package main

import (
	"bytes"
	"context"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"html/template"
	"mime"
	"mime/quotedprintable"
	"net"
	"net/http"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/kardianos/service"
)

const sessionCookieName = "ric930_session"

var errInvalidEmailID = errors.New(
	"invalid email id",
)

type EmailFile struct {
	Name    string    `json:"name"`
	ID      string    `json:"id"`
	Size    int64     `json:"size"`
	ModTime time.Time `json:"mod_time"`
	From    string    `json:"from"`
	To      string    `json:"to"`
	Subject string    `json:"subject"`
	Date    time.Time `json:"date"`
}

type DayEmails struct {
	Date   string      `json:"date"`
	Emails []EmailFile `json:"emails"`
}

type cachedStaticFile struct {
	Data        []byte
	ContentType string
}

type webSession struct {
	LastActivity time.Time
	CSRFToken    string
}

type sessionStore struct {
	mu       sync.Mutex
	sessions map[string]*webSession
	timeout  time.Duration
}

type loginAttempt struct {
	Failures     int
	WindowStart  time.Time
	BlockedUntil time.Time
	LastSeen     time.Time
}

type loginRateLimiter struct {
	mu sync.Mutex

	attempts map[string]*loginAttempt

	maxAttempts int
	window      time.Duration
	lockout     time.Duration
}

type WebServer struct {
	config     *Config
	smtpServer *SMTPServer
	logger     service.Logger
	server     *http.Server

	template    *template.Template
	templateErr error

	loginTemplate    *template.Template
	loginTemplateErr error

	staticFiles map[string]cachedStaticFile

	sessions     *sessionStore
	loginLimiter *loginRateLimiter
}

func newSessionStore(
	timeout time.Duration,
) *sessionStore {
	return &sessionStore{
		sessions: make(
			map[string]*webSession,
		),
		timeout: timeout,
	}
}

func generateSecureToken() (
	string,
	error,
) {
	data := make([]byte, 32)

	if _, err := rand.Read(data); err != nil {
		return "", fmt.Errorf(
			"failed to generate secure token: %w",
			err,
		)
	}

	return base64.RawURLEncoding.
		EncodeToString(data), nil
}

func (s *sessionStore) create() (
	string,
	error,
) {
	sessionID, err := generateSecureToken()
	if err != nil {
		return "", err
	}

	csrfToken, err := generateSecureToken()
	if err != nil {
		return "", err
	}

	now := time.Now()

	s.mu.Lock()
	defer s.mu.Unlock()

	for id, session := range s.sessions {
		if now.Sub(session.LastActivity) >
			s.timeout {

			delete(
				s.sessions,
				id,
			)
		}
	}

	s.sessions[sessionID] = &webSession{
		LastActivity: now,
		CSRFToken:    csrfToken,
	}

	return sessionID, nil
}

func (s *sessionStore) validate(
	sessionID string,
) bool {
	if sessionID == "" {
		return false
	}

	now := time.Now()

	s.mu.Lock()
	defer s.mu.Unlock()

	session, ok := s.sessions[sessionID]
	if !ok {
		return false
	}

	if now.Sub(session.LastActivity) >
		s.timeout {

		delete(
			s.sessions,
			sessionID,
		)

		return false
	}

	// Sliding inactivity timeout.
	session.LastActivity = now

	return true
}

func (s *sessionStore) csrfToken(
	sessionID string,
) (string, bool) {
	if sessionID == "" {
		return "", false
	}

	s.mu.Lock()
	defer s.mu.Unlock()

	session, ok := s.sessions[sessionID]
	if !ok {
		return "", false
	}

	return session.CSRFToken, true
}

func (s *sessionStore) validateCSRF(
	sessionID string,
	token string,
) bool {
	if sessionID == "" ||
		token == "" {

		return false
	}

	s.mu.Lock()
	defer s.mu.Unlock()

	session, ok := s.sessions[sessionID]
	if !ok {
		return false
	}

	return constantTimeStringEqual(
		session.CSRFToken,
		token,
	)
}

func (s *sessionStore) delete(
	sessionID string,
) {
	if sessionID == "" {
		return
	}

	s.mu.Lock()
	defer s.mu.Unlock()

	delete(
		s.sessions,
		sessionID,
	)
}

func newLoginRateLimiter(
	maxAttempts int,
	window time.Duration,
	lockout time.Duration,
) *loginRateLimiter {
	return &loginRateLimiter{
		attempts: make(
			map[string]*loginAttempt,
		),
		maxAttempts: maxAttempts,
		window:      window,
		lockout:     lockout,
	}
}

func (l *loginRateLimiter) allow(
	key string,
) (bool, time.Duration) {
	now := time.Now()

	l.mu.Lock()
	defer l.mu.Unlock()

	for id, attempt := range l.attempts {
		if attempt.BlockedUntil.After(now) {
			continue
		}

		if now.Sub(attempt.LastSeen) >
			l.window+l.lockout {

			delete(
				l.attempts,
				id,
			)
		}
	}

	attempt, ok := l.attempts[key]
	if !ok {
		return true, 0
	}

	attempt.LastSeen = now

	if attempt.BlockedUntil.After(now) {
		return false,
			time.Until(
				attempt.BlockedUntil,
			)
	}

	if now.Sub(attempt.WindowStart) >
		l.window {

		attempt.Failures = 0
		attempt.WindowStart = now
		attempt.BlockedUntil = time.Time{}
	}

	return true, 0
}

func (l *loginRateLimiter) failure(
	key string,
) {
	now := time.Now()

	l.mu.Lock()
	defer l.mu.Unlock()

	attempt, ok := l.attempts[key]
	if !ok {
		attempt = &loginAttempt{
			WindowStart: now,
		}

		l.attempts[key] = attempt
	}

	if now.Sub(attempt.WindowStart) >
		l.window {

		attempt.Failures = 0
		attempt.WindowStart = now
		attempt.BlockedUntil = time.Time{}
	}

	attempt.Failures++
	attempt.LastSeen = now

	if attempt.Failures >=
		l.maxAttempts {

		attempt.BlockedUntil =
			now.Add(l.lockout)
	}
}

func (l *loginRateLimiter) success(
	key string,
) {
	l.mu.Lock()
	defer l.mu.Unlock()

	delete(
		l.attempts,
		key,
	)
}

func remoteIP(
	remoteAddr string,
) string {
	host, _, err := net.SplitHostPort(
		remoteAddr,
	)

	if err == nil {
		return host
	}

	return remoteAddr
}

func NewWebServer(
	cfg *Config,
	smtp *SMTPServer,
) *WebServer {
	sessionTimeout :=
		30 * time.Minute

	if cfg.Web.Auth.SessionTimeout != "" {
		if parsed, err :=
			time.ParseDuration(
				cfg.Web.Auth.SessionTimeout,
			); err == nil &&
			parsed > 0 {

			sessionTimeout = parsed
		}
	}

	loginWindow :=
		5 * time.Minute

	if cfg.Web.Auth.LoginWindow != "" {
		if parsed, err :=
			time.ParseDuration(
				cfg.Web.Auth.LoginWindow,
			); err == nil &&
			parsed > 0 {

			loginWindow = parsed
		}
	}

	loginLockout :=
		15 * time.Minute

	if cfg.Web.Auth.LoginLockout != "" {
		if parsed, err :=
			time.ParseDuration(
				cfg.Web.Auth.LoginLockout,
			); err == nil &&
			parsed > 0 {

			loginLockout = parsed
		}
	}

	maxAttempts :=
		cfg.Web.Auth.LoginMaxAttempts

	if maxAttempts <= 0 {
		maxAttempts = 5
	}

	return &WebServer{
		config:     cfg,
		smtpServer: smtp,

		staticFiles: make(
			map[string]cachedStaticFile,
		),

		sessions: newSessionStore(
			sessionTimeout,
		),

		loginLimiter: newLoginRateLimiter(
			maxAttempts,
			loginWindow,
			loginLockout,
		),
	}
}

func decodeRFC2047(
	encoded string,
) string {
	decoder := new(
		mime.WordDecoder,
	)

	decoded, err :=
		decoder.Decode(encoded)

	if err == nil {
		return decoded
	}

	if strings.Contains(
		encoded,
		"=?UTF-8?B?",
	) {
		parts := strings.Split(
			encoded,
			"?B?",
		)

		if len(parts) >= 2 {
			b64part := strings.TrimSuffix(
				parts[1],
				"?=",
			)

			if decodedBytes, err :=
				base64.StdEncoding.
					DecodeString(
						b64part,
					); err == nil {

				return string(
					decodedBytes,
				)
			}
		}
	}

	return encoded
}

func decodeQuotedPrintable(
	data string,
) string {
	reader :=
		quotedprintable.NewReader(
			strings.NewReader(data),
		)

	buf := new(bytes.Buffer)

	_, _ = buf.ReadFrom(reader)

	return buf.String()
}

func parseEmailHeaders(
	content []byte,
) (
	from string,
	to string,
	subject string,
	date time.Time,
) {
	from = "Unknown"
	to = "Unknown"
	subject = "No subject"
	date = time.Now()

	lines := strings.Split(
		string(content),
		"\n",
	)

	for _, line := range lines {
		line = strings.TrimSpace(
			line,
		)

		lowerLine :=
			strings.ToLower(line)

		switch {
		case strings.HasPrefix(
			lowerLine,
			"from:",
		):
			raw := strings.TrimSpace(
				line[5:],
			)

			raw = strings.Trim(
				raw,
				"\"<> ",
			)

			from = decodeRFC2047(raw)

			if from == "" {
				from = "Unknown"
			}

		case strings.HasPrefix(
			lowerLine,
			"to:",
		):
			raw := strings.TrimSpace(
				line[3:],
			)

			raw = strings.Trim(
				raw,
				"\"<> ",
			)

			to = decodeRFC2047(raw)

			if to == "" {
				to = "Unknown"
			}

		case strings.HasPrefix(
			lowerLine,
			"subject:",
		):
			raw := strings.TrimSpace(
				line[8:],
			)

			subject =
				decodeRFC2047(raw)

			if subject == "" {
				subject = "No subject"
			}

		case strings.HasPrefix(
			lowerLine,
			"date:",
		):
			dateStr :=
				strings.TrimSpace(
					line[5:],
				)

			formats := []string{
				time.RFC1123,
				time.RFC1123Z,
				time.RFC822,
				time.RFC822Z,

				"Mon, 2 Jan 2006 15:04:05 -0700",
				"Mon, 2 Jan 2006 15:04:05 MST",
				"2 Jan 2006 15:04:05 -0700",
			}

			for _, format := range formats {

				parsedDate, err :=
					time.Parse(
						format,
						dateStr,
					)

				if err == nil {
					date = parsedDate
					break
				}
			}
		}
	}

	return
}

func extractMIMEPartBody(
	part string,
) string {
	if idx := strings.Index(
		part,
		"\r\n\r\n",
	); idx != -1 {

		return strings.TrimSuffix(
			part[idx+4:],
			"--",
		)
	}

	if idx := strings.Index(
		part,
		"\n\n",
	); idx != -1 {

		return strings.TrimSuffix(
			part[idx+2:],
			"--",
		)
	}

	return ""
}

func extractEmailBody(
	content []byte,
) string {
	str := string(content)
	boundary := ""

	lines := strings.Split(
		str,
		"\n",
	)

	for i, line := range lines {
		lowerLine :=
			strings.ToLower(line)

		if !strings.HasPrefix(
			lowerLine,
			"content-type:",
		) {
			continue
		}

		if idx := strings.Index(
			line,
			"boundary=",
		); idx != -1 {

			boundary = strings.Trim(
				line[idx+9:],
				"\" \r\n",
			)

			break
		}

		if i+1 < len(lines) &&
			strings.Contains(
				strings.ToLower(
					lines[i+1],
				),
				"boundary=",
			) {

			nextLine := lines[i+1]

			if idx := strings.Index(
				nextLine,
				"boundary=",
			); idx != -1 {

				boundary =
					strings.Trim(
						nextLine[idx+9:],
						"\" \r\n",
					)
			}
		}
	}

	if boundary != "" {
		parts := strings.Split(
			str,
			"--"+boundary,
		)

		// Сначала text/plain.
		for _, part := range parts {
			if !strings.Contains(
				strings.ToLower(part),
				"content-type: text/plain",
			) {
				continue
			}

			body :=
				extractMIMEPartBody(
					part,
				)

			if body == "" {
				continue
			}

			if strings.Contains(
				strings.ToLower(part),
				"content-transfer-encoding: quoted-printable",
			) {
				body =
					decodeQuotedPrintable(
						body,
					)
			}

			return strings.TrimSpace(
				body,
			)
		}

		// Затем text/html.
		for _, part := range parts {
			if !strings.Contains(
				strings.ToLower(part),
				"content-type: text/html",
			) {
				continue
			}

			body :=
				extractMIMEPartBody(
					part,
				)

			if body == "" {
				continue
			}

			if strings.Contains(
				strings.ToLower(part),
				"content-transfer-encoding: quoted-printable",
			) {
				body =
					decodeQuotedPrintable(
						body,
					)
			}

			return strings.TrimSpace(
				body,
			)
		}
	}

	body :=
		extractMIMEPartBody(str)

	if body != "" {
		if strings.Contains(
			strings.ToLower(str),
			"content-transfer-encoding: quoted-printable",
		) {
			body =
				decodeQuotedPrintable(
					body,
				)
		}

		return strings.TrimSpace(
			body,
		)
	}

	return "No content"
}

func listEmails(
	storageDir string,
) ([]DayEmails, error) {
	days := make([]DayEmails, 0)

	entries, err :=
		os.ReadDir(storageDir)

	if err != nil {
		if os.IsNotExist(err) {
			return days, nil
		}

		return nil, err
	}

	for _, entry := range entries {
		if !entry.IsDir() {
			continue
		}

		if _, err := time.Parse(
			"2006-01-02",
			entry.Name(),
		); err != nil {
			continue
		}

		dayPath := filepath.Join(
			storageDir,
			entry.Name(),
		)

		files, err :=
			os.ReadDir(dayPath)

		if err != nil {
			continue
		}

		emails :=
			make([]EmailFile, 0)

		for _, file := range files {
			if file.IsDir() {
				continue
			}

			if file.Type()&
				os.ModeSymlink != 0 {

				continue
			}

			if !strings.EqualFold(
				filepath.Ext(
					file.Name(),
				),
				".eml",
			) {
				continue
			}

			filePath :=
				filepath.Join(
					dayPath,
					file.Name(),
				)

			info, err :=
				file.Info()

			if err != nil {
				continue
			}

			content, err :=
				os.ReadFile(
					filePath,
				)

			if err != nil {
				continue
			}

			from,
				to,
				subject,
				date :=
				parseEmailHeaders(
					content,
				)

			emailID :=
				filepath.ToSlash(
					filepath.Join(
						entry.Name(),
						file.Name(),
					),
				)

			emails = append(
				emails,
				EmailFile{
					Name: file.Name(),
					ID:   emailID,

					Size: info.Size(),

					ModTime: info.ModTime(),

					From:    from,
					To:      to,
					Subject: subject,
					Date:    date,
				},
			)
		}

		sort.Slice(
			emails,
			func(i, j int) bool {
				return emails[i].
					ModTime.
					After(
						emails[j].
							ModTime,
					)
			},
		)

		if len(emails) > 0 {
			days = append(
				days,
				DayEmails{
					Date: entry.Name(),

					Emails: emails,
				},
			)
		}
	}

	sort.Slice(
		days,
		func(i, j int) bool {
			return days[i].Date >
				days[j].Date
		},
	)

	return days, nil
}

func resolveEmailPath(
	storageDir string,
	id string,
) (string, error) {
	id = strings.TrimSpace(id)

	if id == "" {
		return "", fmt.Errorf(
			"%w: empty id",
			errInvalidEmailID,
		)
	}

	if strings.Contains(
		id,
		"\\",
	) {
		return "", fmt.Errorf(
			"%w: backslash is not allowed",
			errInvalidEmailID,
		)
	}

	parts := strings.Split(
		id,
		"/",
	)

	if len(parts) != 2 {
		return "", fmt.Errorf(
			"%w: invalid path structure",
			errInvalidEmailID,
		)
	}

	day := parts[0]
	filename := parts[1]

	if _, err := time.Parse(
		"2006-01-02",
		day,
	); err != nil {
		return "", fmt.Errorf(
			"%w: invalid date directory",
			errInvalidEmailID,
		)
	}

	if filename == "" ||
		filename == "." ||
		filename == ".." ||
		filepath.Base(filename) !=
			filename {

		return "", fmt.Errorf(
			"%w: invalid filename",
			errInvalidEmailID,
		)
	}

	if !strings.EqualFold(
		filepath.Ext(filename),
		".eml",
	) {
		return "", fmt.Errorf(
			"%w: only .eml is allowed",
			errInvalidEmailID,
		)
	}

	storageAbs, err :=
		filepath.Abs(storageDir)

	if err != nil {
		return "", fmt.Errorf(
			"cannot resolve storage directory: %w",
			err,
		)
	}

	storageResolved, err :=
		filepath.EvalSymlinks(
			storageAbs,
		)

	if err != nil {
		return "", fmt.Errorf(
			"cannot resolve storage directory: %w",
			err,
		)
	}

	candidate := filepath.Join(
		storageAbs,
		day,
		filename,
	)

	candidateAbs, err :=
		filepath.Abs(candidate)

	if err != nil {
		return "", fmt.Errorf(
			"cannot resolve email path: %w",
			err,
		)
	}

	candidateResolved, err :=
		filepath.EvalSymlinks(
			candidateAbs,
		)

	if err != nil {
		if os.IsNotExist(err) {
			return "", os.ErrNotExist
		}

		return "", fmt.Errorf(
			"cannot resolve email file: %w",
			err,
		)
	}

	rel, err := filepath.Rel(
		storageResolved,
		candidateResolved,
	)

	if err != nil {
		return "", fmt.Errorf(
			"cannot validate email path: %w",
			err,
		)
	}

	if rel == ".." ||
		strings.HasPrefix(
			rel,
			".."+
				string(
					os.PathSeparator,
				),
		) ||
		filepath.IsAbs(rel) {

		return "", fmt.Errorf(
			"%w: path escapes storage directory",
			errInvalidEmailID,
		)
	}

	info, err :=
		os.Stat(
			candidateResolved,
		)

	if err != nil {
		if os.IsNotExist(err) {
			return "", os.ErrNotExist
		}

		return "", fmt.Errorf(
			"cannot access email: %w",
			err,
		)
	}

	if info.IsDir() {
		return "", fmt.Errorf(
			"%w: email points to directory",
			errInvalidEmailID,
		)
	}

	return candidateResolved, nil
}

func (w *WebServer) loadWebAssets() {
	w.staticFiles =
		make(
			map[string]cachedStaticFile,
		)

	webDir :=
		w.config.Web.Directory

	// index.html
	templatePath :=
		filepath.Join(
			webDir,
			"index.html",
		)

	templateData, err :=
		os.ReadFile(
			templatePath,
		)

	if err != nil {
		w.template = nil

		w.templateErr =
			fmt.Errorf(
				"web template not found: %s: %w",
				templatePath,
				err,
			)

		w.logger.Errorf(
			"%v",
			w.templateErr,
		)
	} else {
		tmpl, err :=
			template.New(
				"index.html",
			).Parse(
				string(
					templateData,
				),
			)

		if err != nil {
			w.template = nil

			w.templateErr =
				fmt.Errorf(
					"failed to parse web template %s: %w",
					templatePath,
					err,
				)

			w.logger.Errorf(
				"%v",
				w.templateErr,
			)
		} else {
			w.template = tmpl
			w.templateErr = nil

			w.logger.Infof(
				"Web template loaded: %s (%d bytes)",
				templatePath,
				len(templateData),
			)
		}
	}

	// login.html
	loginTemplatePath :=
		filepath.Join(
			webDir,
			"login.html",
		)

	loginTemplateData, err :=
		os.ReadFile(
			loginTemplatePath,
		)

	if err != nil {
		w.loginTemplate = nil

		w.loginTemplateErr =
			fmt.Errorf(
				"login template not found: %s: %w",
				loginTemplatePath,
				err,
			)

		w.logger.Errorf(
			"%v",
			w.loginTemplateErr,
		)
	} else {
		tmpl, err :=
			template.New(
				"login.html",
			).Parse(
				string(
					loginTemplateData,
				),
			)

		if err != nil {
			w.loginTemplate = nil

			w.loginTemplateErr =
				fmt.Errorf(
					"failed to parse login template %s: %w",
					loginTemplatePath,
					err,
				)

			w.logger.Errorf(
				"%v",
				w.loginTemplateErr,
			)
		} else {
			w.loginTemplate = tmpl

			w.loginTemplateErr = nil

			w.logger.Infof(
				"Login template loaded: %s (%d bytes)",
				loginTemplatePath,
				len(
					loginTemplateData,
				),
			)
		}
	}

	assets := []struct {
		Name        string
		ContentType string
	}{
		{
			Name: "app.css",

			ContentType: "text/css; charset=utf-8",
		},
		{
			Name: "app.js",

			ContentType: "application/javascript; charset=utf-8",
		},
		{
			Name: "tailwind.js",

			ContentType: "application/javascript; charset=utf-8",
		},
	}

	for _, asset := range assets {

		filePath :=
			filepath.Join(
				webDir,
				asset.Name,
			)

		data, err :=
			os.ReadFile(
				filePath,
			)

		if err != nil {
			w.logger.Warningf(
				"Cannot load web asset %s: %v",
				filePath,
				err,
			)

			continue
		}

		urlPath :=
			"/static/" +
				asset.Name

		w.staticFiles[urlPath] =
			cachedStaticFile{
				Data: data,

				ContentType: asset.ContentType,
			}

		w.logger.Infof(
			"Web asset loaded: %s (%d bytes)",
			filePath,
			len(data),
		)
	}
}

func (w *WebServer) handleStatic(
	wr http.ResponseWriter,
	r *http.Request,
) {
	if r.Method != http.MethodGet &&
		r.Method != http.MethodHead {

		wr.Header().Set(
			"Allow",
			"GET, HEAD",
		)

		http.Error(
			wr,
			"Method not allowed",
			http.StatusMethodNotAllowed,
		)

		return
	}

	file, ok := w.staticFiles[r.URL.Path]

	if !ok {
		http.NotFound(
			wr,
			r,
		)

		return
	}

	wr.Header().Set(
		"Content-Type",
		file.ContentType,
	)

	if r.Method == http.MethodHead {
		return
	}

	if _, err := wr.Write(file.Data); err != nil {
		w.logger.Errorf(
			"Failed to send static file %s: %v",
			r.URL.Path,
			err,
		)
	}
}

func constantTimeStringEqual(
	a string,
	b string,
) bool {
	aHash :=
		sha256.Sum256(
			[]byte(a),
		)

	bHash :=
		sha256.Sum256(
			[]byte(b),
		)

	return subtle.
		ConstantTimeCompare(
			aHash[:],
			bHash[:],
		) == 1
}

func (w *WebServer) setSessionCookie(
	wr http.ResponseWriter,
	sessionID string,
) {
	http.SetCookie(
		wr,
		&http.Cookie{
			Name: sessionCookieName,

			Value: sessionID,

			Path: "/",

			HttpOnly: true,

			Secure: w.config.Web.Auth.
				CookieSecure,

			SameSite: http.SameSiteStrictMode,
		},
	)
}

func (w *WebServer) clearSessionCookie(
	wr http.ResponseWriter,
) {
	http.SetCookie(
		wr,
		&http.Cookie{
			Name: sessionCookieName,

			Value: "",

			Path: "/",

			HttpOnly: true,

			Secure: w.config.Web.Auth.
				CookieSecure,

			SameSite: http.SameSiteStrictMode,

			MaxAge: -1,

			Expires: time.Unix(1, 0),
		},
	)
}

type loginPageData struct {
	Title string
	Error string
}

func (w *WebServer) renderLogin(
	wr http.ResponseWriter,
	status int,
	errorMessage string,
) {
	if w.loginTemplate == nil {
		message :=
			"Login template is unavailable"

		if w.loginTemplateErr != nil {
			message =
				w.loginTemplateErr.
					Error()
		}

		http.Error(
			wr,
			message,
			http.StatusInternalServerError,
		)

		return
	}

	wr.Header().Set(
		"Content-Type",
		"text/html; charset=utf-8",
	)

	wr.Header().Set(
		"Cache-Control",
		"no-store",
	)

	wr.WriteHeader(status)

	if err :=
		w.loginTemplate.Execute(
			wr,
			loginPageData{
				Title: "Fake SMTP Server - Login",

				Error: errorMessage,
			},
		); err != nil {

		w.logger.Errorf(
			"Login template execution error: %v",
			err,
		)
	}
}

func (w *WebServer) handleLogin(
	wr http.ResponseWriter,
	r *http.Request,
) {
	if !w.config.Web.Auth.Enabled {
		http.Redirect(
			wr,
			r,
			"/",
			http.StatusSeeOther,
		)

		return
	}

	switch r.Method {
	case http.MethodGet:
		if cookie, err :=
			r.Cookie(
				sessionCookieName,
			); err == nil {

			if w.sessions.validate(
				cookie.Value,
			) {
				http.Redirect(
					wr,
					r,
					"/",
					http.StatusSeeOther,
				)

				return
			}

			w.clearSessionCookie(
				wr,
			)
		}

		w.renderLogin(
			wr,
			http.StatusOK,
			"",
		)

	case http.MethodPost:
		clientIP :=
			remoteIP(
				r.RemoteAddr,
			)

		allowed, retryAfter :=
			w.loginLimiter.allow(
				clientIP,
			)

		if !allowed {
			seconds := int64(
				(retryAfter + time.Second - 1) / time.Second,
			)

			if seconds < 1 {
				seconds = 1
			}

			wr.Header().Set(
				"Retry-After",
				strconv.FormatInt(
					seconds,
					10,
				),
			)

			w.logger.Warningf(
				"Web login temporarily blocked for %s",
				clientIP,
			)

			w.renderLogin(
				wr,
				http.StatusTooManyRequests,
				"Too many failed login attempts. Try again later.",
			)

			return
		}

		r.Body =
			http.MaxBytesReader(
				wr,
				r.Body,
				16*1024,
			)

		if err :=
			r.ParseForm(); err != nil {

			w.renderLogin(
				wr,
				http.StatusBadRequest,
				"Invalid request",
			)

			return
		}

		username :=
			r.FormValue(
				"username",
			)

		password :=
			r.FormValue(
				"password",
			)

		validUsername :=
			constantTimeStringEqual(
				username,
				w.config.Web.Auth.
					Username,
			)

		validPassword :=
			constantTimeStringEqual(
				password,
				w.config.Web.Auth.
					Password,
			)

		if !validUsername ||
			!validPassword {

			w.loginLimiter.failure(
				clientIP,
			)

			w.logger.Warningf(
				"Failed web login from %s",
				clientIP,
			)

			w.renderLogin(
				wr,
				http.StatusUnauthorized,
				"Invalid username or password",
			)

			return
		}

		w.loginLimiter.success(
			clientIP,
		)

		// Защита от session fixation:
		// старую сессию удаляем.
		if oldCookie, err :=
			r.Cookie(
				sessionCookieName,
			); err == nil {

			w.sessions.delete(
				oldCookie.Value,
			)
		}

		sessionID, err :=
			w.sessions.create()

		if err != nil {
			w.logger.Errorf(
				"Failed to create session: %v",
				err,
			)

			http.Error(
				wr,
				"Failed to create session",
				http.StatusInternalServerError,
			)

			return
		}

		w.setSessionCookie(
			wr,
			sessionID,
		)

		w.logger.Infof(
			"Web login successful from %s",
			clientIP,
		)

		http.Redirect(
			wr,
			r,
			"/",
			http.StatusSeeOther,
		)

	default:
		wr.Header().Set(
			"Allow",
			"GET, POST",
		)

		http.Error(
			wr,
			"Method not allowed",
			http.StatusMethodNotAllowed,
		)
	}
}

func (w *WebServer) handleLogout(
	wr http.ResponseWriter,
	r *http.Request,
) {
	if r.Method !=
		http.MethodPost {

		wr.Header().Set(
			"Allow",
			http.MethodPost,
		)

		http.Error(
			wr,
			"Method not allowed",
			http.StatusMethodNotAllowed,
		)

		return
	}

	r.Body =
		http.MaxBytesReader(
			wr,
			r.Body,
			8*1024,
		)

	if err :=
		r.ParseForm(); err != nil {

		http.Error(
			wr,
			"Invalid request",
			http.StatusBadRequest,
		)

		return
	}

	cookie, err :=
		r.Cookie(
			sessionCookieName,
		)

	if err != nil {
		w.clearSessionCookie(
			wr,
		)

		http.Redirect(
			wr,
			r,
			"/login",
			http.StatusSeeOther,
		)

		return
	}

	csrfToken :=
		r.FormValue(
			"csrf_token",
		)

	if !w.sessions.validateCSRF(
		cookie.Value,
		csrfToken,
	) {
		w.logger.Warningf(
			"Rejected logout with invalid CSRF token from %s",
			r.RemoteAddr,
		)

		http.Error(
			wr,
			"Invalid CSRF token",
			http.StatusForbidden,
		)

		return
	}

	w.sessions.delete(
		cookie.Value,
	)

	w.clearSessionCookie(
		wr,
	)

	http.Redirect(
		wr,
		r,
		"/login",
		http.StatusSeeOther,
	)
}

func (w *WebServer) sessionAuth(
	next http.Handler,
) http.Handler {
	return http.HandlerFunc(
		func(
			wr http.ResponseWriter,
			r *http.Request,
		) {
			if !w.config.Web.Auth.Enabled {
				next.ServeHTTP(
					wr,
					r,
				)

				return
			}

			// Login должен быть публичным.
			if r.URL.Path ==
				"/login" {

				next.ServeHTTP(
					wr,
					r,
				)

				return
			}

			// Статика нужна странице login.
			if strings.HasPrefix(
				r.URL.Path,
				"/static/",
			) {
				next.ServeHTTP(
					wr,
					r,
				)

				return
			}

			cookie, err :=
				r.Cookie(
					sessionCookieName,
				)

			if err == nil &&
				w.sessions.validate(
					cookie.Value,
				) {

				next.ServeHTTP(
					wr,
					r,
				)

				return
			}

			w.clearSessionCookie(
				wr,
			)

			if strings.HasPrefix(
				r.URL.Path,
				"/api/",
			) {
				http.Error(
					wr,
					"Session expired",
					http.StatusUnauthorized,
				)

				return
			}

			http.Redirect(
				wr,
				r,
				"/login",
				http.StatusSeeOther,
			)
		},
	)
}

func (w *WebServer) readEmailByID(
	id string,
) (
	[]byte,
	string,
	error,
) {
	filePath, err :=
		resolveEmailPath(
			w.config.SMTP.StorageDir,
			id,
		)

	if err != nil {
		return nil, "", err
	}

	content, err :=
		os.ReadFile(
			filePath,
		)

	if err != nil {
		return nil, "", err
	}

	return content,
		filePath,
		nil
}

func (w *WebServer) handleEmailError(
	wr http.ResponseWriter,
	r *http.Request,
	id string,
	err error,
) {
	if errors.Is(
		err,
		os.ErrNotExist,
	) {
		http.NotFound(
			wr,
			r,
		)

		return
	}

	if errors.Is(
		err,
		errInvalidEmailID,
	) {
		w.logger.Warningf(
			"Rejected email id %q from %s: %v",
			id,
			r.RemoteAddr,
			err,
		)

		http.Error(
			wr,
			"Invalid email id",
			http.StatusBadRequest,
		)

		return
	}

	w.logger.Errorf(
		"Error accessing email %q: %v",
		id,
		err,
	)

	http.Error(
		wr,
		"Error reading email",
		http.StatusInternalServerError,
	)
}

func (w *WebServer) handleIndex(
	wr http.ResponseWriter,
	r *http.Request,
) {
	if r.URL.Path != "/" {
		http.NotFound(
			wr,
			r,
		)

		return
	}

	if r.Method != http.MethodGet &&
		r.Method != http.MethodHead {

		wr.Header().Set(
			"Allow",
			"GET, HEAD",
		)

		http.Error(
			wr,
			"Method not allowed",
			http.StatusMethodNotAllowed,
		)

		return
	}

	if w.template == nil {
		message :=
			"Web template is unavailable"

		if w.templateErr != nil {
			message =
				w.templateErr.Error()
		}

		http.Error(
			wr,
			message,
			http.StatusInternalServerError,
		)

		return
	}

	days, err :=
		listEmails(
			w.config.SMTP.StorageDir,
		)

	if err != nil {
		w.logger.Errorf(
			"Error listing emails: %v",
			err,
		)

		http.Error(
			wr,
			"Error listing emails",
			http.StatusInternalServerError,
		)

		return
	}

	csrfToken := ""

	if w.config.Web.Auth.Enabled {
		if cookie, err :=
			r.Cookie(
				sessionCookieName,
			); err == nil {

			if token, ok :=
				w.sessions.csrfToken(
					cookie.Value,
				); ok {

				csrfToken = token
			}
		}
	}

	data := struct {
		Days      []DayEmails
		Title     string
		CSRFToken string
	}{
		Days: days,

		Title: "Fake SMTP Server - Emails",

		CSRFToken: csrfToken,
	}

	wr.Header().Set(
		"Content-Type",
		"text/html; charset=utf-8",
	)

	wr.Header().Set(
		"Cache-Control",
		"no-store",
	)

	if r.Method ==
		http.MethodHead {

		return
	}

	if err :=
		w.template.Execute(
			wr,
			data,
		); err != nil {

		w.logger.Errorf(
			"Template execution error: %v",
			err,
		)
	}
}

func (w *WebServer) handleEmailsAPI(
	wr http.ResponseWriter,
	r *http.Request,
) {
	if r.Method !=
		http.MethodGet {

		wr.Header().Set(
			"Allow",
			http.MethodGet,
		)

		http.Error(
			wr,
			"Method not allowed",
			http.StatusMethodNotAllowed,
		)

		return
	}

	days, err :=
		listEmails(
			w.config.SMTP.StorageDir,
		)

	if err != nil {
		w.logger.Errorf(
			"Error listing emails: %v",
			err,
		)

		wr.Header().Set(
			"Content-Type",
			"application/json; charset=utf-8",
		)

		wr.WriteHeader(
			http.StatusInternalServerError,
		)

		_ = json.NewEncoder(
			wr,
		).Encode(
			map[string]string{
				"error": "Error listing emails",
			},
		)

		return
	}

	wr.Header().Set(
		"Content-Type",
		"application/json; charset=utf-8",
	)

	if err :=
		json.NewEncoder(
			wr,
		).Encode(
			days,
		); err != nil {

		w.logger.Errorf(
			"Failed to encode emails JSON: %v",
			err,
		)
	}
}

func (w *WebServer) handleEmailAPI(
	wr http.ResponseWriter,
	r *http.Request,
) {
	if r.Method !=
		http.MethodGet {

		wr.Header().Set(
			"Allow",
			http.MethodGet,
		)

		http.Error(
			wr,
			"Method not allowed",
			http.StatusMethodNotAllowed,
		)

		return
	}

	id :=
		r.URL.Query().
			Get("id")

	if id == "" {
		http.Error(
			wr,
			"Missing id parameter",
			http.StatusBadRequest,
		)

		return
	}

	content, _, err :=
		w.readEmailByID(id)

	if err != nil {
		w.handleEmailError(
			wr,
			r,
			id,
			err,
		)

		return
	}

	body :=
		extractEmailBody(
			content,
		)

	wr.Header().Set(
		"Content-Type",
		"text/html; charset=utf-8",
	)

	if _, err :=
		wr.Write(
			[]byte(body),
		); err != nil {

		w.logger.Errorf(
			"Failed to send email %q: %v",
			id,
			err,
		)
	}
}

func (w *WebServer) handleViewAPI(
	wr http.ResponseWriter,
	r *http.Request,
) {
	if r.Method !=
		http.MethodGet {

		wr.Header().Set(
			"Allow",
			http.MethodGet,
		)

		http.Error(
			wr,
			"Method not allowed",
			http.StatusMethodNotAllowed,
		)

		return
	}

	id :=
		r.URL.Query().
			Get("id")

	if id == "" {
		http.Error(
			wr,
			"Missing id parameter",
			http.StatusBadRequest,
		)

		return
	}

	content, _, err :=
		w.readEmailByID(id)

	if err != nil {
		w.handleEmailError(
			wr,
			r,
			id,
			err,
		)

		return
	}

	body :=
		extractEmailBody(
			content,
		)

	wr.Header().Set(
		"Content-Type",
		"text/html; charset=utf-8",
	)

	if _, err :=
		wr.Write(
			[]byte(body),
		); err != nil {

		w.logger.Errorf(
			"Failed to send email view %q: %v",
			id,
			err,
		)
	}
}

func (w *WebServer) handleDownloadAPI(
	wr http.ResponseWriter,
	r *http.Request,
) {
	if r.Method !=
		http.MethodGet {

		wr.Header().Set(
			"Allow",
			http.MethodGet,
		)

		http.Error(
			wr,
			"Method not allowed",
			http.StatusMethodNotAllowed,
		)

		return
	}

	id :=
		r.URL.Query().
			Get("id")

	if id == "" {
		http.Error(
			wr,
			"Missing id parameter",
			http.StatusBadRequest,
		)

		return
	}

	content,
		filePath,
		err :=
		w.readEmailByID(id)

	if err != nil {
		w.handleEmailError(
			wr,
			r,
			id,
			err,
		)

		return
	}

	filename :=
		filepath.Base(
			filePath,
		)

	disposition :=
		mime.FormatMediaType(
			"attachment",
			map[string]string{
				"filename": filename,
			},
		)

	wr.Header().Set(
		"Content-Type",
		"message/rfc822",
	)

	wr.Header().Set(
		"Content-Disposition",
		disposition,
	)

	wr.Header().Set(
		"Content-Length",
		strconv.Itoa(
			len(content),
		),
	)

	if _, err :=
		wr.Write(content); err != nil {

		w.logger.Errorf(
			"Failed to download email %q: %v",
			id,
			err,
		)
	}
}

func (w *WebServer) Start() error {
	if !w.config.Web.Enabled {
		w.logger.Info(
			"Web server disabled in config",
		)

		return nil
	}

	// HTML/CSS/JS читаются один раз при запуске.
	w.loadWebAssets()

	mux := http.NewServeMux()

	mux.HandleFunc(
		"/static/",
		w.handleStatic,
	)

	mux.HandleFunc(
		"/login",
		w.handleLogin,
	)

	mux.HandleFunc(
		"/logout",
		w.handleLogout,
	)

	mux.HandleFunc(
		"/api/emails",
		w.handleEmailsAPI,
	)

	mux.HandleFunc(
		"/api/email",
		w.handleEmailAPI,
	)

	mux.HandleFunc(
		"/api/view",
		w.handleViewAPI,
	)

	mux.HandleFunc(
		"/api/download",
		w.handleDownloadAPI,
	)

	mux.HandleFunc(
		"/",
		w.handleIndex,
	)

	addr := fmt.Sprintf(
		":%d",
		w.config.Web.Port,
	)

	w.server =
		&http.Server{
			Addr: addr,

			Handler: w.sessionAuth(mux),

			ReadTimeout: 10 * time.Second,

			WriteTimeout: 10 * time.Second,

			IdleTimeout: 60 * time.Second,

			ReadHeaderTimeout: 5 * time.Second,
		}

	// net.Listen выполняется синхронно,
	// поэтому занятый порт сразу вернёт ошибку из Start().
	listener, err :=
		net.Listen(
			"tcp",
			addr,
		)

	if err != nil {
		w.server = nil

		return fmt.Errorf(
			"failed to listen on %s: %w",
			addr,
			err,
		)
	}

	w.logger.Infof(
		"Web server listening on http://0.0.0.0:%d",
		w.config.Web.Port,
	)

	w.logger.Infof(
		"Web directory: %s",
		w.config.Web.Directory,
	)

	w.logger.Infof(
		"Web Auth enabled: %v",
		w.config.Web.Auth.Enabled,
	)

	if w.config.Web.Auth.Enabled {
		w.logger.Infof(
			"Web session inactivity timeout: %s",
			w.config.Web.Auth.SessionTimeout,
		)

		w.logger.Infof(
			"Web session cookie Secure: %v",
			w.config.Web.Auth.CookieSecure,
		)

		w.logger.Infof(
			"Web login rate limit: %d attempts / %s, lockout %s",
			w.config.Web.Auth.LoginMaxAttempts,
			w.config.Web.Auth.LoginWindow,
			w.config.Web.Auth.LoginLockout,
		)
	}

	go func() {
		err :=
			w.server.Serve(
				listener,
			)

		if err != nil &&
			!errors.Is(
				err,
				http.ErrServerClosed,
			) {

			w.logger.Errorf(
				"Web server error: %v",
				err,
			)
		}
	}()

	return nil
}

func (w *WebServer) Stop() error {
	if w.server == nil {
		return nil
	}

	w.logger.Info(
		"Stopping web server...",
	)

	ctx, cancel :=
		context.WithTimeout(
			context.Background(),
			5*time.Second,
		)

	defer cancel()

	err :=
		w.server.Shutdown(ctx)

	w.server = nil

	if err != nil {
		return fmt.Errorf(
			"failed to stop web server: %w",
			err,
		)
	}

	return nil
}