lib: cobs: fix for non-zero delimiters

[hlord2000]

Adds fix for case where custom delimiter is equal to overhead byte
Adds tests for custom delimiters
Simplified implementation

Signed-off-by: Helmut Lord [email protected]

[hlord2000]

@pdgendt per comment in #101004

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[pdgendt]

There isn't an official specification of how the delimiter/sentinel byte should be used for encoding/decoding AFAIK.
In my go library I took inspiration from the rust cobs library used in Tokio.

The overhead byte encodes the length of the block, but leaps the delimiter/sentinel value. For decoding the overhead length byte, you simply have to subtract 1 if it is larger than the delimiter/sentinel, otherwise you don't.

[pdgendt]

Searching further on the topic of a custom delimiter I found the paper https://conferences.sigcomm.org/sigcomm/1997/papers/p062.pdf
Where the encoded output stream is XOR'd with the delimiter value. I think this is an even more elegant solution (and I will update my own library accordingly).