[tz] Proposal: validation text file with releases

[Stephen Colebourne]

FWIW, I think such a format would be very useful. Effectively, it is a
unit test for others to confirm that they interpret the rules the same
way as intended.

It is similar to what I produced when trying to demonstrate the amount
of change being caused by apparently "minor" changes to the data:
https://github.com/jodastephen/tzdiff/commits/master

Any output of this type should indeed just consist of a simple text
file with ISO-8601 format timestamps.

Stephen



On 11 July 2015 at 11:35, Jon Skeet <skeet at pobox.com> wrote:
> Background: I'm the primary developer for Noda Time which consumes the tz
> data. I'm currently refactoring the code to do this... and I've come across
> some code (originally ported from Joda Time) which I now understand in terms
> of what it's doing, but not exactly why.
>
> For a little while now, the Noda Time source repo has included a text dump
> file, containing a text dump of every transition (up to 2100, at the moment)
> for every time zone. It looks like this, picking just one example:
>
> Zone: Africa/Maseru
> LMT: [StartOfTime, 1892-02-07T22:08:00Z) +01:52 (+00)
> SAST: [1892-02-07T22:08:00Z, 1903-02-28T22:30:00Z) +01:30 (+00)
> SAST: [1903-02-28T22:30:00Z, 1942-09-20T00:00:00Z) +02 (+00)
> SAST: [1942-09-20T00:00:00Z, 1943-03-20T23:00:00Z) +03 (+01)
> SAST: [1943-03-20T23:00:00Z, 1943-09-19T00:00:00Z) +02 (+00)
> SAST: [1943-09-19T00:00:00Z, 1944-03-18T23:00:00Z) +03 (+01)
> SAST: [1944-03-18T23:00:00Z, EndOfTime) +02 (+00)
>
> I use this file for confidence when refactoring my time zone handling code -
> if the new code comes up with the same set of transitions as the old code,
> it's probably okay. (This is just one line of defence, of course - there are
> unit tests, though not as many as I'd like.)
>
> It strikes me that having a similar file (I'm not wedded to the format, but
> it should have all the same information, one way or another) released
> alongside the main data files would be really handy for all implementors -
> it would be a good way of validating consistency across multiple platforms,
> with the release data being canonical. For any platforms which didn't want
> to actually consume the rules as rules, but just wanted a list of
> transitions, it could even effectively replace their use of the data.
>
> One other benefit: diffing the dump between two releases would make it clear
> what had changed in effect, rather than just in terms of rules.
>
> One sticking point is size. The current file for Noda Time is about 4MB,
> although it zips down to about 300K. Some thoughts around this:
>
> We wouldn't need to distribute it in the same file as the data - just as we
> have data and code file, there could be a "textdump" file or whatever we'd
> want to call it. These could be retroactively generated for previous
> releases, too.
> As you can see, there's redundancy in the format above, in that it's a list
> of "zone intervals" (as I call them in Noda Time) rather than a list of
> transitions - the end of each interval is always the start of the next
> interval.
> For zones which settle into an infinite daylight saving pattern, I currently
> generate from the start of time to 2100 (and then a single zone interval for
> the end of time as Noda Time understands it; we'd need to work out what form
> that would take, if any). If we decided that "year of release + 30 years"
> was enough, that would cut down the size considerably.
>
> Any thoughts? If the feeling is broadly positive, the next step would be to
> nail down the text format, then find a willing victim/volunteer to write the
> C code. (You really don't want me writing C...)
>
> Jon
>