feat: add Ground Station UART baseline

internal/protocol/rc.go

@@ -0,0 +1,86 @@
+package protocol
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+)
+
+const (
+	Magic0  byte = 'G'
+	Magic1  byte = 'S'
+	Version byte = 1
+
+	TypeRCChannels byte = 1
+
+	Channels = 16
+	Header   = 10
+	CRCSize  = 2
+	RCSize   = Header + Channels*2 + CRCSize
+)
+
+var (
+	ErrFrameLength = errors.New("groundstation: invalid frame length")
+	ErrMagic       = errors.New("groundstation: invalid frame magic")
+	ErrVersion     = errors.New("groundstation: unsupported frame version")
+	ErrCRC         = errors.New("groundstation: invalid frame crc")
+	ErrType        = errors.New("groundstation: unexpected frame type")
+)
+
+type RCFrame struct {
+	Sequence  uint16
+	Timestamp uint32
+	Channels  [Channels]int16
+}
+
+func EncodeRC(frame RCFrame) []byte {
+	buf := make([]byte, RCSize)
+	buf[0], buf[1], buf[2], buf[3] = Magic0, Magic1, Version, TypeRCChannels
+	binary.LittleEndian.PutUint16(buf[4:6], frame.Sequence)
+	binary.LittleEndian.PutUint32(buf[6:10], frame.Timestamp)
+	for i, value := range frame.Channels {
+		binary.LittleEndian.PutUint16(buf[Header+i*2:Header+i*2+2], uint16(value))
+	}
+	binary.LittleEndian.PutUint16(buf[RCSize-CRCSize:], CRC16CCITT(buf[:RCSize-CRCSize]))
+	return buf
+}
+
+func DecodeRC(buf []byte) (RCFrame, error) {
+	var frame RCFrame
+	if len(buf) != RCSize {
+		return frame, ErrFrameLength
+	}
+	if buf[0] != Magic0 || buf[1] != Magic1 {
+		return frame, ErrMagic
+	}
+	if buf[2] != Version {
+		return frame, ErrVersion
+	}
+	if buf[3] != TypeRCChannels {
+		return frame, ErrType
+	}
+	if got, want := binary.LittleEndian.Uint16(buf[RCSize-CRCSize:]), CRC16CCITT(buf[:RCSize-CRCSize]); got != want {
+		return frame, fmt.Errorf("%w: got %04x want %04x", ErrCRC, got, want)
+	}
+	frame.Sequence = binary.LittleEndian.Uint16(buf[4:6])
+	frame.Timestamp = binary.LittleEndian.Uint32(buf[6:10])
+	for i := range frame.Channels {
+		frame.Channels[i] = int16(binary.LittleEndian.Uint16(buf[Header+i*2 : Header+i*2+2]))
+	}
+	return frame, nil
+}
+
+func CRC16CCITT(data []byte) uint16 {
+	crc := uint16(0xffff)
+	for _, b := range data {
+		crc ^= uint16(b) << 8
+		for bit := 0; bit < 8; bit++ {
+			if crc&0x8000 != 0 {
+				crc = (crc << 1) ^ 0x1021
+			} else {
+				crc <<= 1
+			}
+		}
+	}
+	return crc
+}

internal/protocol/rc_test.go

@@ -0,0 +1,26 @@
+package protocol
+
+import "testing"
+
+func TestRCFrameRoundTrip(t *testing.T) {
+	input := RCFrame{Sequence: 42, Timestamp: 123456}
+	for i := range input.Channels {
+		input.Channels[i] = int16(i*100 - 750)
+	}
+
+	got, err := DecodeRC(EncodeRC(input))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != input {
+		t.Fatalf("round trip mismatch: got %#v want %#v", got, input)
+	}
+}
+
+func TestRCFrameRejectsBadCRC(t *testing.T) {
+	packet := EncodeRC(RCFrame{})
+	packet[Header] ^= 1
+	if _, err := DecodeRC(packet); err == nil {
+		t.Fatal("expected CRC error")
+	}
+}