rzcobs

Reverse-Zerocompressing-COBS encoding (rzCOBS) is a variant of rCOBS which aditionally compresses messages known to contain many zero bytes.

Standard COBS/rCOBS encoding:

00000000 => end of frame
nnnnnnnn => output n-1 bytes from stream, output 0x00
11111111 => output 254 bytes from stream

rzCOBS encoding:

00000000 => end of frame
0xxxxxxx => foreach x from LSB to MSB: if x=0 output 1 byte from stream, if x=1 output 0x00
1nnnnnnn => output n+7 bytes from stream, output 0x00
11111111 => output 134 bytes from stream

Zero-compression is achieved by splitting the input in chunks of 7 bytes and encoding in a bitfield which are zeros, and then transmitting only the non-zero bytes. If there are hore than 7 non-zero bytes in a row, encoding works COBS-like, where the number of non-zero bytes is emitted. The maximum overhead is therefore ceil(n/134) bytes, for a message of n bytes.

When a message is encoded and then decoded, the result is the original message, with up to 6 zero bytes appended. Higher layer protocols must be able to deal with these appended zero bytes.

License

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• 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.

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