// This file is part of https://github.com/racingmars/go3270/
// Copyright 2020 by Matthew R. Wilson, licensed under the MIT license. See
// LICENSE in the project root for license information.

package go3270

import (
	"net"
	"time"
)

const (
	binary       = 0
	send         = 1
	se           = 240 // f0
	sb           = 250 // fa
	will         = 251 // fb
	wont         = 252 // fc
	do           = 253 // fd
	dont         = 254 // fe
	iac          = 255 // ff
	terminalType = 24  // 18
	eor          = 25  // 19
)

// NegotiateTelnet will naively (e.g. not checking client responses) negotiate
// the options necessary for tn3270 on a new telnet connection, conn.
func NegotiateTelnet(conn net.Conn) error {
	conn.Write([]byte{iac, do, terminalType})
	conn.Write([]byte{iac, sb, terminalType, send, iac, se})
	conn.Write([]byte{iac, do, eor})
	conn.Write([]byte{iac, do, binary})
	conn.Write([]byte{iac, will, eor, iac, will, binary})
	flushConnection(conn, time.Second*5)
	return nil
}

// UnNegotiateTelnet will naively (e.g. not checking client responses) attempt
// to restore the telnet options state to what it was before NegotiateTelnet()
// was called.
func UnNegotiateTelnet(conn net.Conn, timeout time.Duration) error {
	conn.Write([]byte{iac, wont, eor, iac, wont, binary})
	conn.Write([]byte{iac, dont, binary})
	conn.Write([]byte{iac, dont, eor})
	conn.Write([]byte{iac, dont, terminalType})
	flushConnection(conn, timeout)
	return nil
}

// flushConnection discards all bytes that it can read from conn, allowing up
// to the duration timeout for the first byte to be read.
func flushConnection(conn net.Conn, timeout time.Duration) error {
	defer conn.SetReadDeadline(time.Time{})
	buffer := make([]byte, 1024)
	for {
		conn.SetReadDeadline(time.Now().Add(timeout))
		n, err := conn.Read(buffer)
		if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
			debugf("nothing to flush\n")
			return nil
		}
		if err != nil {
			debugf("error while flushing: %v\n", err)
			return err
		}
		debugf("%d bytes read while flushing connection\n", n)
		// for follow-up reads, reduce the timeout
		timeout = time.Second / 2
	}
}