Add lightweight timestamp-to-ISO conversion function

[sffc]

For time zone calculations, we need to store local datetimes (also known as civil datetimes or plain datetimes) for metazone cutoffs in the data provider.

If we store them as an ISO string, like "1988-04-03 04:01", we need either 12 or 16 bytes. It would be more efficient to store them as a "local timestamp", or the duration between the local datetime and 1970-01-01T00:00, most likely measured in minutes and stored in a u32. (Note: we should be very careful on using the term "local timestamp", because that can be really confusing terminology.)

I think this conversion function could go in icu_calendar.

[sffc]

CC @Manishearth

[sffc]

@samchen61661: Manish will post a suggestion on how to easily implement this function in icu_calendar.

Note that we likely want this to work bi-directionally. ISO-to-timestamp is probably what we need first.

[Manishearth]

We don't have this but it ought to be easy to implement, we already have internal "from_fixed" and "to_fixed" functions which convert to and from "fixed" dates, counted as days since 1 AD. So adding this function to DateTime<Iso> would involve pulling out the h/m/s, then taking a difference between the fixed date and the fixed date for the unix epoch, and then multiplying and adding.

icu4x/components/calendar/src/iso.rs

Lines 444 to 499 in 38f2736

	// Fixed is day count representation of calendars starting from Jan 1st of year 1.
	// The fixed calculations algorithms are from the Calendrical Calculations book.
	//
	// Lisp code reference: https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1167-L1189
	pub(crate) fn fixed_from_iso(date: IsoDateInner) -> i32 {
	// Calculate days per year
	let mut fixed: i32 = EPOCH - 1 + 365 * (date.year.0 - 1);
	// Adjust for leap year logic
	fixed += ((date.year.0 - 1) / 4) - ((date.year.0 - 1) / 100) + ((date.year.0 - 1) / 400);
	// Days of current year
	fixed += (367 * (date.month.0 as i32) - 362) / 12;
	// Leap year adjustment for the current year
	fixed += if date.month.0 <= 2 {
	0
	} else if Self::is_leap_year(date.year) {
	-1
	} else {
	-2
	};
	// Days passed in current month
	fixed + (date.day.0 as i32)
	}
	fn fixed_from_iso_integers(year: i32, month: i32, day: i32) -> i32 {
	#[allow(clippy::unwrap_used)] // TODO(#1668) Clippy exceptions need docs or fixing.
	Self::fixed_from_iso(
	*Date::new_iso_date_from_integers(year, month as u8, day as u8)
	.unwrap()
	.inner(),
	)
	}
	// Lisp code reference: https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1191-L1217
	fn iso_year_from_fixed(date: i32) -> i32 {
	// 400 year cycles have 146097 days
	let n_400 = date / 146097;
	let date = date % 146097;
	// 100 year cycles have 36524 days
	let n_100 = date / 36524;
	let date = date % 36524;
	// 4 year cycles have 1461 days
	let n_4 = date / 1461;
	let date = date % 1461;
	let n_1 = date / 365;
	let year = 400 * n_400 + 100 * n_100 + 4 * n_4 + n_1;
	if n_100 == 4 || n_1 == 4 {
	year
	} else {
	year + 1
	}
	}

[sffc]

Note: I think the desired output for the string-to-integer function is a signed number of minutes since Januay 1, 1970 in local time. Probably an i32.

[sffc]

@samchen61661 Is this complete?

[samchen61661]

@samchen61661 Is this complete?

Yeah, marked it as closed