242 lines
5.8 KiB
Rust
242 lines
5.8 KiB
Rust
//! The compile time leap second table and the calendar helpers that place
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//! its entries on the Modified Julian Date scale.
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/// One leap second announcement.
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///
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/// The entry records the date on which a new value of `TAI - UTC` took
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/// effect. It does not record the size of the inserted second, because in
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/// every announcement to date the inserted second was exactly one second
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/// and the new offset is one greater than the previous one.
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#[derive(Debug, Clone, Copy, PartialEq)]
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pub struct LeapSecond {
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/// Year of the effective date, in the proleptic Gregorian calendar.
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pub year: i32,
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/// Month of the effective date, in `1..=12`.
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pub month: u32,
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/// Day of the effective date, in `1..=31`.
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pub day: u32,
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/// The value of `TAI - UTC` in seconds from this date onward.
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pub tai_minus_utc: f64,
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}
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impl LeapSecond {
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/// Returns the Modified Julian Date of the effective instant.
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///
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/// Leap seconds take effect at 00:00:00 UTC, so the effective instant
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/// is the start of the given calendar day.
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pub const fn effective_mjd(self) -> f64 {
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mjd_from_ymd(self.year, self.month, self.day)
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}
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}
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/// The leap second table, ordered by effective date.
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///
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/// Source: IERS Bulletin C. The first entry is 1972-01-01, when the
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/// current scheme began with an initial offset of 10 s inherited from
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/// earlier adjustments.
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pub const LEAP_SECONDS: &[LeapSecond] = &[
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LeapSecond {
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year: 1972,
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month: 1,
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day: 1,
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tai_minus_utc: 10.0,
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},
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LeapSecond {
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year: 1972,
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month: 7,
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day: 1,
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tai_minus_utc: 11.0,
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},
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LeapSecond {
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year: 1973,
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month: 1,
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day: 1,
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tai_minus_utc: 12.0,
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},
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LeapSecond {
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year: 1974,
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month: 1,
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day: 1,
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tai_minus_utc: 13.0,
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},
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LeapSecond {
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year: 1975,
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month: 1,
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day: 1,
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tai_minus_utc: 14.0,
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},
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LeapSecond {
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year: 1976,
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month: 1,
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day: 1,
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tai_minus_utc: 15.0,
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},
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LeapSecond {
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year: 1977,
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month: 1,
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day: 1,
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tai_minus_utc: 16.0,
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},
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LeapSecond {
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year: 1978,
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month: 1,
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day: 1,
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tai_minus_utc: 17.0,
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},
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LeapSecond {
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year: 1979,
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month: 1,
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day: 1,
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tai_minus_utc: 18.0,
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},
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LeapSecond {
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year: 1980,
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month: 1,
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day: 1,
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tai_minus_utc: 19.0,
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},
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LeapSecond {
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year: 1981,
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month: 7,
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day: 1,
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tai_minus_utc: 20.0,
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},
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LeapSecond {
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year: 1982,
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month: 7,
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day: 1,
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tai_minus_utc: 21.0,
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},
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LeapSecond {
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year: 1983,
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month: 7,
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day: 1,
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tai_minus_utc: 22.0,
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},
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LeapSecond {
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year: 1985,
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month: 7,
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day: 1,
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tai_minus_utc: 23.0,
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},
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LeapSecond {
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year: 1988,
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month: 1,
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day: 1,
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tai_minus_utc: 24.0,
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},
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LeapSecond {
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year: 1990,
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month: 1,
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day: 1,
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tai_minus_utc: 25.0,
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},
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LeapSecond {
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year: 1991,
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month: 1,
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day: 1,
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tai_minus_utc: 26.0,
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},
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LeapSecond {
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year: 1992,
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month: 7,
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day: 1,
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tai_minus_utc: 27.0,
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},
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LeapSecond {
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year: 1993,
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month: 7,
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day: 1,
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tai_minus_utc: 28.0,
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},
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LeapSecond {
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year: 1994,
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month: 7,
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day: 1,
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tai_minus_utc: 29.0,
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},
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LeapSecond {
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year: 1996,
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month: 1,
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day: 1,
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tai_minus_utc: 30.0,
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},
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LeapSecond {
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year: 1997,
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month: 7,
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day: 1,
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tai_minus_utc: 31.0,
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},
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LeapSecond {
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year: 1999,
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month: 1,
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day: 1,
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tai_minus_utc: 32.0,
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},
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LeapSecond {
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year: 2006,
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month: 1,
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day: 1,
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tai_minus_utc: 33.0,
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},
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LeapSecond {
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year: 2009,
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month: 1,
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day: 1,
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tai_minus_utc: 34.0,
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},
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LeapSecond {
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year: 2012,
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month: 7,
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day: 1,
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tai_minus_utc: 35.0,
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},
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LeapSecond {
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year: 2015,
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month: 7,
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day: 1,
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tai_minus_utc: 36.0,
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},
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LeapSecond {
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year: 2017,
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month: 1,
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day: 1,
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tai_minus_utc: 37.0,
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},
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];
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/// The Modified Julian Date of the first table entry, 1972-01-01.
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///
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/// Queries earlier than this instant are rejected because UTC did not yet
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/// use integer second offsets.
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pub const TABLE_COVERS_FROM_MJD: f64 = mjd_from_ymd(
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LEAP_SECONDS[0].year,
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LEAP_SECONDS[0].month,
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LEAP_SECONDS[0].day,
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);
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/// The value of `TAI - UTC` on and after the last table entry.
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///
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/// This is the offset that remains in force until a new leap second is
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/// announced.
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pub const TAI_MINUS_UTC_AFTER_LAST_ENTRY: f64 = LEAP_SECONDS[LEAP_SECONDS.len() - 1].tai_minus_utc;
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/// Converts a proleptic Gregorian calendar date to a Modified Julian Date.
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///
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/// The result is the MJD of 00:00:00 on the given day. The algorithm is the
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/// Fliegel-Van Flandern integer conversion, which is exact for every date
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/// the Gregorian calendar is defined on and does not need a lookup table.
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///
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/// The computation is done in `i64` and converted once at the end, so the
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/// result is exact for the range of dates in which a leap second can
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/// plausibly occur.
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pub const fn mjd_from_ymd(year: i32, month: u32, day: u32) -> f64 {
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let a = (14 - month as i64) / 12;
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let y = year as i64 + 4800 - a;
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let m = month as i64 + 12 * a - 3;
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let jdn = day as i64 + (153 * m + 2) / 5 + 365 * y + y / 4 - y / 100 + y / 400 - 32045;
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// The Julian Day Number counts days from noon, and MJD 0 is the
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// midnight starting 1858-11-17, whose Julian Day Number is 2 400 001.
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(jdn - 2_400_001) as f64
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}
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