skyview/internal/beast/parser.go

package beast

import (
	"bufio"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"time"
)

// Beast message types
const (
	BeastModeAC    = 0x31 // '1' - Mode A/C
	BeastModeS     = 0x32 // '2' - Mode S Short (56 bits)
	BeastModeSLong = 0x33 // '3' - Mode S Long (112 bits)
	BeastStatusMsg = 0x34 // '4' - Status message
	BeastEscape    = 0x1A // Escape character
)

// Message represents a Beast format message
type Message struct {
	Type       byte
	Timestamp  uint64 // 48-bit timestamp in 12MHz ticks
	Signal     uint8  // Signal level (RSSI)
	Data       []byte // Mode S data
	ReceivedAt time.Time
	SourceID   string // Identifier for the source receiver
}

// Parser handles Beast binary format parsing
type Parser struct {
	reader   *bufio.Reader
	sourceID string
}

// NewParser creates a new Beast format parser
func NewParser(r io.Reader, sourceID string) *Parser {
	return &Parser{
		reader:   bufio.NewReader(r),
		sourceID: sourceID,
	}
}

// ReadMessage reads and parses a single Beast message
func (p *Parser) ReadMessage() (*Message, error) {
	// Look for escape character
	for {
		b, err := p.reader.ReadByte()
		if err != nil {
			return nil, err
		}
		if b == BeastEscape {
			break
		}
	}

	// Read message type
	msgType, err := p.reader.ReadByte()
	if err != nil {
		return nil, err
	}

	// Validate message type
	var dataLen int
	switch msgType {
	case BeastModeAC:
		dataLen = 2
	case BeastModeS:
		dataLen = 7
	case BeastModeSLong:
		dataLen = 14
	case BeastStatusMsg:
		// Status messages have variable length, skip for now
		return p.ReadMessage()
	default:
		return nil, fmt.Errorf("unknown message type: 0x%02x", msgType)
	}

	// Read timestamp (6 bytes, 48-bit)
	timestampBytes := make([]byte, 8)
	if _, err := io.ReadFull(p.reader, timestampBytes[2:]); err != nil {
		return nil, err
	}
	timestamp := binary.BigEndian.Uint64(timestampBytes)

	// Read signal level (1 byte)
	signal, err := p.reader.ReadByte()
	if err != nil {
		return nil, err
	}

	// Read Mode S data
	data := make([]byte, dataLen)
	if _, err := io.ReadFull(p.reader, data); err != nil {
		return nil, err
	}

	// Unescape data if needed
	data = p.unescapeData(data)

	return &Message{
		Type:       msgType,
		Timestamp:  timestamp,
		Signal:     signal,
		Data:       data,
		ReceivedAt: time.Now(),
		SourceID:   p.sourceID,
	}, nil
}

// unescapeData removes escape sequences from Beast data
func (p *Parser) unescapeData(data []byte) []byte {
	result := make([]byte, 0, len(data))
	i := 0
	for i < len(data) {
		if i < len(data)-1 && data[i] == BeastEscape && data[i+1] == BeastEscape {
			result = append(result, BeastEscape)
			i += 2
		} else {
			result = append(result, data[i])
			i++
		}
	}
	return result
}

// ParseStream continuously reads messages from the stream
func (p *Parser) ParseStream(msgChan chan<- *Message, errChan chan<- error) {
	for {
		msg, err := p.ReadMessage()
		if err != nil {
			if err != io.EOF && !errors.Is(err, io.ErrClosedPipe) {
				errChan <- fmt.Errorf("parser error from %s: %w", p.sourceID, err)
			}
			return
		}
		msgChan <- msg
	}
}

// GetSignalStrength converts signal byte to dBFS
func (msg *Message) GetSignalStrength() float64 {
	// Beast format: signal level is in units where 255 = 0 dBFS
	// Typical range is -50 to 0 dBFS
	if msg.Signal == 0 {
		return -50.0 // Minimum detectable signal
	}
	return float64(msg.Signal) * (-50.0 / 255.0)
}

// GetICAO24 extracts the ICAO 24-bit address from Mode S messages
func (msg *Message) GetICAO24() (uint32, error) {
	if msg.Type == BeastModeAC {
		return 0, errors.New("Mode A/C messages don't contain ICAO address")
	}

	if len(msg.Data) < 4 {
		return 0, errors.New("insufficient data for ICAO address")
	}

	// ICAO address is in bytes 1-3 of Mode S messages
	icao := uint32(msg.Data[1])<<16 | uint32(msg.Data[2])<<8 | uint32(msg.Data[3])
	return icao, nil
}

// GetDownlinkFormat returns the downlink format (first 5 bits)
func (msg *Message) GetDownlinkFormat() uint8 {
	if len(msg.Data) == 0 {
		return 0
	}
	return (msg.Data[0] >> 3) & 0x1F
}

// GetTypeCode returns the message type code for extended squitter messages
func (msg *Message) GetTypeCode() (uint8, error) {
	df := msg.GetDownlinkFormat()
	if df != 17 && df != 18 { // Extended squitter
		return 0, errors.New("not an extended squitter message")
	}

	if len(msg.Data) < 5 {
		return 0, errors.New("insufficient data for type code")
	}

	return (msg.Data[4] >> 3) & 0x1F, nil
}