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/// The [`CobsDecoder`] type is used to decode a stream of bytes to a
/// given mutable output slice. This is often useful when heap data
/// structures are not available, or when not all message bytes are
/// received at a single point in time.
#[derive(Debug)]
pub struct CobsDecoder<'a> {
/// Destination slice for decoded message
dest: &'a mut [u8],
/// Index of next byte to write in `dest`
dest_idx: usize,
/// Decoder state as an enum
state: DecoderState,
}
/// The [`DecoderState`] is used to track the current state of a
/// streaming decoder. This struct does not contain the output buffer
/// (or a reference to one), and can be used when streaming the decoded
/// output to a custom data type.
#[derive(Debug)]
pub enum DecoderState {
/// State machine has not received any non-zero bytes
Idle,
/// 1-254 bytes, can be header or 00
Grab(u8),
/// 255 bytes, will be a header next
GrabChain(u8),
}
fn add(to: &mut [u8], idx: usize, data: u8) -> Result<(), ()> {
*to.get_mut(idx)
.ok_or_else(|| ())? = data;
Ok(())
}
/// [`DecodeResult`] represents the possible non-error outcomes of
/// pushing an encoded data byte into the [`DecoderState`] state machine
pub enum DecodeResult {
/// The given input byte did not prompt an output byte, either because the
/// state machine is still idle, or we have just processed a header byte.
/// More data is needed to complete the message.
NoData,
/// We have received a complete and well-encoded COBS message. The
/// contents of the associated output buffer may now be used
DataComplete,
/// The following byte should be appended to the current end of the decoded
/// output buffer.
/// More data is needed to complete the message.
DataContinue(u8),
}
impl DecoderState {
/// Push a single encoded byte into the state machine. If the input was
/// unexpected, such as an early end of a framed message segment, an Error will
/// be returned, and the current associated output buffer contents should be discarded.
///
/// If a complete message is indicated, the decoding state machine will automatically
/// reset itself to the Idle state, and may be used to begin decoding another message.
///
/// NOTE: Sentinel value must be included in the input to this function for the
/// decoding to complete
pub fn feed(&mut self, data: u8) -> Result<DecodeResult, ()> {
use DecoderState::*;
use DecodeResult::*;
let (ret, state) = match (&self, data) {
// Currently Idle, received a terminator, ignore, stay idle
(Idle, 0x00) => (Ok(NoData), Idle),
// Currently Idle, received a byte indicating the
// next 255 bytes have no zeroes, so we will have 254 unmodified
// data bytes, then an overhead byte
(Idle, 0xFF) => (Ok(NoData), GrabChain(0xFE)),
// Currently Idle, received a byte indicating there will be a
// zero that must be modified in the next 1..=254 bytes
(Idle, n) => (Ok(NoData), Grab(n - 1)),
// We have reached the end of a data run indicated by an overhead
// byte, AND we have recieved the message terminator. This was a
// well framed message!
(Grab(0), 0x00) => (Ok(DataComplete), Idle),
// We have reached the end of a data run indicated by an overhead
// byte, and the next segment of 254 bytes will have no modified
// sentinel bytes
(Grab(0), 0xFF) => {
(Ok(DataContinue(0)), GrabChain(0xFE))
},
// We have reached the end of a data run indicated by an overhead
// byte, and we will treat this byte as a modified sentinel byte.
// place the sentinel byte in the output, and begin processing the
// next non-sentinel sequence
(Grab(0), n) => {
(Ok(DataContinue(0)), Grab(n - 1))
},
// We were not expecting the sequence to terminate, but here we are.
// Report an error due to early terminated message
(Grab(_), 0) => {
(Err(()), Idle)
}
// We have not yet reached the end of a data run, decrement the run
// counter, and place the byte into the decoded output
(Grab(i), n) => {
(Ok(DataContinue(n)), Grab(*i - 1))
},
// We have reached the end of a data run indicated by an overhead
// byte, AND we have recieved the message terminator. This was a
// well framed message!
(GrabChain(0), 0x00) => {
(Ok(DataComplete), Idle)
}
// We have reached the end of a data run, and we will begin another
// data run with an overhead byte expected at the end
(GrabChain(0), 0xFF) => (Ok(NoData), GrabChain(0xFE)),
// We have reached the end of a data run, and we will expect `n` data
// bytes unmodified, followed by a sentinel byte that must be modified
(GrabChain(0), n) => (Ok(NoData), Grab(n - 1)),
// We were not expecting the sequence to terminate, but here we are.
// Report an error due to early terminated message
(GrabChain(_), 0) => {
(Err(()), Idle)
}
// We have not yet reached the end of a data run, decrement the run
// counter, and place the byte into the decoded output
(GrabChain(i), n) => {
(Ok(DataContinue(n)), GrabChain(*i - 1))
},
};
*self = state;
ret
}
}
impl<'a> CobsDecoder<'a> {
/// Create a new streaming Cobs Decoder. Provide the output buffer
/// for the decoded message to be placed in
pub fn new(dest: &'a mut [u8]) -> CobsDecoder<'a> {
CobsDecoder {
dest,
dest_idx: 0,
state: DecoderState::Idle,
}
}
/// Push a single byte into the streaming CobsDecoder. Return values mean:
///
/// * Ok(None) - State machine okay, more data needed
/// * Ok(Some(N)) - A message of N bytes was successfully decoded
/// * Err(M) - Message decoding failed, and M bytes were written to output
///
/// NOTE: Sentinel value must be included in the input to this function for the
/// decoding to complete
pub fn feed(&mut self, data: u8) -> Result<Option<usize>, usize> {
match self.state.feed(data) {
Err(()) => Err(self.dest_idx),
Ok(DecodeResult::NoData) => Ok(None),
Ok(DecodeResult::DataContinue(n)) => {
add(self.dest, self.dest_idx, n).map_err(|_| self.dest_idx)?;
self.dest_idx += 1;
Ok(None)
}
Ok(DecodeResult::DataComplete) => {
Ok(Some(self.dest_idx))
}
}
}
/// Push a slice of bytes into the streaming CobsDecoder. Return values mean:
///
/// * Ok(None) - State machine okay, more data needed
/// * Ok(Some((N, M))) - A message of N bytes was successfully decoded,
/// using M bytes from `data` (and earlier data)
/// * Err(J) - Message decoding failed, and J bytes were written to output
///
/// NOTE: Sentinel value must be included in the input to this function for the
/// decoding to complete
pub fn push(&mut self, data: &[u8]) -> Result<Option<(usize, usize)>, usize> {
for (consumed_idx, d) in data.iter().enumerate() {
let x = self.feed(*d);
if let Some(decoded_bytes_ct) = x? {
// convert from index to number of bytes consumed
return Ok(Some((decoded_bytes_ct, consumed_idx + 1)));
}
}
Ok(None)
}
}
// This needs to be a macro because `src` and `dst` could be the same or different.
macro_rules! decode_raw (
($src:ident, $dst:ident) => ({
let mut source_index = 0;
let mut dest_index = 0;
// Stop at the first terminator, if any
let src_end = if let Some(end) = $src.iter().position(|b| *b == 0) {
end
} else {
$src.len()
};
while source_index < src_end {
let code = $src[source_index];
if source_index + code as usize > src_end && code != 1 {
return Err(());
}
source_index += 1;
// TODO: There are potential `panic!`s in these dest_index offsets
for _ in 1..code {
$dst[dest_index] = $src[source_index];
source_index += 1;
dest_index += 1;
}
if 0xFF != code && source_index < src_end {
$dst[dest_index] = 0;
dest_index += 1;
}
}
DecodeReport {
dst_used: dest_index,
src_used: source_index,
}
})
);
/// Decodes the `source` buffer into the `dest` buffer.
///
/// This function uses the typical sentinel value of 0.
///
/// # Failures
///
/// This will return `Err(())` if there was a decoding error. Otherwise,
/// it will return `Ok(n)` where `n` is the length of the decoded message.
pub fn decode(source: &[u8], dest: &mut[u8]) -> Result<usize, ()> {
let mut dec = CobsDecoder::new(dest);
// Did we decode a message, using some or all of the buffer?
match dec.push(source).or(Err(()))? {
Some((d_used, _s_used)) => return Ok(d_used),
None => {},
}
// If we consumed the entire buffer, but did NOT get a message,
// AND the message did not end with a zero, try providing one to
// complete the decoding.
if source.last() != Some(&0) {
// Explicitly push sentinel of zero
if let Some((d_used, _s_used)) = dec.push(&[0]).or(Err(()))? {
return Ok(d_used)
}
}
// Nope, no early message, no missing terminator, just failed to decode
Err(())
}
/// A report of the source and destination bytes used during in-place decoding
#[derive(Debug)]
pub struct DecodeReport {
// The number of source bytes used, NOT INCLUDING the sentinel byte,
// if there was one.
pub src_used: usize,
// The number of bytes of the source buffer that now include the
// decoded result
pub dst_used: usize,
}
/// Decodes a message in-place.
///
/// This is the same function as `decode_in_place`, but provides a report
/// of both the number of source bytes consumed as well as the size of the
/// destination used.
pub fn decode_in_place_report(buff: &mut[u8]) -> Result<DecodeReport, ()> {
Ok(decode_raw!(buff, buff))
}
/// Decodes a message in-place.
///
/// This is the same function as `decode`, but replaces the encoded message
/// with the decoded message instead of writing to another buffer.
///
/// The returned `usize` is the number of bytes used for the DECODED value,
/// NOT the number of source bytes consumed during decoding.
pub fn decode_in_place(buff: &mut[u8]) -> Result<usize, ()> {
Ok(decode_raw!(buff, buff).dst_used)
}
/// Decodes the `source` buffer into the `dest` buffer using an arbitrary sentinel value.
///
/// This is done by XOR-ing each byte of the source message with the chosen sentinel value,
/// which transforms the message into the same message encoded with a sentinel value of 0.
/// Then the regular decoding transformation is performed.
///
/// The returned `usize` is the number of bytes used for the DECODED value,
/// NOT the number of source bytes consumed during decoding.
pub fn decode_with_sentinel(source: &[u8], dest: &mut[u8], sentinel: u8) -> Result<usize, ()> {
for (x, y) in source.iter().zip(dest.iter_mut()) {
*y = *x ^ sentinel;
}
decode_in_place(dest)
}
/// Decodes a message in-place using an arbitrary sentinel value.
///
/// The returned `usize` is the number of bytes used for the DECODED value,
/// NOT the number of source bytes consumed during decoding.
pub fn decode_in_place_with_sentinel(buff: &mut[u8], sentinel: u8) -> Result<usize, ()> {
for x in buff.iter_mut() {
*x ^= sentinel;
}
decode_in_place(buff)
}
#[cfg(feature = "use_std")]
/// Decodes the `source` buffer into a vector.
pub fn decode_vec(source: &[u8]) -> Result<Vec<u8>, ()> {
let mut decoded = vec![0; source.len()];
match decode(source, &mut decoded[..]) {
Ok(n) => {
decoded.truncate(n);
Ok(decoded)
},
Err(()) => Err(()),
}
}
#[cfg(feature = "use_std")]
/// Decodes the `source` buffer into a vector with an arbitrary sentinel value.
pub fn decode_vec_with_sentinel(source: &[u8], sentinel: u8) -> Result<Vec<u8>, ()> {
let mut decoded = vec![0; source.len()];
match decode_with_sentinel(source, &mut decoded[..], sentinel) {
Ok(n) => {
decoded.truncate(n);
Ok(decoded)
},
Err(()) => Err(()),
}
}