Description
In src/tz/zic.rs, there is a complete parser for the zic input format. zic is a tool maintained by the IANA Time Zone Database. It is the tool that converts a plain text description of time zone rules into the binary format used in /usr/share/zoneinfo. For example, here are the DST rules for the United States and the time zone definition of America/New_York (taken from the northamerica file in the data (or complete) tzdb distribution):
# Rule NAME FROM TO - IN ON AT SAVE LETTER/S
Rule US 1918 1919 - Mar lastSun 2:00 1:00 D
Rule US 1918 1919 - Oct lastSun 2:00 0 S
Rule US 1942 only - Feb 9 2:00 1:00 W # War
Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
Rule US 1945 only - Sep 30 2:00 0 S
Rule US 1967 2006 - Oct lastSun 2:00 0 S
Rule US 1967 1973 - Apr lastSun 2:00 1:00 D
Rule US 1974 only - Jan 6 2:00 1:00 D
Rule US 1975 only - Feb lastSun 2:00 1:00 D
Rule US 1976 1986 - Apr lastSun 2:00 1:00 D
Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D
Rule US 2007 max - Mar Sun>=8 2:00 1:00 D
Rule US 2007 max - Nov Sun>=1 2:00 0 S
# Rule NAME FROM TO - IN ON AT SAVE LETTER
Rule NYC 1920 only - Mar lastSun 2:00 1:00 D
Rule NYC 1920 only - Oct lastSun 2:00 0 S
Rule NYC 1921 1966 - Apr lastSun 2:00 1:00 D
Rule NYC 1921 1954 - Sep lastSun 2:00 0 S
Rule NYC 1955 1966 - Oct lastSun 2:00 0 S
# Zone NAME STDOFF RULES FORMAT [UNTIL]
#STDOFF -4:56:01.6
Zone America/New_York -4:56:02 - LMT 1883 Nov 18 17:00u
-5:00 US E%sT 1920
-5:00 NYC E%sT 1942
-5:00 US E%sT 1946
-5:00 NYC E%sT 1967
-5:00 US E%sT
The src/tz/zic.rs already knows how to parse all of the above. My original plan was to have Jiff support reading from these Zic input files as an alternative to the standard binary TZif based zoneinfo database. But once I got to the point of actually turning the rules into something actionable, I stumbled quite a bit. The zic.c code in particular that I believe we want to port is here.
The main reason for supporting this format (in addition to TZif) is that it is very compact. And potentially opens the door to defining custom time zones via a human readable semi-standard format. It would also hopefully enable jiff-cli to compile its own time zone database without using the tzdb C toolchain, although that isn't nearly as important.
@jhpratt expressed interest in collaborating on this. I think the main issue I see with collaboration here is that I would want the Zic compiler to be inside Jiff itself. In theory, it could be split into a jiff-zic crate or something, but at least as of present, the Zic data types all make pretty extensive use of Jiff's civil time datatypes and ranged integer types. So splitting this out would either mean splitting Zic's civil times into a separate that that both jiff and jiff-zic could depend upon, or it would mean copying at least some parts of the civil time code. (For example, the code that converts between civil time and timestamps, and the code for determining the nth weekday. And maybe other things.)