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Subject: Re: [boost] [date_time] Time zone improvements
From: Jeff Garland (jeff_at_[hidden])
Date: 2008-12-01 09:08:32

Rutger ter Borg wrote:
> Jeff Garland wrote:

This thread is in slow motion...but keeping it going...

>> Very cool :-) One question -- is there a portable solution where a user
>> could deploy a files onto a Windows system or is it a Unix only solution?
> Portable, big endian binary files.
> The variant I'm proposing is also based on the POSIX standard


> "TZ
> This variable shall represent timezone information. The contents of the
> environment variable named TZ shall be used by the [snip] functions, and by
> various utilities, to override the default timezone. The value of TZ has one
> of the two forms (spaces inserted for clarity):
> :characters
> or:
> std offset dst offset, rule
> "


> The second variant is what's currently in Boost.Date_Time as Posix Time Zone,
> having a direct specification of the time zone. I'll be talking about the
> first variant, containing :characters. Although this is implementaiton
> specific, most POSIX-compatible systems have implemented the Zoneinfo
> database for this variant, e.g., Linux-based systems, many BSDs, Mac OS X,
> etc. The :characters point to any other zoneinfo-compatible binary file,
> either with an absolute or relative path.
> A natural step for changing Boost.Date_Time would be to make changes such that
> the Posix Time Zone supports both formats. I.e.,
> variant 1) posix_time_zone( ":/etc/localtime" ); or
> posix_time_zone( ":/usr/share/zoneinfo/Europe/Amsterdam" );
> variant 2) posix_time_zone( "EST-5EDT,M4.1.0,M10.5.0" );
> I think both should work as it is from the same standard, and seems to be a
> natural and consistent starting point to look at changing the interface. Do
> you agree?

Ok, again it would be wonderful if this could also be deployed to a properly
configure Windows system -- preferably without changing the string.

>> Actually, I believe the abstract timezone is able to support this, but
>> there
>> isn't a concrete implementation. It would be great to have the concrete
>> implementation to prove out the abstraction (obviously from the text below
>> you think there needs to be changes).
> As far as I can tell, the abstract time zone class currently is not able to
> represent variant 1): the abstract zone class is geared towards a
> single-lined definition of a time zone, whereas variant 1) may be regarded as
> a series of definitions, i.e., a map of
> < transition_time_point_in_utc, { gmt_offset, abbreviation, is_dst, ... } >
> looking up utc_to_local is a matter of doing a lower_bound search on the
> transition time points.

Look carefully at the abstract time zone. It's designed so that when you ask
for the local_start_time and the local_end_time for dst you need to provide
the year as a parameter so that subclasses can provide the historic
conversions -- so it looks like this:

   time_zone_base tz = ...
   ptime t = tz.dst_local_start_time(2008);

But, it's possible that to fully support support Olsen you'll need dst_offset
and maybe some others to be expanded to support a year parameter as well. In
any case I think we should be able to do that in a backward compatible fashion.

> The right-hand side of the map above is widely
> implemented with a struct called ttinfo.
> A proposal from my side would be to change the abstract time zone class in
> such way that it is able to represent both variants. The easiest approach
> here is probably to introduce the ttinfo structure, and make the current
> library working with that.
> variant 1): has a map of transition points, returns a (pointer to a) ttinfo
> structure. May hold more than 2 different ttinfo structures.
> variant 2): always has 2 ttinfo structures (std/dst). Uses
> dst-calculations/functions to lookup the transitions point, returns the right
> (pointer to a) ttinfo structure.
> What do you think?

At first blush, I'd like to make as few changes to the abstract timezone as
possible and would prefer to simply add a new subclass, but I'll keep an open
mind as we work thru the details.


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