PlainMonthDay fixes in reference code #3190
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In the tabular Hijri calendars, the months have a set number of days
with only one month varying between 29 and 30 days in leap years. In
the observational Hijri calendar (islamic-umalqura) any month can have
30 days.
This applies that information to make maxLengthOfMonthCodeInAnyYear() in
helperIslamic more accurate. It prevents a slow search roundtripping
through 20 years when code like this is executed:
Temporal.PlainMonthDay.from({
calendar: 'islamic-tbla',
monthCode: 'M10',
day: 30
})
This PlainMonthDay shouldn't exist. The change makes it fail immediately
rather than exhausting the search space.
This is just a polyfill optimization. The spec text remains correct.
In hindsight, when I wrote the maxLengthOfMonthCodeInAnyYear() code in e977eff I put it in a slightly odd place. There is no need to check overflow in each iteration of the search loop, we can check it at the beginning. And in fact this code was buggy: PlainMonthDays created from out-of-range days with overflow: 'constrain' would sometimes get wrong reference years. For example: Temporal.PlainMonthDay.from({ calendar: 'gregory', monthCode: 'M12', day: 32 }) This would get the incorrect reference year 1971, and would actually produce a different PlainMonthDay instance from the same call with day: 31, which would get the correct reference year 1972. This was a polyfill-only bug. No change needed to the spec text.
We need to add some new tests to the expected failures file: they expose a bug in ICU4C where the wrong leap month was calculated for the Chinese calendar in 1987.
Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #3190 +/- ##
==========================================
+ Coverage 96.67% 96.74% +0.06%
==========================================
Files 22 22
Lines 10357 10348 -9
Branches 1859 1859
==========================================
- Hits 10013 10011 -2
+ Misses 294 287 -7
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Fixes a bug in the determination of PlainMonthDay reference years, exposed by new test262 tests. Updates the test262 commit to pull in these new tests.
The spec is correct, no change to the spec text needed.