}
}

// New builds a server. It does not listen until Start is called.
func New(store *preset.Store, mgr netcfg.Manager, paths config.Paths, settings config.Settings) *Server {
	return &Server{store: store, mgr: mgr, paths: paths, settings: settings}
}

// OnSettingsChange registers a callback fired after settings are saved.
func (s *Server) OnSettingsChange(f func(config.Settings)) { s.onSettingsChange = f }

// OnPresetsChange registers a callback fired after any preset mutation.
func (s *Server) OnPresetsChange(f func()) { s.onPresetsChange = f }

// URL returns the address to open, including the session token. It is empty
// when the server is not running.
func (s *Server) URL() string {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.ln == nil {
		return ""
	}
	return fmt.Sprintf("http://127.0.0.1:%d/?t=%s", s.ln.Addr().(*net.TCPAddr).Port, s.token)
}

// Start binds a random loopback port and serves until Stop or an idle timeout.
// Calling it while already running just returns the existing URL, so clicking
// "Manage presets…" twice reuses the one session.
func (s *Server) Start() (string, error) {
	s.mu.Lock()
	if s.ln != nil {
		url := fmt.Sprintf("http://127.0.0.1:%d/?t=%s", s.ln.Addr().(*net.TCPAddr).Port, s.token)
		s.lastBeat = time.Now()
		s.mu.Unlock()
		return url, nil
	}

	// Loopback only. This binds no external interface at any point, which
	// matters given the app spends its life on customer networks.
	ln, err := net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		s.mu.Unlock()
		return "", fmt.Errorf("binding a local port: %w", err)
	}

	token, err := newToken()
	if err != nil {
		ln.Close()
		s.mu.Unlock()
		return "", err
	}

	s.ln = ln
	s.token = token
	s.lastBeat = time.Now()
	s.stop = make(chan struct{})
	s.srv = &http.Server{
		Handler:           s.routes(),
		ReadHeaderTimeout: 10 * time.Second,
	}
	stop := s.stop
	srv := s.srv
	port := ln.Addr().(*net.TCPAddr).Port
	s.mu.Unlock()

	go func() {
		if err := srv.Serve(ln); err != nil && err != http.ErrServerClosed {
			log.Printf("editor server stopped: %v", err)
		}
	}()
	go s.watchIdle(stop)

	log.Printf("editor server listening on 127.0.0.1:%d", port)
	return fmt.Sprintf("http://127.0.0.1:%d/?t=%s", port, token), nil
}

// Stop shuts the server down. It is safe to call when not running.
func (s *Server) Stop() {
	s.mu.Lock()
	srv, stop := s.srv, s.stop
	s.srv, s.ln, s.stop, s.token = nil, nil, nil, ""
	s.mu.Unlock()

	if stop != nil {
		close(stop)
	}
	if srv == nil {
		return
	}
	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
	defer cancel()
	_ = srv.Shutdown(ctx)
	log.Printf("editor server shut down")
}

func (s *Server) watchIdle(stop <-chan struct{}) {
	t := time.NewTicker(30 * time.Second)
	defer t.Stop()
	for {
		select {
		case <-stop:
			return
		case <-t.C:
			s.mu.Lock()
			idle := time.Since(s.lastBeat)
			s.mu.Unlock()
			if idle > idleTimeout {
				log.Printf("editor server idle for %s, shutting down", idle.Round(time.Second))
				s.Stop()
				return
			}
		}
	}
}

func newToken() (string, error) {
	var b [32]byte
	if _, err := rand.Read(b[:]); err != nil {
		return "", fmt.Errorf("generating a session token: %w", err)
	}
	return hex.EncodeToString(b[:]), nil
}

// Settings returns the current settings.
func (s *Server) Settings() config.Settings {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.settings
}

// SetSettings replaces the settings the server serves, for when something
// outside the editor changes them.
func (s *Server) SetSettings(c config.Settings) {
	s.mu.Lock()
	s.settings = c
	s.mu.Unlock()
}

func (s *Server) beat() {
	s.mu.Lock()
	s.lastBeat = time.Now()
	s.mu.Unlock()
}

// --- routing ---

func (s *Server) routes() http.Handler {
	mux := http.NewServeMux()

	assets, err := fs.Sub(webFS, "web")
	if err != nil {
		// Only reachable if the embed directive and the directory disagree,
		// which is a build-time mistake, not a runtime condition.
		panic(err)
	}
	mux.Handle("GET /", http.FileServer(http.FS(assets)))

	mux.HandleFunc("POST /api/heartbeat", func(w http.ResponseWriter, r *http.Request) {
		writeJSON(w, http.StatusOK, map[string]any{"ok": true})
	})

	mux.HandleFunc("GET /api/presets", s.handleListPresets)
	mux.HandleFunc("POST /api/presets", s.handlePutPreset)
	mux.HandleFunc("PUT /api/presets/{id}", s.handlePutPreset)
	mux.HandleFunc("DELETE /api/presets/{id}", s.handleDeletePreset)

	mux.HandleFunc("GET /api/adapters", s.handleAdapters)
	mux.HandleFunc("GET /api/settings", s.handleGetSettings)
	mux.HandleFunc("PUT /api/settings", s.handlePutSettings)

	mux.HandleFunc("GET /api/export", s.handleExport)
	mux.HandleFunc("POST /api/import", s.handleImport)

	return s.withAuth(mux)
}

// withAuth gates every request on the session token and refreshes the idle
// timer.
//
// The token arrives once in the URL and is then stored in a SameSite=Strict
// cookie. Mutating requests additionally require the token in a header, which
// a cross-site page cannot set: a cookie alone would let any website in the
// browser POST to this port and rewrite the user's presets.
func (s *Server) withAuth(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		s.mu.Lock()
		token := s.token
		s.mu.Unlock()
		if token == "" {
			http.Error(w, "server is shutting down", http.StatusServiceUnavailable)
			return
		}

		if q := r.URL.Query().Get("t"); q != "" && tokenEqual(q, token) {
			http.SetCookie(w, &http.Cookie{
				Name:     "ipswap_session",
				Value:    token,
				Path:     "/",
				HttpOnly: true,
				SameSite: http.SameSiteStrictMode,
			})
			s.beat()
			next.ServeHTTP(w, r)
			return
		}

		c, err := r.Cookie("ipswap_session")
		if err != nil || !tokenEqual(c.Value, token) {
			http.Error(w, "unauthorised: reopen the editor from the tray menu", http.StatusUnauthorized)
			return
		}

		if r.Method != http.MethodGet && r.Method != http.MethodHead {
			if !tokenEqual(r.Header.Get(tokenHeader), token) {
				http.Error(w, "missing session header", http.StatusForbidden)
				return
			}
		}

		s.beat()
		next.ServeHTTP(w, r)
	})
}

func tokenEqual(got, want string) bool {
	return subtle.ConstantTimeCompare([]byte(got), []byte(want)) == 1
}

// --- handlers ---

func (s *Server) handleListPresets(w http.ResponseWriter, r *http.Request) {
	writeJSON(w, http.StatusOK, map[string]any{
		"presets": s.store.All(),
		"groups":  s.store.Groups(),
	})
}

func (s *Server) handlePutPreset(w http.ResponseWriter, r *http.Request) {
	var p preset.Preset
	if err := readJSON(r, &p); err != nil {
		writeErr(w, http.StatusBadRequest, err)
		return
	}
	if id := r.PathValue("id"); id != "" {
		p.ID = id
	}

	saved, err := s.store.Put(p)
	if err != nil {
		writeErr(w, http.StatusBadRequest, err)
		return
	}
	s.notifyPresets()
	writeJSON(w, http.StatusOK, saved)
}

func (s *Server) handleDeletePreset(w http.ResponseWriter, r *http.Request) {
	if err := s.store.Delete(r.PathValue("id")); err != nil {
		writeErr(w, http.StatusNotFound, err)
		return
	}
	s.notifyPresets()
	writeJSON(w, http.StatusOK, map[string]any{"ok": true})
}

func (s *Server) handleAdapters(w http.ResponseWriter, r *http.Request) {
	adapters, err := s.mgr.Adapters()
	if err != nil {
		writeErr(w, http.StatusInternalServerError, err)
		return
	}

	type row struct {
		netcfg.Adapter
		Current string `json:"current"`
	}
	out := make([]row, 0, len(adapters))
	for _, a := range adapters {
		r := row{Adapter: a}
		if live, err := s.mgr.Current(a.Name); err == nil {
			r.Current = live.Summary()
		}
		out = append(out, r)
	}
	writeJSON(w, http.StatusOK, map[string]any{"adapters": out})
}

func (s *Server) handleGetSettings(w http.ResponseWriter, r *http.Request) {
	writeJSON(w, http.StatusOK, s.Settings())
}

func (s *Server) handlePutSettings(w http.ResponseWriter, r *http.Request) {
	c := s.Settings()
	if err := readJSON(r, &c); err != nil {
		writeErr(w, http.StatusBadRequest, err)
		return
	}
	if err := c.Save(s.paths.Config); err != nil {
		writeErr(w, http.StatusInternalServerError, err)
		return
	}
	s.SetSettings(c)
	if s.onSettingsChange != nil {
		s.onSettingsChange(c)
	}
	writeJSON(w, http.StatusOK, c)
}

func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
	var groups []string
	if g := strings.TrimSpace(r.URL.Query().Get("groups")); g != "" {
		groups = strings.Split(g, ",")
	}

	b, err := s.store.Export(groups...)
	if err != nil {
		writeErr(w, http.StatusInternalServerError, err)
		return
	}

	w.Header().Set("Content-Type", "application/json")
	w.Header().Set("Content-Disposition", `attachment; filename="ipswap-presets.json"`)
	w.Write(b)
}

func (s *Server) handleImport(w http.ResponseWriter, r *http.Request) {
	mode := preset.ImportMode(r.URL.Query().Get("mode"))
	if mode == "" {
		mode = preset.ImportMerge
	}

	body, err := io.ReadAll(io.LimitReader(r.Body, 8<<20))
	if err != nil {
		writeErr(w, http.StatusBadRequest, err)
		return
	}

	res, err := s.store.Import(body, mode)
	if err != nil {
		// Import validates the whole file first, so a rejection here means
		// nothing on disk changed.
		writeErr(w, http.StatusBadRequest, err)
		return
	}
	s.notifyPresets()
	writeJSON(w, http.StatusOK, res)
}

func (s *Server) notifyPresets() {
	if s.onPresetsChange != nil {
		s.onPresetsChange()
	}
}

// --- helpers ---

func writeJSON(w http.ResponseWriter, status int, v any) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	if err := json.NewEncoder(w).Encode(v); err != nil {
		log.Printf("writing response: %v", err)
	}
}

func writeErr(w http.ResponseWriter, status int, err error) {
	writeJSON(w, status, map[string]string{"error": err.Error()})
}

func readJSON(r *http.Request, v any) error {
	dec := json.NewDecoder(io.LimitReader(r.Body, 1<<20))
	if err := dec.Decode(v); err != nil {
		return fmt.Errorf("invalid request body: %w", err)
	}
	return nil
}