[math] Fix crc8_ccitt discarding the MSB of every input byte - #1371
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[math] Fix crc8_ccitt discarding the MSB of every input byte#1371TudorFanaru18 wants to merge 1 commit into
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The non-AVR path transliterates the avr-libc inline assembly literally:
1: lsl %0 ; shift left, MSB -> CARRY
brcc 2f ; branch on CARRY, i.e. the *pre-shift* MSB
eor %0, %2
`lsl` preserves the departing bit in the carry flag, so `brcc` tests bit 7
of the value before the shift. C has no carry flag: `data <<= 1` on a
uint8_t discards that bit, so the following `if (data & 0x80)` tests the
pre-shift bit 6 instead and bit 7 is never examined.
Over all 32768 (crc, data) pairs this made
crc8_ccitt_update(crc, data) == crc8_ccitt_update(crc, data ^ 0x80)
hold universally, left only 128 of 256 output values reachable, and made
single-bit errors in bit 7 of any byte undetectable.
Adds crc_test, which the module picks up by glob. There were no tests for
this header, which is why the fault went unnoticed; the new suite pins the
published check values for all three functions and asserts that flipping
the MSB of an input byte changes the CRC-8.
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Draft: CI fails (by design). The 5 failing assertions in communication/amnb/message_test.cpp are the subject of #1370 : AMNB's test fixtures encode the uncorrected CRC values, so fixing the CRC changes AMNB header CRCs on the wire.
Whether to update the fixtures, note it in the changelog, or bump a protocol version is the open question.
Fixes #1370