package main

import (
	"bytes"
	"crypto/ed25519"
	"crypto/sha512"
	"crypto/subtle"
	"embed"
	"encoding/base64"
	"encoding/hex"
	"flag"
	"fmt"
	"html/template"
	"io"
	"log"
	"net/http"
	"os"
	"path/filepath"
	"strings"
	"time"
)

var (
	port        = flag.Int("port", 42069, "port to listen on")
	secretsFile = flag.String("secrets", "secrets", "path to the secrets file")
	notepadFile = flag.String("notepad", "notepad", "path to save user notes")

	serverStartTime = time.Now()
)

var (
	notepadPassword []byte
	secretKey       ed25519.PrivateKey
)

func loadSecrets() error {
	secrets, err := os.ReadFile(*secretsFile)
	if err != nil {
		return err
	}
	for _, line := range bytes.Split(bytes.TrimSpace(secrets), []byte("\n")) {
		if pw, ok := bytes.CutPrefix(line, []byte("notepad-password=")); ok {
			notepadPassword = make([]byte, hex.DecodedLen(len(pw)))
			if _, err := hex.Decode(notepadPassword, pw); err != nil {
				return err
			}
		} else if key, ok := bytes.CutPrefix(line, []byte("secret-key=")); ok {
			seed := make([]byte, hex.DecodedLen(len(key)))
			if _, err := hex.Decode(seed, key); err != nil {
				return err
			}
			if len(seed) != ed25519.SeedSize {
				return fmt.Errorf("invalid ed25519 key")
			}
			secretKey = ed25519.NewKeyFromSeed(seed)
		}
	}
	return nil
}

var (
	//go:embed templates/404.html.template
	notFoundString   string
	notFoundTemplate = template.Must(template.New("notFound").Parse(notFoundString))
)

func notFound(w http.ResponseWriter, r *http.Request) {
	w.WriteHeader(http.StatusNotFound)
	type args struct {
		Path string
	}
	if err := notFoundTemplate.Execute(w, args{Path: r.URL.Path}); err != nil {
		log.Printf("Warning: notFound: %s", err)
	}
}

var (
	//go:embed templates/outline.html.template
	outlineString   string
	outlineTemplate = template.Must(template.New("outline").Parse(outlineString))

	//go:embed templates/index.html.template
	indexString   string
	indexTemplate = template.Must(template.Must(outlineTemplate.Clone()).New("body").Parse(indexString))
)

func index(w http.ResponseWriter, _ *http.Request) {
	if err := indexTemplate.ExecuteTemplate(w, "outline", nil); err != nil {
		log.Printf("Warning: index: %s", err)
	}
}

var (
	//go:embed templates/pong.html.template
	pongString   string
	pongTemplate = template.Must(template.New("pong").Parse(pongString))
)

func pong(w http.ResponseWriter, _ *http.Request) {
	if err := pongTemplate.Execute(w, nil); err != nil {
		log.Printf("Warning: pong: %s", err)
	}
}

//go:embed static
var staticFiles embed.FS

func serveStaticFile(w http.ResponseWriter, r *http.Request, path string) {
	f, err := staticFiles.Open(path)
	if err != nil {
		notFound(w, r)
		return
	}
	defer f.Close()
	http.ServeContent(w, r, path, serverStartTime, f.(io.ReadSeeker))
}

func static(w http.ResponseWriter, r *http.Request) {
	serveStaticFile(w, r, strings.TrimPrefix(r.URL.Path, "/"))
}

func favicon(w http.ResponseWriter, r *http.Request) {
	serveStaticFile(w, r, "static/favicon.ico")
}

const cookieExpiration = 400 * 24 * time.Hour

func attachCookie(w http.ResponseWriter) error {
	nowBytes, err := time.Now().MarshalBinary()
	if err != nil {
		return err
	}
	sig := ed25519.Sign(secretKey, nowBytes)
	http.SetCookie(w, &http.Cookie{
		Name:        "auth",
		Value:       base64.RawStdEncoding.EncodeToString(append(nowBytes, sig...)),
		Path:        "/notepad",
		Expires:     time.Now().Add(cookieExpiration),
		Secure:      true,
		HttpOnly:    true,
		SameSite:    http.SameSiteStrictMode,
		Partitioned: true,
	})
	return nil
}

func cookieOK(cookie string) bool {
	bytes, err := base64.RawStdEncoding.DecodeString(cookie)
	if err != nil {
		return false
	}
	if len(bytes) < ed25519.SignatureSize {
		return false
	}
	msg, sig := bytes[:len(bytes)-ed25519.SignatureSize], bytes[len(bytes)-ed25519.SignatureSize:]
	if !ed25519.Verify(secretKey.Public().(ed25519.PublicKey), msg, sig) {
		return false
	}
	var t time.Time
	if err := t.UnmarshalBinary(msg); err != nil {
		return false
	}
	return time.Since(t) < cookieExpiration
}

func authenticate(next http.Handler) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if cookie, err := r.Cookie("auth"); err == nil && cookieOK(cookie.Value) {
			attachCookie(w)
			next.ServeHTTP(w, r)
			return
		}
		_, password, ok := r.BasicAuth()
		if !ok {
			w.Header().Set("WWW-Authenticate", `Basic realm="notepad"`)
			http.Error(w, "Unauthorized", http.StatusUnauthorized)
			return
		}
		pwHash := sha512.Sum512([]byte(password))
		if subtle.ConstantTimeCompare(pwHash[:], notepadPassword) == 0 {
			w.Header().Set("WWW-Authenticate", `Basic realm="notepad"`)
			http.Error(w, "Unauthorized", http.StatusUnauthorized)
			return
		}
		attachCookie(w)
		next.ServeHTTP(w, r)
	}
}

var (
	//go:embed templates/note.html.template
	notepadString   string
	notepadTemplate = template.Must(template.Must(outlineTemplate.Clone()).New("body").Parse(notepadString)).Lookup("outline")
)

func notepad(w http.ResponseWriter, r *http.Request) {
	var currentContent string
	if currentContentBytes, err := os.ReadFile(*notepadFile); err != nil {
		currentContent = fmt.Sprintf("Error reading notepad file: %s", err)
	} else {
		currentContent = string(currentContentBytes)
	}
	type args struct {
		Content string
	}
	if err := notepadTemplate.Execute(w, args{Content: currentContent}); err != nil {
		log.Printf("Warning: notepad: %s", err)
	}
}

func saveNote(content []byte) error {
	f, err := os.CreateTemp(filepath.Dir(*notepadFile), "notepad")
	if err != nil {
		return err
	}
	defer f.Close()
	if _, err = f.Write(content); err != nil {
		return err
	}
	if err := f.Close(); err != nil {
		return err
	}
	return os.Rename(f.Name(), *notepadFile)
}

func autosave(w http.ResponseWriter, r *http.Request) {
	msg := "Saved ✓"
	if err := saveNote([]byte(r.FormValue("content"))); err != nil {
		msg = fmt.Sprintf("Error: %s", err)
	}
	if err := notepadTemplate.ExecuteTemplate(w, "saveIndicator", msg); err != nil {
		log.Printf("Warning: autosave: %s", err)
	}
}

func main() {
	flag.Parse()

	if err := loadSecrets(); err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(1)
	}

	http.HandleFunc("GET /pong", pong)
	http.Handle("GET /notepad", authenticate(http.HandlerFunc(notepad)))
	http.HandleFunc("POST /notepad/autosave", autosave)
	http.HandleFunc("GET /static/", static)
	http.HandleFunc("GET /favicon.ico", favicon)
	http.HandleFunc("GET /{$}", index)
	http.HandleFunc("GET /", notFound)

	addr := fmt.Sprintf(":%d", *port)
	log.Printf("Listening on %q", addr)
	log.Fatal(http.ListenAndServe(addr, nil))
}