feature request: update_bit()

[SebKuzminsky]

I need to checksum data that isn't an integer multiple of 8 bits in length.

I'm working on an interface library for a SPI device that uses CRC to validate frames on the SPI bus. The frames are made up of fields that don't start and end on byte boundaries.

Here's a diagram showing what i mean:

The microcontroller performs a SPI transfer of 6+n bytes. The first two bytes sent by the micro contain the command (zero-padded on the left). The next two bytes contain a 15-bit checksum of the command, with a 0 in the LSB.

The device receives the command and validates it with the checksum. If that passes it sends n bytes of data, followed by a two byte trailer consisting of 6 bits of "command count" and 10 bits of checksum covering the n payload bytes and the 6 bits of "command count".

As far as i can tell, there's not a way in crc-rs to compute a checksum that doesn't cover an integer number of bytes.

It's easy enough to implement my own amateurish crc code to do this, but it'd be nice if i could just use your awesome library.

I'd be happy to try to implement this in crc-rs if you think it'd be a useful addition.

[akhilles]

I would like to support this use-case, but it really depends on the API. Maybe we can add something like this to Digest?

fn update_bits<const BITS: usize>(&mut self, byte: u8) {}

with compile-time enforcement of BITS < 8.