//! UTC timestamps without a chrono dependency: Howard Hinnant's
//! civil_from_days for the calendar math, unix-epoch seconds everywhere
//! else (the run ledger stores epochs and renders through [`fmt_ts`]).

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

/// Unix-epoch seconds right now (UTC).
pub(crate) fn now_secs() -> i64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_secs() as i64)
        .unwrap_or(0)
}

/// `YYYY-MM-DD_HHMMSS` in UTC (bench report file names).
pub(crate) fn utc_stamp() -> String {
    let secs = now_secs();
    let (h, m, s) = ((secs % 86400) / 3600, (secs % 3600) / 60, secs % 60);
    let (y, mo, d) = civil_date(secs / 86400);
    format!(
        "{y:04}-{mo:02}-{d:02}_{h:02}{m:02}{s:02}",
        y = y,
        mo = mo,
        d = d
    )
}

/// `YYYY-MM-DD HH:MM` in UTC (run-ledger display).
pub(crate) fn fmt_ts(secs: i64) -> String {
    let (h, m, _) = ((secs % 86400) / 3600, (secs % 3600) / 60, secs % 60);
    let (y, mo, d) = civil_date(secs / 86400);
    format!("{y:04}-{mo:02}-{d:02} {h:02}:{m:02}")
}

/// Days since 1970-01-01 → (year, month, day); Howard Hinnant's civil_from_days.
pub(crate) fn civil_date(z: i64) -> (i64, i64, i64) {
    let z = z + 719_468;
    let era = if z >= 0 { z } else { z - 146_096 } / 146_097;
    let doe = z - era * 146_097;
    let yoe = (doe - doe / 1_460 + 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;
    let mo = if mp < 10 { mp + 3 } else { mp - 9 };
    let y = if mo <= 2 { y + 1 } else { y };
    (y, mo, d)
}

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

    #[test]
    fn stamp_format() {
        let s = utc_stamp();
        assert_eq!(s.len(), 17, "{s}");
        assert_eq!(&s[4..5], "-");
        assert_eq!(&s[7..8], "-");
        assert_eq!(&s[10..11], "_");
    }

    #[test]
    fn civil_date_known_values() {
        assert_eq!(civil_date(0), (1970, 1, 1));
        assert_eq!(civil_date(19_723), (2024, 1, 1));
        assert_eq!(civil_date(20_818), (2026, 12, 31));
    }

    #[test]
    fn fmt_ts_renders_date_and_minute() {
        // 2026-09-14 10:03:00 UTC
        assert_eq!(fmt_ts(1_789_380_180), "2026-09-14 10:03");
    }
}