// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

// echolot-server: the probe server (spec: docs/probe-protocol.md).
//
// Modes:
//   - container ("docker mode"): autodetected (or --docker / ECHOLOT_DOCKER=1);
//     config comes from ECHOLOT_* env vars; systemd/self-update are refused.
//   - native: same flags/env, plus --install-systemd / --uninstall-systemd
//     and opt-in --self-update against a Gitea releases API.
package main

import (
	"context"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"errors"
	"fmt"
	"log/slog"
	"math/big"
	"net"
	"net/http"
	"os"
	"os/signal"
	"path/filepath"
	"strconv"
	"syscall"
	"time"

	"echo-lot.app/server/internal/config"
	"echo-lot.app/server/internal/control"
	"echo-lot.app/server/internal/dataplane"
	"echo-lot.app/server/internal/selfupdate"
	"echo-lot.app/server/internal/session"
	"echo-lot.app/server/internal/store"
	"echo-lot.app/server/internal/system"
)

// Version is stamped via -ldflags "-X main.Version=v1.2.3" in CI.
var Version = "dev"

func main() {
	if err := run(); err != nil {
		slog.Error("fatal", "err", err)
		os.Exit(1)
	}
}

func run() error {
	cfg, actions, err := config.Load(os.Args[1:])
	if err != nil {
		return err
	}
	control.Version = Version

	switch {
	case actions.Version:
		fmt.Println(Version)
		return nil
	case actions.InstallSystemd:
		// The unit runs this same binary in serve mode with env-based config.
		return system.InstallSystemd(nil)
	case actions.UninstallSystemd:
		return system.UninstallSystemd()
	case actions.SelfUpdate:
		return selfupdate.Run(cfg.SelfUpdateAPI, Version)
	}
	return serve(cfg)
}

func serve(cfg *config.Config) error {
	slog.Info("echolot-server starting", "version", Version, "mode",
		map[bool]string{true: "container", false: "native"}[cfg.Docker],
		"state_dir", cfg.StateDir)

	st, err := store.Open(cfg.StateDir)
	if err != nil {
		return fmt.Errorf("state store: %w", err)
	}
	cert, err := loadOrCreateCert(cfg)
	if err != nil {
		return fmt.Errorf("tls: %w", err)
	}
	pin, err := control.SpkiPinB64(cert)
	if err != nil {
		return err
	}
	slog.Info("control-plane certificate", "pin-sha256", pin)

	sessions := session.NewManager(15 * time.Minute)
	ctl := &control.Server{
		Store: st, Sessions: sessions, Name: cfg.Name,
		UDPPort: mustPort(cfg.UDPListen), TCPPort: mustPort(cfg.TCPListen), PinB64: pin,
	}

	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
	defer stop()
	errCh := make(chan error, 4)

	// Control plane (HTTPS, pin-based trust)
	ctlSrv := &http.Server{
		Addr: cfg.ControlListen, Handler: ctl.Handler(),
		TLSConfig:         &tls.Config{Certificates: []tls.Certificate{cert}, MinVersion: tls.VersionTLS12},
		ReadHeaderTimeout: 10 * time.Second,
	}
	go func() { errCh <- fmt.Errorf("control: %w", ctlSrv.ListenAndServeTLS("", "")) }()

	// Admin/health (plain HTTP, localhost by default; spec §7)
	admin := http.NewServeMux()
	admin.HandleFunc("GET /healthz", func(w http.ResponseWriter, _ *http.Request) {
		fmt.Fprintf(w, `{"ok":true,"version":%q}`, Version)
	})
	// TODO(spec §7): enrollment token management + device list. Until the
	// admin UI exists, mint tokens with: echolot-admin (or curl on this
	// listener once the endpoint lands).
	admin.HandleFunc("POST /admin/enroll-tokens", func(w http.ResponseWriter, r *http.Request) {
		tok, err := st.NewEnrollToken(24*time.Hour, r.URL.Query().Get("note"))
		if err != nil {
			http.Error(w, err.Error(), 500)
			return
		}
		fmt.Fprintf(w, `{"token":%q,"expires_in_s":86400}`+"\n", tok)
	})
	adminSrv := &http.Server{Addr: cfg.AdminListen, Handler: admin, ReadHeaderTimeout: 10 * time.Second}
	go func() { errCh <- fmt.Errorf("admin: %w", adminSrv.ListenAndServe()) }()

	// UDP data plane
	udpAddr, err := net.ResolveUDPAddr("udp", cfg.UDPListen)
	if err != nil {
		return err
	}
	udpConn, err := net.ListenUDP("udp", udpAddr)
	if err != nil {
		return fmt.Errorf("udp listen: %w", err)
	}
	dp := &dataplane.Server{Sessions: sessions}
	go func() { errCh <- fmt.Errorf("udp: %w", dp.Serve(udpConn)) }()

	slog.Info("listening",
		"control", cfg.ControlListen, "admin", cfg.AdminListen, "udp", cfg.UDPListen)

	select {
	case <-ctx.Done():
		slog.Info("shutting down")
		shutCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
		defer cancel()
		_ = ctlSrv.Shutdown(shutCtx)
		_ = adminSrv.Shutdown(shutCtx)
		_ = udpConn.Close()
		return nil
	case err := <-errCh:
		return err
	}
}

func mustPort(listen string) int {
	_, p, err := net.SplitHostPort(listen)
	if err != nil {
		return 0
	}
	n, _ := strconv.Atoi(p)
	return n
}

// loadOrCreateCert loads the configured cert/key, or generates a self-signed
// ECDSA P-256 cert into the state dir on first start. Self-signed is
// first-class per spec §1 — clients pin the SPKI, they don't chase CAs.
func loadOrCreateCert(cfg *config.Config) (tls.Certificate, error) {
	certPath, keyPath := cfg.TLSCert, cfg.TLSKey
	if certPath == "" {
		certPath = filepath.Join(cfg.StateDir, "tls-cert.pem")
		keyPath = filepath.Join(cfg.StateDir, "tls-key.pem")
	}
	if c, err := tls.LoadX509KeyPair(certPath, keyPath); err == nil {
		return c, nil
	} else if cfg.TLSCert != "" {
		// Explicitly configured paths must exist — do not silently overwrite.
		return tls.Certificate{}, fmt.Errorf("loading configured cert %s: %w", certPath, err)
	}

	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	if err != nil {
		return tls.Certificate{}, err
	}
	serial, _ := rand.Int(rand.Reader, big.NewInt(1).Lsh(big.NewInt(1), 62))
	tmpl := x509.Certificate{
		SerialNumber: serial,
		Subject:      pkix.Name{CommonName: "echolot-server"},
		NotBefore:    time.Now().Add(-time.Hour),
		// Long-lived on purpose: trust is the SPKI pin from enrollment, not
		// cert validity; rotation happens via next_pins in the profile.
		NotAfter:              time.Now().AddDate(10, 0, 0),
		KeyUsage:              x509.KeyUsageDigitalSignature,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		BasicConstraintsValid: true,
	}
	der, err := x509.CreateCertificate(rand.Reader, &tmpl, &tmpl, &key.PublicKey, key)
	if err != nil {
		return tls.Certificate{}, err
	}
	keyDer, err := x509.MarshalECPrivateKey(key)
	if err != nil {
		return tls.Certificate{}, err
	}
	if err := os.MkdirAll(cfg.StateDir, 0o700); err != nil && !errors.Is(err, os.ErrExist) {
		return tls.Certificate{}, err
	}
	certPem := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: der})
	keyPem := pem.EncodeToMemory(&pem.Block{Type: "EC PRIVATE KEY", Bytes: keyDer})
	if err := os.WriteFile(certPath, certPem, 0o644); err != nil {
		return tls.Certificate{}, err
	}
	if err := os.WriteFile(keyPath, keyPem, 0o600); err != nil {
		return tls.Certificate{}, err
	}
	slog.Info("generated self-signed certificate", "cert", certPath)
	return tls.X509KeyPair(certPem, keyPem)
}