[tz] Proposal: Sections for different kinds of backward links

Paul Eggert eggert at cs.ucla.edu
Thu Nov 18 03:37:52 UTC 2021

On 11/17/21 18:29, Stephen Colebourne via tz wrote:
> Having "Singapore" (an obsolete ID) in `primary`
> really isn't useful info and you might as well not bother with the
> change. The whole point is to provide a way to prune obsolete IDs like
> that.

For many years the files zone1970.tab and zone.tab have provided a way 
to do that. Either of these files suffice to prune away obsolete IDs, 
for a particular meaning of "obsolete", by using the rule that if the 
file contains a name, it's not obsolete.

The two files differ because they use different definition of 
"obsolete". zone1970.tab is intended to correspond to the current 
guidelines (one name per alike-since-1970 region), whereas zone.tab is 
intended to correspond to older guidelines (a name for each 
alike-since-1970 region in each country). If neither of these files 
correspond to the definition of "obsolete" that you need, it'd be 
helpful to see examples of where they both go amiss so that we can think 
about how to remedy the situtation.

> If you want downstream consumers to have useful info from this change
> you would need a list of all obsolete IDs, all posixish IDs and the
> minimal set of primary IDs (ie. excluding spelling and obsolete
> variants) .

Here is a simple definition that differs a bit from what I sent earlier, 
but has the virtue of not requiring reorganizing the data: obsolete IDs 
are those not listed in zone.tab or zone1970.tab (your pick). Posixish 
IDs start with "Etc/". The minimal set of primary IDs consists of those 
listed in zone.tab or zone1970.tab (your pick). These categories 
overlap, but the overlap shouldn't be much of a problem.

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