ostraka-core 1.1.0

Domain types for Ostraka: tasks, gates, verdicts and run records.
Documentation 
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//! Timestamps for records, without taking a dependency for it.
//!
//! Records are read by people and diffed by machines, so they carry RFC 3339
//! UTC rather than an epoch count. The conversion is Howard Hinnant's
//! civil-from-days algorithm, which is exact for every date this will ever see.

use std::time::{SystemTime, UNIX_EPOCH};

/// The current instant as RFC 3339 UTC, to the second.
pub fn now_rfc3339() -> String {
    let secs = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_secs() as i64)
        .unwrap_or(0);
    rfc3339_from_unix(secs)
}

/// Formats a Unix timestamp as RFC 3339 UTC.
pub fn rfc3339_from_unix(secs: i64) -> String {
    let days = secs.div_euclid(86_400);
    let rem = secs.rem_euclid(86_400);
    let (year, month, day) = civil_from_days(days);
    let (h, m, s) = (rem / 3600, (rem % 3600) / 60, rem % 60);
    format!("{year:04}-{month:02}-{day:02}T{h:02}:{m:02}:{s:02}Z")
}

fn civil_from_days(z: i64) -> (i64, u32, u32) {
    let z = z + 719_468;
    let era = z.div_euclid(146_097);
    let doe = z.rem_euclid(146_097);
    let yoe = (doe - doe / 1460 + doe / 36_524 - doe / 146_096) / 365;
    let y = yoe + era * 400;
    let doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
    let mp = (5 * doy + 2) / 153;
    let d = (doy - (153 * mp + 2) / 5 + 1) as u32;
    let m = if mp < 10 { mp + 3 } else { mp - 9 } as u32;
    (if m <= 2 { y + 1 } else { y }, m, d)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn known_instants_format_correctly() {
        assert_eq!(rfc3339_from_unix(0), "1970-01-01T00:00:00Z");
        assert_eq!(rfc3339_from_unix(1_000_000_000), "2001-09-09T01:46:40Z");
        // A leap day, which is where naive date arithmetic usually goes wrong.
        assert_eq!(rfc3339_from_unix(1_709_164_800), "2024-02-29T00:00:00Z");
    }

    #[test]
    fn the_clock_produces_a_plausible_timestamp() {
        let now = now_rfc3339();
        assert_eq!(now.len(), 20);
        assert!(now.ends_with('Z'));
        assert!(now.starts_with("20"), "unexpected year in {now}");
    }
}