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physics: make leap second lookup reject instants inside a leap second

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ApsisRS contributors 2026-09-26 13:19:51 +08:00
parent 8b5213747a
commit ce0f341cbd
1 changed files with 16 additions and 32 deletions
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48
crates/physics/apsis-leap/src/time.rs
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@ -55,45 +55,29 @@ pub fn tai_mjd_from_utc_mjd(utc_mjd: f64) -> Option<f64> {
/// Converts a TAI Modified Julian Date to the corresponding UTC Modified
/// Julian Date.
///
/// The offset that applies depends on the UTC instant being sought, so the
/// value is recovered by a monotone iteration. The iteration starts from
/// the smallest possible answer, obtained with the largest offset in the
/// table, and increases until the offset stops changing. Because the offset
/// is a non-decreasing step function of UTC, the sequence is non-decreasing
/// and reaches the fixed point in as many steps as there are offset changes
/// between the seed and the answer, which is a small number.
/// Each candidate offset from the table is tried in turn. A candidate is
/// accepted when the offset in force at the resulting UTC instant is the
/// same offset that produced it. At most one offset can satisfy that
/// condition for a given input, so the search is unambiguous.
///
/// Returns `None` when the instant precedes the start of the table, or when
/// the result would fall before that boundary.
/// Returns `None` in two cases. The first is an instant before the start of
/// the table, where UTC carried fractional offsets that this table does not
/// describe. The second is an instant that falls strictly inside an
/// inserted leap second. Such an instant has no UTC representation, because
/// the UTC clock labels that interval `23:59:60`, a value that does not
/// exist on the continuous Modified Julian Date scale.
pub fn utc_mjd_from_tai_mjd(tai_mjd: f64) -> Option<f64> {
if !tai_mjd.is_finite() {
return None;
}
// Seed with the largest offset, which gives the smallest candidate UTC.
let mut utc_mjd = tai_mjd - TAI_MINUS_UTC_AFTER_LAST_ENTRY / SECONDS_PER_DAY;
for _ in 0..64 {
let offset = offset_for_iteration(utc_mjd);
let next = tai_mjd - offset / SECONDS_PER_DAY;
if next == utc_mjd {
break;
for entry in LEAP_SECONDS {
let offset = entry.tai_minus_utc;
let candidate = tai_mjd - offset / SECONDS_PER_DAY;
if tai_minus_utc_at_mjd(candidate) == Some(offset) {
return Some(candidate);
}
utc_mjd = next;
}
if utc_mjd < TABLE_COVERS_FROM_MJD {
return None;
}
Some(utc_mjd)
}
/// Returns the offset in force near `utc_mjd`, using the first table entry
/// for instants below the start of the table.
///
/// This is used only while iterating in [`utc_mjd_from_tai_mjd`], where a
/// candidate may transiently precede the boundary even though the answer
/// does not. Callers that need to distinguish an out of range instant from
/// a valid one must use [`tai_minus_utc_at_mjd`].
fn offset_for_iteration(utc_mjd: f64) -> f64 {
tai_minus_utc_at_mjd(utc_mjd).unwrap_or(LEAP_SECONDS[0].tai_minus_utc)
None
}
/// Converts a proleptic Gregorian calendar date at 00:00:00 to a Modified