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frame.go
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frame.go
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package spartn
import (
"bufio"
"errors"
)
const (
FramePreamble uint8 = 0x73
)
// Frame is used to encapsulate encoded SPARTN Messages, supplying
// encryption and authentication information.
type Frame struct {
Preamble uint8 // 8
MessageType uint8 // 7
PayloadLength uint16 // 10
EAF bool // 1
MessageCRCType MessageCRCType // 2
CRC uint8 // 4
MessageSubtype uint8 // 4
TimeTagType bool // 1
TimeTag uint32 // TimeTagType ? 16bits : 32bits
SolutionID uint8 // 7
SolutionProcessorID uint8 // 4
EncryptionID uint8 // 4
EncryptionSequenceNumber uint8 // 6
AuthenticationIndicator uint8 // 3
EmbeddedAuthenticationLength uint8 // 3
MessagePayload []byte // SizeOf MessageLength - padded
EmbeddedAuthenticationData []byte // SizeOf EmbeddedAuthenticationLength
MessageCRC uint32 // 8 - 32
}
// DeserializeFrame only discards a single byte from the given Reader
// if no valid preamble is found, or if the Frame CRC is invalid.
//
// Discards bytes from Reader whether or not the MessageCRC is valid.
func DeserializeFrame(r *bufio.Reader) (frame Frame, err error) {
frame.Preamble, err = r.ReadByte()
if err != nil {
return frame, err
}
if frame.Preamble != FramePreamble {
return frame, errors.New("invalid preamble")
}
frameHeader, err := DeserializeFrameStart(r, &frame)
if ; err != nil {
return frame, err
}
payloadDescription, err := DeserializePayloadDescriptionBlock(r, &frame)
if ; err != nil {
return frame, err
}
frame.MessagePayload = make([]byte, frame.PayloadLength)
_, err = r.Read(frame.MessagePayload)
if err != nil {
return frame, err
}
frame.EmbeddedAuthenticationData = make([]byte, frame.EmbeddedAuthenticationLength)
_, err = r.Read(frame.EmbeddedAuthenticationData)
if err != nil {
return frame, err
}
frame.MessageCRC, err = DeserializeMessageCRC(frame.MessageCRCType, r)
if err != nil {
return frame, err
}
// TODO: Implement Serialize method for Frame, instead of collecting and joining parsed []byte
crc, _ := frame.MessageCRCType.CalculateCRC(append(frameHeader, append(
payloadDescription, append(
frame.MessagePayload, frame.EmbeddedAuthenticationData...)...)...))
if crc != frame.MessageCRC {
return frame, errors.New("message CRC does not match computed value")
}
return frame, err
}
// DeserializeFrameStart discards bytes from Reader only if CRC is valid.
func DeserializeFrameStart(r *bufio.Reader, frame *Frame) (frameHeader []byte, err error) {
frameHeader, err = r.Peek(3)
if err != nil {
return frameHeader, err
}
frame.MessageType = frameHeader[0] >> 1
frame.PayloadLength = (uint16(frameHeader[0]&0x01) << 9 &
uint16(frameHeader[1]) << 1 &
uint16(frameHeader[2]) >> 7)
frame.EAF = (frameHeader[2] & 0x40) >> 6 == 0x01
frame.MessageCRCType = MessageCRCType((frameHeader[2] & 0x30) >> 4)
frame.CRC = frameHeader[2] & 0x0F
if uint8(FrameHash.CalculateCRC(frameHeader[:2])) != frame.CRC {
return frameHeader, errors.New("frame CRC did not match calculated value")
}
_, err = r.Discard(3)
return frameHeader, err
}
func DeserializePayloadDescriptionBlock(r *bufio.Reader, frame *Frame) (parsedBytes []byte, err error) {
// Consider only peeking 6 bytes ahead until TimeTagType is checked
payloadDescription, err := r.Peek(8)
if err != nil {
return parsedBytes, err
}
parsedBytes = payloadDescription
frame.MessageSubtype = payloadDescription[0] >> 4
frame.TimeTagType = (payloadDescription[0] & 0x04) >> 3 == 0x01
frame.TimeTag = (
uint32(payloadDescription[0] & 0x07) << 13 &
uint32(payloadDescription[1]) << 5 &
uint32(payloadDescription[2] & 0xF8) >> 3)
if frame.TimeTagType { // parse 16 more bits into TimeTag
frame.TimeTag = (
frame.TimeTag << 16 &
uint32(payloadDescription[2] & 0x07) << 13 &
uint32(payloadDescription[3]) << 5 &
uint32(payloadDescription[4] & 0xF8) >> 3)
_, err = r.Discard(8)
payloadDescription = payloadDescription[4:]
} else {
_, err = r.Discard(6)
parsedBytes = payloadDescription[:6]
payloadDescription = payloadDescription[2:]
}
if err != nil {
return parsedBytes, err
}
frame.SolutionID = (
(payloadDescription[0] & 0x07) << 4 &
(payloadDescription[1] & 0xF0) >> 4)
frame.SolutionProcessorID = payloadDescription[1] & 0x0F
frame.EncryptionID = (payloadDescription[2] & 0xF0) >> 4
frame.EncryptionSequenceNumber = (
(payloadDescription[2] & 0x0F) << 4 &
(payloadDescription[3] & 0xC0) >> 6)
frame.AuthenticationIndicator = (payloadDescription[3] & 0x38) >> 3
frame.EmbeddedAuthenticationLength = payloadDescription[3] & 0x7
return parsedBytes, nil
}