Fixes a broken build left by 777a6a8, and the test it breaks is the interesting
part.
texture::tests::alpha_is_scaled_from_eight_bit_halves
assert_eq!(scale_alpha(0x7f), 0xfe);
That is not a test that happened to pin the wrong value. The test's NAME asserts
the wrong rule -- "scaled from eight bit halves" is the `a * 2` implementation
written down as intent, with 0xFE as its proof. Someone looked at this, thought
"alpha is a half-scale byte, double it", and then locked the conclusion into both
the identifier and the assertion.
So the previous commit's claim that `a * 2` was an implementation detail was wrong
in a way that matters: it had a name, a doc comment and a test defending it. That
is not an accident that escaped review, it is a convention that was adopted.
Two things follow.
The rule is now /128 * 255, and the test says so in its name. It also pins 0x7E ->
0xFB, a value the doubling cannot produce and which the disc does contain.
And 777a6a8 was pushed with a failing test in it. I only caught it because the
count came back as 100 rather than 278 and the discrepancy was worth one more
command -- `cargo test -q` plus a grep for `^test result: ok` sums only the
binaries that passed, so a failure is invisible in the total unless you go and
look at the raw lines. I had been using that filtered sum to report "278 tests"
for several commits without knowing it could hide a red build.
That is worth more than the bug it found. The measurement I had been reporting as
evidence of health was itself an instance of the defect this whole stretch has been
about: a quantity that should have had one definition, expressed as a filtered sum,
agreeing with the wrong answer and nobody comparing it against anything.
278 passing with the disc present, clippy clean.
ReaperOfVeriod
4bf552dab0