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rcobs

Reverse-COBS encoding (rCOBS) is a variant of COBS encoding designed to allow encoding with zero lookahead.

COBS and rCOBS are useful for framing a stream of messages for transmission over a pipe of bytes (like UART or TCP). COBS encoding ensures encoded messages don't contain 0x00 bytes, so the 0x00 byte can then be used as a message separator.

Standard COBS splits the input into zero-byte-separated chunks of up to 254 bytes, and prefixes them with a length byte. This requires a lookahead of up to 254 bytes when doing streaming encoding.

rCOBS outputs the length byte at the end of the chunk instead of at the start, completely eliminating the lookahead requirement for encoding. The tradeoff is decoding now has to be done starting from the end of the message backwards, so streaming decode is not possible at all. The message has to be read in its entirety into a buffer before it can be decoded.

This makes rCOBS ideal for situations where data is encoded in constrained, embedded systems and decoded in more capable systems, where the full buffering requirement is not a problem, such as:

Logging
Crash reporting
Telemetry/monitoring
Sensor data feeds

This crate implements a streaming no-std-friendly encoder, and a non-streaming std-only decoder (enabled with std Cargo feature). no-std decoding is intentionally not provided: if you need to decode on embedded devices, standard COBS is a better fit for you.

Examples

Chunks are delimited by double-spaces for your convenience.

Message:        11 22 33 44
COBS-encoded:   05 11 22 33 44
rCOBS-encoded:  11 22 33 44 05

Message:        11 22 00  33
COBS-encoded:   03 11 22  02 33
rCOBS-encoded:  11 22 03  33 02

Message:        11 00  00  00  42 42 42
COBS-encoded:   02 11  01  01  04 42 42 42
rCOBS-encoded:  11 02  01  01  42 42 42 04

Credits

Idea of reversing COBS to allow encoding with no lookahead by @jamesmunns. Implementation by @dirbaio.

License

This work is licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.