Three pack_language mutants and two detect_rate boundary mutants survive
mutation testing with no test able to close them, and it's not for lack
of trying: they're equivalent by construction. Recording the proofs next
to the code so nobody re-chases them:
- (b[0] - 0x60) as u16, shifted << 10 then truncated to u16, is congruent
mod 65536 to (b[0] + 0x60) as u16 shifted the same way, because
0x60 * 2 * 1024 is an exact multiple of 65536. The same swap on the
second letter (shifted only << 5) is NOT equivalent, which is why only
the first letter's mutant survives.
- The two | with ^ mutations that OR the three packed fields together
are equivalent because the fields (a lowercase letter minus 0x60, so
1..=26) always fit in 5 bits and never share a set bit once shifted
into their 0/5/10 positions.
- detect_rate's tolerance and tie-break comparisons only diverge from
their <= mutants on an exact 0.5 fps distance or an exact tie, and a
brute-force search over every achievable integer-nanosecond median
found no case that lands on either boundary bit-exactly.
Matthew Jackson
698ba36ae4