TELNET or TELNETS(TELNET over SSL) to koukoku.shadan.open.ad.jp in Go

main.go

package main

import (
	"bufio"
	"bytes"
	"crypto/tls"
	"fmt"
	"io"
	"log"
	"net"
	"os"

	"golang.org/x/text/encoding/japanese"
	"golang.org/x/text/transform"
)

const (
	host = "koukoku.shadan.open.ad.jp"
)

var (
	addrTelnet  = fmt.Sprintf("%s:23", host)
	addrTelnetS = fmt.Sprintf("%s:992", host)
)

// If you want to connect with TELNET, run `go run main.go t`.
// If you want to connect with TELNETS, run `go run main.go s`.
// This program should be run on a terminal with the character encoding set to UTF-8.
func main() {
	var conn net.Conn
	var isNotUTF8 bool
	var err error

	if len(os.Args) != 2 {
		log.Fatalf("expected argument length is 2 but actual is %d", len(os.Args))
	}

	switch os.Args[1] {
	case "t":
		// TELNET
		conn, err = net.Dial("tcp", addrTelnet)
		isNotUTF8 = true
	case "s":
		// TELNETS
		conn, err = tls.Dial("tcp", addrTelnetS, &tls.Config{})
	default:
		log.Fatalf("invalid argument %q\n", os.Args[1])
	}
	if err != nil {
		log.Fatalf("failed to connect to %q with %q mode. err: %v\n", host, os.Args[1], err)
	}
	defer conn.Close()

	telnet(conn, isNotUTF8)
}

func telnet(conn net.Conn, isNotUTF8 bool) {
	reader := bufio.NewReader(conn)
	for {
		line, err := reader.ReadString('\n')
		if err != nil {
			break
		}

		if isNotUTF8 {
			line, err = toUTF8(line)
			if err != nil {
				log.Fatalf("failed to decode to utf8. err: %v\n", err)
			}
		}

		fmt.Print(line)
	}
}

func toUTF8(org string) (string, error) {
	decoded, err := io.ReadAll(transform.NewReader(bytes.NewReader([]byte(org)), japanese.ShiftJIS.NewDecoder()))
	if err != nil {
		return "", err
	}

	return string(decoded), nil
}