src.dualinventive.com/go/lib/cp3000/cp3000.go

// Package cp3000 provides functionality to talk to legacy servers using the deplicated cp3000 protocol
package cp3000

import (
	"bytes"
	"fmt"
	"io"
	"strconv"
)

// Type is the CP3000 messagetype
type Type byte

// state is the CP3000 decoder state
type state int

const (
	stateStart state = iota
	stateCmd
	stateArgs
	stateChecksum
	stateEnd
)

const (
	// TypeReply is a CP3000 reply
	TypeReply Type = '!'
	// TypeCommand is a CP3000 command (request or publish)
	TypeCommand Type = '$'
)

// Msg represents a CP3000 message
type Msg struct {
	Command Command
	Params  []string
	Type    Type
}

// Bytes return the CP3000 raw data
func (m Msg) Bytes() []byte {
	// Build the buffer. The buffer contains the command and params
	var buffer bytes.Buffer
	_, err := fmt.Fprintf(&buffer, "%c%s", m.Type, m.Command)
	if err != nil {
		return nil
	}
	for _, param := range m.Params {
		_, err = fmt.Fprintf(&buffer, ",%s", strconv.QuoteToASCII(param))
		if err != nil {
			return nil
		}
	}
	// Write the checksum after the buffer
	_, err = fmt.Fprintf(&buffer, "*%02X\n", calcChecksum(buffer))
	if err != nil {
		return nil
	}

	return buffer.Bytes()
}

func (m Msg) String() string {
	return string(m.Bytes())
}

// Decode decodes CP3000 commands
//nolint: gocyclo
func Decode(line string) (*Msg, error) {
	msg := &Msg{}
	var arg bytes.Buffer
	var quoted bool
	var checksumBytes []byte
	checksum := 0
	state := stateStart
	buffer := bytes.NewBufferString(line)

	var err error
	var b, prev byte

	for err == nil {
		b, err = buffer.ReadByte()
		// except the start and the checksum everything is used for the checksum
		if state != stateStart && state != stateChecksum {
			// if we find an asterisk outside a quoted section it is the start-of-checksum
			//  this charater must be skipped!
			if !(b == '*' && !quoted) {
				checksum ^= int(b)
			}
		}

		switch state {
		case stateStart:
			// a message always starts with $ or !
			if b != byte(TypeReply) && b != byte(TypeCommand) {
				return nil, fmt.Errorf("no cmd start: %c", b)
			}
			msg.Type = Type(b)
			state = stateCmd
		case stateCmd:
			// a command has a variable length and ends on:
			// - a comma (i.e. start of arguments)
			// - an asterisk (i.e. begin of checksum)
			if b == ',' {
				state = stateArgs
			} else if b == '*' {
				state = stateChecksum
			} else {
				msg.Command += Command(b)
			}
		case stateArgs:
			// Arguments are none or multiple chunks split by comma's and ended by the
			//  beginning of the checksum ('*'). Strings MAY be quoted "bla\"" and must be
			//  escaped.
			if quoted || (b != ',' && b != '*') {
				if b == '"' && prev != '\\' {
					quoted = !quoted
				}
				erri := arg.WriteByte(b)
				_ = erri
			} else {
				newArg, qerr := strconv.Unquote(arg.String())
				if qerr != nil {
					// it is not required to have quoted strings, allow strings where unquote fails
					newArg = arg.String()
				}
				msg.Params = append(msg.Params, newArg)
				// beginning of checksum
				if b == '*' {
					state = stateChecksum
				}
				arg.Reset()
			}
			prev = b
		case stateChecksum:
			// the checksum is 2 bytes which is the result of all XOR-ed bytes
			checksumBytes = append(checksumBytes, b)
			if len(checksumBytes) == 2 {
				if fmt.Sprintf("%02X", checksum) != string(checksumBytes) {
					return nil, fmt.Errorf("checksum mismatch (recv %s, calc %02X)", checksumBytes, checksum)
				}
				state = stateEnd
			}
		case stateEnd:
			// nothing to do anymore
		}
	}

	// When something else occurred than io.EOF, return that error
	if err != nil && err != io.EOF {
		return nil, err
	}
	if state != stateEnd {
		return nil, fmt.Errorf("incomplete message")
	}

	return msg, nil
}

// SendReply sends a reply via CP3000
func SendReply(w io.Writer, reply uint8, params ...string) error {
	cmd := Msg{
		Type:    TypeReply,
		Command: replyToCommand(reply),
		Params:  params,
	}

	// Write to w
	_, err := w.Write(cmd.Bytes())
	return err
}

// Send will create a cp3000 command with the given parameters and writes to w.
// W can be a TCP connection (for direct communication) or any other communication.
func Send(w io.Writer, command Command, params ...string) error {
	cmd := Msg{
		Type:    TypeCommand,
		Command: command,
		Params:  params,
	}

	// Write to w
	_, err := w.Write(cmd.Bytes())
	return err
}

// calcChecksum will calculate the cp3000 checksum of the given buffer.
// The buffer may start with $ and end with * (start and end of the protocol)
func calcChecksum(buffer bytes.Buffer) uint8 {
	var checksum int

	if b, err := buffer.ReadByte(); err == nil {
		if b != '$' && b != '!' {
			checksum ^= int(b)
		}
	}

	var b byte
	var err error
	quoted := false
	for ; err == nil; b, err = buffer.ReadByte() {
		if b == '"' {
			quoted = !quoted
		}
		if b == '*' && !quoted {
			break
		}
		checksum ^= int(b)
	}

	return uint8(checksum & 0xff)
}