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improved DLE encoder

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This commit is contained in:
Robin Müller 2021-08-17 15:05:29 +02:00
parent d92a796705
commit 4b72e246c3
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GPG Key ID: 11D4952C8CCEF814
2 changed files with 79 additions and 54 deletions
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67
src/fsfw/globalfunctions/DleEncoder.cpp
View File

@ -1,6 +1,7 @@
#include "fsfw/globalfunctions/DleEncoder.h"
DleEncoder::DleEncoder() {}
DleEncoder::DleEncoder(bool escapeStxEtx, bool escapeCr): escapeStxEtx(escapeStxEtx),
escapeCr(escapeCr) {}
DleEncoder::~DleEncoder() {}
@ -17,26 +18,28 @@ ReturnValue_t DleEncoder::encode(const uint8_t* sourceStream,
++encodedIndex;
}
while (encodedIndex < maxDestLen and sourceIndex < sourceLen)
{
while (encodedIndex < maxDestLen and sourceIndex < sourceLen) {
nextByte = sourceStream[sourceIndex];
// STX, ETX and CR characters in the stream need to be escaped with DLE
if (nextByte == STX_CHAR or nextByte == ETX_CHAR or nextByte == CARRIAGE_RETURN) {
if (encodedIndex + 1 >= maxDestLen) {
return STREAM_TOO_SHORT;
}
else {
destStream[encodedIndex] = DLE_CHAR;
++encodedIndex;
/* Escaped byte will be actual byte + 0x40. This prevents
* STX, ETX, and carriage return characters from appearing
* in the encoded data stream at all, so when polling an
* encoded stream, the transmission can be stopped at ETX.
* 0x40 was chosen at random with special requirements:
* - Prevent going from one control char to another
* - Prevent overflow for common characters */
destStream[encodedIndex] = nextByte + 0x40;
}
if ((nextByte == STX_CHAR or nextByte == ETX_CHAR) or
(this->escapeCr and nextByte == CARRIAGE_RETURN)) {
if(this->escapeStxEtx) {
if (encodedIndex + 1 >= maxDestLen) {
return STREAM_TOO_SHORT;
}
else {
destStream[encodedIndex] = DLE_CHAR;
++encodedIndex;
/* Escaped byte will be actual byte + 0x40. This prevents
* STX, ETX, and carriage return characters from appearing
* in the encoded data stream at all, so when polling an
* encoded stream, the transmission can be stopped at ETX.
* 0x40 was chosen at random with special requirements:
* - Prevent going from one control char to another
* - Prevent overflow for common characters */
destStream[encodedIndex] = nextByte + 0x40;
}
}
}
// DLE characters are simply escaped with DLE.
else if (nextByte == DLE_CHAR) {
@ -90,16 +93,22 @@ ReturnValue_t DleEncoder::decode(const uint8_t *sourceStream,
destStream[decodedIndex] = nextByte;
}
else {
/* The next byte is a STX, DTX or 0x0D character which
* was escaped by a DLE character. The actual byte was
* also encoded by adding + 0x40 to prevent having control chars,
* in the stream at all, so we convert it back. */
if (nextByte == 0x42 or nextByte == 0x43 or nextByte == 0x4D) {
destStream[decodedIndex] = nextByte - 0x40;
}
else {
return DECODING_ERROR;
}
if(this->escapeStxEtx) {
/* The next byte is a STX, DTX or 0x0D character which
* was escaped by a DLE character. The actual byte was
* also encoded by adding + 0x40 to prevent having control chars,
* in the stream at all, so we convert it back. */
if ((nextByte == STX_CHAR + 0x40 or nextByte == ETX_CHAR + 0x40) or
(this->escapeCr and nextByte == CARRIAGE_RETURN + 0x40)) {
destStream[decodedIndex] = nextByte - 0x40;
}
else {
return DECODING_ERROR;
}
}
else {
return DECODING_ERROR;
}
}
++encodedIndex;
}

66
src/fsfw/globalfunctions/DleEncoder.h
View File

@ -1,7 +1,7 @@
#ifndef FRAMEWORK_GLOBALFUNCTIONS_DLEENCODER_H_
#define FRAMEWORK_GLOBALFUNCTIONS_DLEENCODER_H_
#include "../returnvalues/HasReturnvaluesIF.h"
#include "fsfw/returnvalues/HasReturnvaluesIF.h"
#include <cstddef>
/**
@ -13,9 +13,9 @@
*
* This encoder can be used to achieve a basic transport layer when using
* char based transmission systems.
* The passed source strean is converted into a encoded stream by adding
* The passed source stream is converted into a encoded stream by adding
* a STX marker at the start of the stream and an ETX marker at the end of
* the stream. Any STX, ETX, DLE and CR occurrences in the source stream are
* the stream. Any STX, ETX, DLE and CR occurrences in the source stream can be
* escaped by a DLE character. The encoder also replaces escaped control chars
* by another char, so STX, ETX and CR should not appear anywhere in the actual
* encoded data stream.
@ -26,38 +26,45 @@
*/
class DleEncoder: public HasReturnvaluesIF {
private:
DleEncoder();
virtual ~DleEncoder();
DleEncoder(bool escapeStxEtx = true, bool escapeCr = false);
virtual ~DleEncoder();
public:
static constexpr uint8_t INTERFACE_ID = CLASS_ID::DLE_ENCODER;
static constexpr ReturnValue_t STREAM_TOO_SHORT = MAKE_RETURN_CODE(0x01);
static constexpr ReturnValue_t DECODING_ERROR = MAKE_RETURN_CODE(0x02);
static constexpr uint8_t INTERFACE_ID = CLASS_ID::DLE_ENCODER;
static constexpr ReturnValue_t STREAM_TOO_SHORT = MAKE_RETURN_CODE(0x01);
static constexpr ReturnValue_t DECODING_ERROR = MAKE_RETURN_CODE(0x02);
//! Start Of Text character. First character is encoded stream
static constexpr uint8_t STX_CHAR = 0x02;
//! End Of Text character. Last character in encoded stream
static constexpr uint8_t ETX_CHAR = 0x03;
//! Data Link Escape character. Used to escape STX, ETX and DLE occurrences
//! in the source stream.
static constexpr uint8_t DLE_CHAR = 0x10;
static constexpr uint8_t CARRIAGE_RETURN = 0x0D;
//! Start Of Text character. First character is encoded stream
static constexpr uint8_t STX_CHAR = 0x02;
//! End Of Text character. Last character in encoded stream
static constexpr uint8_t ETX_CHAR = 0x03;
//! Data Link Escape character. Used to escape STX, ETX and DLE occurrences
//! in the source stream.
static constexpr uint8_t DLE_CHAR = 0x10;
static constexpr uint8_t CARRIAGE_RETURN = 0x0D;
/**
* Encodes the give data stream by preceding it with the STX marker
* and ending it with an ETX marker. STX, ETX and DLE characters inside
* the stream are escaped by DLE characters and also replaced by adding
* 0x40 (which is reverted in the decoding process).
* and ending it with an ETX marker. DLE characters inside
* the stream are escaped by DLE characters. STX, ETX and CR characters can be escaped with a
* DLE character as well. The escaped characters are also encoded by adding
* 0x40 (which is reverted in the decoding process). This is performed so the source stream
* does not have STX/ETX/CR occurrences anymore, so the receiving side can simply parse for
* start and end markers
* @param sourceStream
* @param sourceLen
* @param destStream
* @param maxDestLen
* @param encodedLen
* @param addStxEtx
* Adding STX and ETX can be omitted, if they are added manually.
* @param addStxEtx Adding STX start marker and ETX end marker can be omitted,
* if they are added manually
* @param escapeStxEtx STX and ETX occurrences in the given source stream will be escaped and
* encoded by adding 0x40
* @param escapeCr CR characters in the given source stream will be escaped and encoded
* by adding 0x40
* @return
*/
static ReturnValue_t encode(const uint8_t *sourceStream, size_t sourceLen,
ReturnValue_t encode(const uint8_t *sourceStream, size_t sourceLen,
uint8_t *destStream, size_t maxDestLen, size_t *encodedLen,
bool addStxEtx = true);
@ -69,11 +76,20 @@ public:
* @param destStream
* @param maxDestStreamlen
* @param decodedLen
* @param escapeStxEtx STX and ETX characters were escaped in the encoded stream and need to
* be decoded back as well
* @param escapeCr CR characters were escaped in the encoded stream and need to
* be decoded back as well by subtracting 0x40
* @return
*/
static ReturnValue_t decode(const uint8_t *sourceStream,
size_t sourceStreamLen, size_t *readLen, uint8_t *destStream,
size_t maxDestStreamlen, size_t *decodedLen);
ReturnValue_t decode(const uint8_t *sourceStream,
size_t sourceStreamLen, size_t *readLen, uint8_t *destStream,
size_t maxDestStreamlen, size_t *decodedLen);
private:
bool escapeStxEtx;
bool escapeCr;
};
#endif /* FRAMEWORK_GLOBALFUNCTIONS_DLEENCODER_H_ */