package main

import (
	"bytes"
	"fmt"
	"io"
	"net"
	"strings"
	"time"

	"go.bug.st/serial"
)

// transport is a bidirectional byte stream to a Lutron interface. Both the
// serial and telnet implementations satisfy it.
type transport io.ReadWriteCloser

// openTransport dials a device using its configured transport.
func openTransport(cfg *DeviceConfig) (transport, error) {
	switch cfg.Transport {
	case "serial":
		return openSerial(cfg.Serial)
	case "telnet":
		return openTelnet(cfg.Telnet)
	default:
		return nil, fmt.Errorf("unknown transport %q", cfg.Transport)
	}
}

// openSerial opens the serial port at the configured baud rate.
func openSerial(cfg SerialConfig) (transport, error) {
	port, err := serial.Open(cfg.Device, &serial.Mode{BaudRate: cfg.Baud})
	if err != nil {
		return nil, err
	}
	return port, nil
}

// telnetLoginTimeout bounds the login handshake so a wrong host can't wedge the
// connect path indefinitely.
const telnetLoginTimeout = 10 * time.Second

// openTelnet dials the device over TCP and performs the Lutron integration
// login handshake, returning a stream with telnet IAC negotiation filtered out.
func openTelnet(cfg TelnetConfig) (transport, error) {
	addr := cfg.Address
	if _, _, err := net.SplitHostPort(addr); err != nil {
		// No port given; default to the telnet port.
		addr = net.JoinHostPort(addr, "23")
	}
	conn, err := net.DialTimeout("tcp", addr, telnetLoginTimeout)
	if err != nil {
		return nil, err
	}
	// Keep the TCP connection alive. A QSE-CI-NWK-E telnet link can sit idle for
	//  long stretches (we only write on changes), and a router/NAT may drop an
	//  idle connection without keepalives (per Lutron App Note 048618).
	if tcp, ok := conn.(*net.TCPConn); ok {
		tcp.SetKeepAlive(true)
		tcp.SetKeepAlivePeriod(30 * time.Second)
	}
	tc := &telnetConn{Conn: conn}

	// Perform the login handshake. The QSE-CI-NWK-E prompts "login:" and, on some
	//  units, "password:"; we answer each as it appears.
	if err := tc.login(cfg.Username, cfg.Password); err != nil {
		conn.Close()
		return nil, err
	}
	return tc, nil
}

// telnetConn wraps a net.Conn and strips telnet IAC command sequences from the
// inbound stream, auto-refusing any option negotiation so the server proceeds.
type telnetConn struct {
	net.Conn
	pending []byte // Bytes held back mid-IAC-sequence across Read calls.
}

// Telnet command bytes (RFC 854) used by the IAC filter.
const (
	iac  = 255
	iacD = 254 // DONT
	iacC = 253 // DO
	iacW = 252 // WONT
	iacL = 251 // WILL
	iacS = 250 // SB (subnegotiation begin)
	iacE = 240 // SE (subnegotiation end)
)

// login reads prompts and sends the credentials as they are requested. It
// returns once credentials are handled and the integration prompt is seen.
func (t *telnetConn) login(username, password string) error {
	t.SetReadDeadline(time.Now().Add(telnetLoginTimeout))
	defer t.SetReadDeadline(time.Time{})

	var acc []byte
	buf := make([]byte, 256)
	sentUser, sentPass := false, false
	for {
		n, err := t.Read(buf)
		if n > 0 {
			acc = append(acc, buf[:n]...)
			lower := strings.ToLower(string(acc))
			if !sentUser && username != "" && strings.Contains(lower, "login:") {
				if _, err := t.Write([]byte(username + "\r\n")); err != nil {
					return err
				}
				sentUser = true
				acc = acc[:0]
				continue
			}
			// The QSE-CI-NWK-E prompts "passphrase:" when a login passphrase is set;
			//  older/other units use "password:". Answer either.
			if !sentPass && password != "" && (strings.Contains(lower, "passphrase:") || strings.Contains(lower, "password:")) {
				if _, err := t.Write([]byte(password + "\r\n")); err != nil {
					return err
				}
				sentPass = true
				acc = acc[:0]
				continue
			}
			// Once credentials are handled and the unit echoes a prompt, the link
			//  is ready for integration commands.
			if strings.Contains(lower, "qse>") || strings.Contains(lower, "gnet>") || strings.Contains(lower, "qnet>") {
				return nil
			}
			if (sentUser || username == "") && (sentPass || password == "") && len(acc) > 0 {
				// No recognizable prompt, but credentials are handled; assume ready.
				return nil
			}
		}
		if err != nil {
			if username == "" && password == "" {
				// Some units open straight into the command stream with no prompt.
				return nil
			}
			return fmt.Errorf("telnet login: %w", err)
		}
	}
}

// Read returns inbound data with telnet IAC sequences removed.
func (t *telnetConn) Read(p []byte) (int, error) {
	n, err := t.Conn.Read(p)
	if n == 0 {
		return 0, err
	}
	data := append(t.pending, p[:n]...)
	t.pending = nil
	clean, leftover := t.filterIAC(data)
	t.pending = leftover
	copy(p, clean)
	return len(clean), err
}

// filterIAC removes IAC command sequences from data, replying to DO/WILL with
// WONT/DONT. Bytes belonging to an incomplete trailing sequence are returned as
// leftover to be prepended to the next read.
func (t *telnetConn) filterIAC(data []byte) (clean, leftover []byte) {
	out := make([]byte, 0, len(data))
	for i := 0; i < len(data); {
		b := data[i]
		if b != iac {
			out = append(out, b)
			i++
			continue
		}
		// Need at least the command byte to proceed.
		if i+1 >= len(data) {
			return out, data[i:]
		}
		cmd := data[i+1]
		switch cmd {
		case iac:
			// Escaped 0xFF is a literal data byte.
			out = append(out, iac)
			i += 2
		case iacC, iacL, iacD, iacW:
			// Option negotiation: the option byte is required too.
			if i+2 >= len(data) {
				return out, data[i:]
			}
			t.refuse(cmd, data[i+2])
			i += 3
		case iacS:
			// Subnegotiation: skip until IAC SE.
			end := bytes.Index(data[i:], []byte{iac, iacE})
			if end < 0 {
				return out, data[i:]
			}
			i += end + 2
		default:
			// Two-byte command without an option.
			i += 2
		}
	}
	return out, nil
}

// refuse answers an option request by declining it, keeping the server from
// waiting on a negotiation we don't implement.
func (t *telnetConn) refuse(cmd, option byte) {
	var reply byte
	switch cmd {
	case iacC: // Server asks us to DO -> we WONT.
		reply = iacW
	case iacL: // Server WILL -> we DONT.
		reply = iacD
	default:
		return
	}
	t.Conn.Write([]byte{iac, reply, option})
}