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use crate::{Dt, Scale, clamp_i128_to_i64};
use chrono::{DateTime, Duration, TimeDelta, Utc};
impl Dt {
/// Creates a `Dt` from a `chrono::DateTime<chrono::Utc>`.
///
/// This is the exact reverse of [`Dt::to_chrono_datetime_utc`].
///
/// - The resulting `Dt` is expressed in the TAI scale
/// (the library's canonical internal scale).
/// - Sub-nanosecond attoseconds are set to zero.
/// - If the `DateTime` is outside the range representable as an `i64`
/// number of nanoseconds since the Unix epoch, the value is clamped
/// to exactly the maximum/minimum nanosecond value (`i64::MAX` /
/// `i64::MIN` ns) rather than saturating to `Dt` extremes.
pub fn from_chrono_datetime_utc(dt: DateTime<Utc>) -> Self {
match dt.timestamp_nanos_opt() {
Some(ns) => Dt::UNIX_EPOCH.add(Dt::from_ns(ns as i128, Scale::TAI)),
None => {
let ns = if dt > DateTime::<Utc>::UNIX_EPOCH {
i64::MAX
} else {
i64::MIN
};
Dt::UNIX_EPOCH.add(Dt::from_ns(ns as i128, Scale::TAI))
}
}
}
/// Creates a `Dt` from a `chrono::Duration` / `TimeSpan` (nanosecond precision).
///
/// This is the exact reverse of [`Dt::to_chrono_duration`].
///
/// - The conversion is **lossless** when the chrono duration fits inside an `i64`
/// number of nanoseconds.
/// - Uses existing `from_ns` helper.
/// - If `num_nanoseconds()` returns `None` (the chrono value is outside the
/// range that chrono itself can represent as nanoseconds), we clamp to
/// **exactly** the maximum/minimum nanosecond value that chrono can store
/// (`i64::MAX` / `i64::MIN` nanoseconds) rather than saturating to
/// `Dt::MAX` / `Dt::MIN`.
pub fn from_chrono_duration(dur: Duration) -> Self {
match dur.num_nanoseconds() {
Some(ns) => Self::from_ns(ns as i128, Scale::TAI),
None => {
let ns = if dur > Duration::zero() {
i64::MAX
} else {
i64::MIN
};
Self::from_ns(ns as i128, Scale::TAI)
}
}
}
/// Converts this `Dt` to a `chrono::DateTime<chrono::Utc>`.
///
/// This is the main/default conversion method for absolute instants.
///
/// - The `Dt` is first converted to TAI internally (respecting all
/// scales, leap seconds, and relativistic models).
/// - The duration since the Unix epoch (1970-01-01 00:00:00 UTC) is then
/// computed.
/// - Sub-nanosecond attoseconds are truncated toward zero.
/// - Saturates at the minimum/maximum representable `DateTime<Utc>`
/// (roughly years 1678–2262) if the instant is out of range.
/// Never returns an error.
pub fn to_chrono_datetime_utc(&self) -> DateTime<Utc> {
let span_since_epoch = self.to_diff_raw(Dt::UNIX_EPOCH);
let total_nanos = span_since_epoch.to_attos() / 1_000_000_000i128;
let nanos = clamp_i128_to_i64(total_nanos);
DateTime::<Utc>::from_timestamp_nanos(nanos)
}
/// Converts this `Span` to a `chrono::Duration` (nanosecond precision).
///
/// - Sub-nanosecond attoseconds are **truncated toward zero**.
/// - The conversion is fully exact up to the nanosecond (128-bit integer arithmetic).
/// - **Saturates** at `chrono::Duration::MIN` / `chrono::Duration::MAX`
/// (roughly ±292 million years) if the value is out of range.
/// Never returns an error.
pub fn to_chrono_duration(&self) -> Duration {
let total_nanos = self.to_attos() / 1_000_000_000i128;
let nanos = clamp_i128_to_i64(total_nanos);
// `TimeDelta::nanoseconds` is infallible and returns exactly the
// `chrono::Duration` alias.
TimeDelta::nanoseconds(nanos).into()
}
}