fmt: improve strptime so that it can parse adjacent values more easily #70
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Previously, in strptime, we just skipped over ASCII digits and flags when they might appear. But in the course of trying to fix #62, it looks like we're going to want to have parsing respect padding settings. So this commit refactors parsing to actually extract the padding settings in the same way that formatting does.
In order to make #62 work, we need some way of "limiting" the parsing of numbers. So for example, when parsing a year, we shouldn't try to parse more than 4 digits. And when parsing a month, we shouldn't try to parse more than 2 digits. This isn't a big deal, except it ends up being inconsistent with using the padding settings during formatting. Say one does `%3m` for example to format the month number with up to two leading zeroes. Well, if parsing limits itself to 2 digits, then it can't parse this month! And indeed, this appears to be how C's `strptime` and `strftime` work. They aren't consistent with one another! So we make parsing "aware" of the padding setting such that `%3m` can happily parse `005`. This also gets around the thorny issue of backcompat concerns if we ended up expanding our years past -9999..=9999. Users could do, for example, `%5Y` to use a 5 digit year for *both* parsing and formatting. Boom. Fixes #62
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Previously,
strptimeworked greedily. That is, specifiers like%Ymatched as many ASCII digits as possible, and then tried to parse all
of them into a year value.
But this meant that using
%Y%m%dto parse something like20240730couldn't work:
%Ywould consume all of20240730and then of coursefail since the value exceeds the year boundaries of Jiff.
Instead, we tweak how parsing works so that it only consumes the
maximum possible number of digits given its boundaries.
Fixes #62