package main

import (
	"bufio"
	"bytes"
	"context"
	"crypto/tls"
	"errors"
	"io"
	"net"
	"slices"
	"strings"

	"github.com/jackc/pgx/v5/pgxpool"
	"go.lindenii.runxiyu.org/lindenii-common/clog"
	"go.lindenii.runxiyu.org/lindenii-common/mailkit"
)

type mx_server_state_t uint

const (
	mx_server_state_begin mx_server_state_t = iota
	mx_server_state_helo
	mx_server_state_mail
	mx_server_state_rcpt
)

type mx_recv_session struct {
	buf_conn           *bufio.ReadWriter
	net_conn           net.Conn
	tls_conn           *tls.Conn
	my_server_name     string
	tls_config         *tls.Config
	db                 *pgxpool.Pool
	remote_server_name string
	current_mail_from  string
	current_rcpt_to    []string
	server_state       mx_server_state_t
}

func (session *mx_recv_session) handle(ctx context.Context) error {
	session.buf_conn = bufio.NewReadWriter(bufio.NewReader(session.net_conn), bufio.NewWriter(session.net_conn))
	config_consistent_run(func() {
		session.my_server_name = config.Server_name
		session.tls_config = config._tls_config
		session.db = global_db
	})
	_, _ = session.buf_conn.WriteString("220 " + session.my_server_name + " " + VERSION + "\r\n")
	_ = session.buf_conn.Flush()
	session.server_state = mx_server_state_begin
	for {
		line, err := session.buf_conn.ReadString('\n')
		if err != nil {
			if err == io.EOF {
				return err_connection_handler_eof
			}
			return err
		}
		line = strings.TrimSuffix(line, "\n")
		line = strings.TrimSuffix(line, "\r")
		cmd_end := strings.IndexByte(line, ' ')
		var param_start int
		if cmd_end == -1 {
			cmd_end = len(line)
			param_start = len(line)
		} else {
			param_start = cmd_end + 1
		}
		cmd := strings.ToUpper(line[:cmd_end])
		param := line[param_start:]
	switch_cmd:
		switch cmd {
		case "STARTTLS":
			if param != "" {
				_, _ = session.buf_conn.WriteString("501 5.5.4 Syntax error (no parameters allowed)\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			if session.tls_conn != nil {
				_, _ = session.buf_conn.WriteString("554 5.5.1 Error: TLS already active\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			_, _ = session.buf_conn.WriteString("220 2.0.0 Ready to start TLS\r\n")
			_ = session.buf_conn.Flush()
			session.tls_conn = tls.Server(session.net_conn, session.tls_config)
			session.buf_conn = bufio.NewReadWriter(bufio.NewReader(session.tls_conn), bufio.NewWriter(session.tls_conn))
			session.server_state = mx_server_state_begin
			session.current_mail_from = ""
			session.current_rcpt_to = []string{""}
		case "HELO":
			if param == "" { // TODO: actually validate the hostname
				_, _ = session.buf_conn.WriteString("501 Syntax: HELO hostname\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			session.remote_server_name = param
			_ = session.remote_server_name // TODO
			session.server_state = mx_server_state_helo
			_, _ = session.buf_conn.WriteString("250 " + session.my_server_name + "\r\n")
			_ = session.buf_conn.Flush()
		case "MAIL":
			switch session.server_state {
			case mx_server_state_begin:
				_, _ = session.buf_conn.WriteString("503 5.5.1 Error: send HELO/EHLO first\r\n")
				_ = session.buf_conn.Flush()
				break switch_cmd
			case mx_server_state_helo:
				break
			case mx_server_state_mail:
				_, _ = session.buf_conn.WriteString("503 5.5.1 Error: nested MAIL command\r\n")
				_ = session.buf_conn.Flush()
				break switch_cmd
			}
			if len(param) <= len("FROM:") || strings.ToUpper(param[:len("FROM:")]) != "FROM:" {
				_, _ = session.buf_conn.WriteString("501 5.5.4 Syntax: MAIL FROM:<address>\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			session.current_mail_from = param[len("FROM:"):]
			session.current_rcpt_to = []string{}
			session.server_state = mx_server_state_mail
			_, _ = session.buf_conn.WriteString("250 2.1.0 Ok\r\n")
			_ = session.buf_conn.Flush()
			// TODO: Address validation
		case "RCPT":
			if session.server_state != mx_server_state_mail && session.server_state != mx_server_state_rcpt {
				_, _ = session.buf_conn.WriteString("503 5.5.1 Error: need MAIL command\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			if len(param) <= len("TO:") || strings.ToUpper(param[:len("TO:")]) != "TO:" {
				_, _ = session.buf_conn.WriteString("501 5.5.4 Syntax: RCPT TO:<address>\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			recipient_address, _, _ := mailkit.Strip_angle_brackets(param[len("TO:"):])
			var count int
			err := session.db.QueryRow(ctx, "SELECT COUNT (*) FROM addresses WHERE address = $1", recipient_address).Scan(&count)
			if err != nil {
				_, _ = session.buf_conn.WriteString("451 Internal error: " + err.Error() + "\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			if count == 0 {
				_, _ = session.buf_conn.WriteString("550 5.1.1 Recipient address rejected: Local recipients not found: " + recipient_address + "\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			session.current_rcpt_to = append(session.current_rcpt_to, recipient_address)
			session.server_state = mx_server_state_rcpt
			_, _ = session.buf_conn.WriteString("250 2.1.5 Ok\r\n")
			_ = session.buf_conn.Flush()
		case "DATA":
			if session.server_state != mx_server_state_rcpt {
				_, _ = session.buf_conn.WriteString("503 5.5.1 Error: need RCPT command\r\n")
				_ = session.buf_conn.Flush()
				break
			}
			_, _ = session.buf_conn.WriteString("354 End data with <CR><LF>.<CR><LF>\r\n")
			_ = session.buf_conn.Flush()
			var current_data []byte
			for {
				tmp, err := session.buf_conn.ReadSlice('\r')
				if err != nil {
					return err
				}

				// session.buf_conn.ReadSlice returns an internal buffer that gets
				// overwritten on the next reader operation. So we must
				// make a copy; also we have to allocate data_part to
				// the correct length because [[builtin.copy]] copies
				// min(len(dst), len(src)) items.
				data_part := make([]byte, len(tmp))
				copy(data_part, tmp)

				next_four, err := session.buf_conn.Peek(4)
				if err != nil {
					return err
				}
				if bytes.Equal(next_four, []byte{'\n', '.', '\r', '\n'}) {
					current_data = slices.Concat(current_data, data_part[:len(data_part)-1])
					break
				}
				current_data = slices.Concat(current_data, data_part)
			}
			_, err := session.buf_conn.Discard(4)
			if err != nil {
				return err
			}
			err = deliver_local(ctx, session.db, session.current_mail_from, session.current_rcpt_to, current_data, session.current_rcpt_to)
			var err_local_recipients_not_found *err_local_recipients_not_found_t
			switch {
			case errors.As(err, &err_local_recipients_not_found):
				_, _ = session.buf_conn.WriteString("550 5.1.1 Recipient address rejected: " + err_local_recipients_not_found.Error() + "\r\n")
			case err != nil:
				_, _ = session.buf_conn.WriteString("500 2.0.0 Error: " + err.Error() + "\r\n")
			default:
				_, _ = session.buf_conn.WriteString("250 2.0.0 Ok: Accepted\r\n")
			}
			_ = session.buf_conn.Flush()
			session.server_state = mx_server_state_helo
		case "QUIT":
			_, _ = session.buf_conn.WriteString("221 2.0.0 Bye\r\n")
			_ = session.buf_conn.Flush()
			if session.tls_conn != nil {
				session.tls_conn.Close()
			}
			session.net_conn.Close()
			return nil
		case "NOOP":
			_, _ = session.buf_conn.WriteString("250 2.0.0 Ok\r\n")
			_ = session.buf_conn.Flush()
		case "RSET":
			if session.server_state != mx_server_state_begin {
				session.server_state = mx_server_state_helo
			}
			_, _ = session.buf_conn.WriteString("250 2.0.0 Ok\r\n")
			_ = session.buf_conn.Flush()
		default:
			_, _ = session.buf_conn.WriteString("500 5.5.2 Error: command not recognized\r\n")
			_ = session.buf_conn.Flush()
		}
	}
}

func mx_new_session(ctx context.Context, net_conn net.Conn) error {
	session := mx_recv_session{
		net_conn: net_conn,
	}
	return session.handle(ctx)
}

func serve_mx() {
	var mx_net, mx_addr string
	config_consistent_run(func() {
		mx_net = config.MX.Net
		mx_addr = config.MX.Addr
	})
	listener, err := net.Listen(mx_net, mx_addr)
	if err != nil {
		clog.Fatal(1, "MX: Cannot listen: "+err.Error())
	}
	defer listener.Close()
	clog.Info("MX: Listening via " + mx_net + " on " + mx_addr)

	for {
		conn, err := listener.Accept()
		if err != nil {
			clog.Error("MX: Cannot accept connection: " + err.Error())
		}
		clog.Info("MX: Accepted connection from " + conn.RemoteAddr().String())

		go func() {
			ctx, cancel := context.WithCancel(context.Background())
			defer cancel()
			err := mx_new_session(ctx, conn)
			if err != nil {
				if err == err_connection_handler_eof {
					clog.Info("MX: Connection for " + conn.RemoteAddr().String() + " closed with EOF")
				} else {
					clog.Error("MX: Connection handler for " + conn.RemoteAddr().String() + " returned error: " + err.Error())
				}
			} else {
				clog.Info("MX: Connection for " + conn.RemoteAddr().String() + " closed gracefully")
			}
		}()
	}
}