How is zone-precedence.txt generated?

[guillerodriguez]

I see that zone-precedence.txt is used as a tie breaker when there are more than one candidate zone.
But how is this file generated? What criteria are used to determine precedence?

[mattjohnsonpint]

I add to it manually when needed. I list all the zones that are in conflict, and sort them by population as reported on Wikipedia, from highest to lowest. Only the sort order within the conflicting zones is used, (I don't bother sorting the entire file)

There's likely a better way - but basically I'm saying that when building the list of selection zones (as used by GetTimeZoneIdsForCountry), that if more than one time zone shares a name, then associate the name with the zone with the largest population.

[guillerodriguez]

Ah, I see. Thanks for the info.

I was looking for a way to drop "redundant zones" for cases where you don't care about historical changes. i.e. for any given country, do not show multiple timezones that would result in the same rules for present and future times. Of course, the question is, when there are multiple options, which one to keep.

Keeping the zone with the largest population makes sense, although I wonder whether this actually yields the "right" results in all cases.

The process I came up with so far is to look up the zone in CLDR's windowsZones.xml file, as this already takes care of many of these cases. For example, IANA's zone.tab file lists three zones for Spain: Europe/Madrid, Atlantic/Canary, and Africa/Ceuta. In practice there are only two effective timezones in Spain (if one doesn't care about historical differences), and Ceuta just follows the mainland (Madrid) time. And indeed, in windowsZones.xml we have this:

<mapZone other="Romance Standard Time" territory="ES" type="Europe/Madrid Africa/Ceuta"/>

i.e. Africa/Ceuta is listed as an "alias" of Europe/Madrid.

This procedure already gets me from 102 duplicate zones across 20 countries to only 18 duplicates across 7 countries. So I was exploring how others have solved this issue, to see if I can find a solution for these remaining 18 duplicate zones (hopefully one that does not involve manually deciding which ones to keep :))

Thanks again.