[tz] Proposal: validation text file with releases

[Arthur David Olson]

A possibility here is to store the output of a "zdump -v" command; I've
used "zdump -v" output for regression testing; I believe Paul Eggert has
done so as well. The "-c" option of zdump could be used to limit the range
of the output.

    @dashdashado

On Sat, Jul 11, 2015 at 6:35 AM, Jon Skeet <skeet at pobox.com> wrote:

> Background: I'm the primary developer for Noda Time <http://nodatime.org> 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
> <https://github.com/nodatime/nodatime/blob/master/src/NodaTime.Test/TestData/tzdb-dump.txt>,
> 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
>
>
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