catcher 0.12.0

//! Today's date, without a crate.
//!
//! std knows the seconds since the epoch and nothing about time zones, and
//! the daily note has to be dated where the user is, not at Greenwich. The
//! calendar arithmetic is Howard Hinnant's civil-from-days; the offset comes
//! from `TZ` when it is a fixed offset, and otherwise from one `date +%z` at
//! startup — the only source of the zone database std cannot read. A machine
//! without `date` falls back to UTC, and a zone that changes offset while
//! catcher is running is noticed at the next launch, not before.

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

/// Today, as (year, month, day) in local time.
pub fn today() -> (i32, u32, u32) {
    let secs = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_secs() as i64)
        .unwrap_or(0);
    let local = secs + i64::from(local_offset());
    civil_from_days(local.div_euclid(86_400))
}

/// `YYYY-MM-DD`.
pub fn iso(y: i32, m: u32, d: u32) -> String {
    format!("{y:04}-{m:02}-{d:02}")
}

/// The long form a heading wants: `Tuesday 1 September 2026`.
pub fn long(y: i32, m: u32, d: u32) -> String {
    format!(
        "{} {d} {} {y}",
        weekday(y, m, d),
        MONTHS[(m as usize).saturating_sub(1) % 12]
    )
}

/// The date `n` days on (or back, when negative).
pub fn shift(y: i32, m: u32, d: u32, n: i64) -> (i32, u32, u32) {
    civil_from_days(days_from_civil(y, m, d) + n)
}

const MONTHS: [&str; 12] = [
    "January",
    "February",
    "March",
    "April",
    "May",
    "June",
    "July",
    "August",
    "September",
    "October",
    "November",
    "December",
];

const WEEKDAYS: [&str; 7] = [
    "Thursday",
    "Friday",
    "Saturday",
    "Sunday",
    "Monday",
    "Tuesday",
    "Wednesday",
];

/// Day zero, 1970-01-01, was a Thursday.
fn weekday(y: i32, m: u32, d: u32) -> &'static str {
    WEEKDAYS[days_from_civil(y, m, d).rem_euclid(7) as usize]
}

/// Days since 1970-01-01 to (y, m, d): the Gregorian calendar as a sequence
/// of 400-year eras, each starting on the 1st of March so the leap day lands
/// last. Hinnant's `days_from_civil`.
fn days_from_civil(y: i32, m: u32, d: u32) -> i64 {
    let y = i64::from(y) - i64::from(m <= 2);
    let era = y.div_euclid(400);
    let yoe = y - era * 400;
    let mp = i64::from((m + 9) % 12);
    let doy = (153 * mp + 2) / 5 + i64::from(d) - 1;
    let doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
    era * 146_097 + doe - 719_468
}

/// The inverse: (y, m, d) from days since the epoch. Hinnant's `civil_from_days`.
fn civil_from_days(z: i64) -> (i32, u32, u32) {
    let z = z + 719_468;
    let era = z.div_euclid(146_097);
    let doe = z - era * 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;
    ((y + i64::from(m <= 2)) as i32, m, d)
}

/// Work out the offset before the terminal goes raw: it costs a process, and
/// a child spawned under raw mode inherits a terminal it does not expect.
/// `today` finds it anyway if this was never called.
pub fn init() {
    local_offset();
}

/// Seconds east of UTC, found once and kept: the answer costs a process.
fn local_offset() -> i32 {
    static OFFSET: OnceLock<i32> = OnceLock::new();
    *OFFSET.get_or_init(|| {
        std::env::var("TZ")
            .ok()
            .and_then(|tz| offset_from_tz(&tz))
            .or_else(|| date_z().and_then(|z| offset_from_z(&z)))
            .unwrap_or(0)
    })
}

/// What `date +%z` prints: `+0100`, `-0700`.
fn date_z() -> Option<String> {
    let out = std::process::Command::new("date")
        .arg("+%z")
        .output()
        .ok()?;
    out.status
        .success()
        .then(|| String::from_utf8_lossy(&out.stdout).trim().to_string())
}

/// `+HHMM` or `-HHMM` to seconds east.
fn offset_from_z(z: &str) -> Option<i32> {
    let (sign, rest) = match z.chars().next()? {
        '+' => (1, &z[1..]),
        '-' => (-1, &z[1..]),
        _ => return None,
    };
    if rest.len() != 4 || !rest.chars().all(|c| c.is_ascii_digit()) {
        return None;
    }
    let h: i32 = rest[..2].parse().ok()?;
    let m: i32 = rest[2..].parse().ok()?;
    Some(sign * (h * 3600 + m * 60))
}

/// The fixed-offset spellings of `TZ` — `UTC`, `UTC0`, `EST5`, `UTC-3`,
/// `GMT+02:00`. POSIX puts the sign the other way round: `EST5` is five hours
/// *west*. A named zone (`Europe/London`) needs the database, so `None`.
fn offset_from_tz(tz: &str) -> Option<i32> {
    let tz = tz.trim().trim_start_matches(':');
    let name_len = tz.chars().take_while(|c| c.is_ascii_alphabetic()).count();
    if name_len < 3 || tz.contains('/') {
        return None;
    }
    let rest = &tz[name_len..];
    if rest.is_empty() {
        return matches!(&tz[..3], "UTC" | "GMT").then_some(0);
    }
    let (west, rest) = match rest.chars().next()? {
        '-' => (false, &rest[1..]),
        '+' => (true, &rest[1..]),
        _ => (true, rest),
    };
    let mut parts = rest.split(':');
    let h: i32 = parts.next()?.parse().ok()?;
    let m: i32 = parts.next().map_or(Some(0), |p| p.parse().ok())?;
    let secs = h * 3600 + m * 60;
    Some(if west { -secs } else { secs })
}

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

    #[test]
    fn the_epoch_and_leap_days_round_trip_through_hinnant() {
        assert_eq!(civil_from_days(0), (1970, 1, 1));
        assert_eq!(days_from_civil(1970, 1, 1), 0);
        assert_eq!(civil_from_days(-1), (1969, 12, 31));
        assert_eq!(civil_from_days(days_from_civil(2000, 2, 29)), (2000, 2, 29));
        assert_eq!(civil_from_days(days_from_civil(2026, 9, 1)), (2026, 9, 1));
        // 2026-09-01 is 20,697 days on
        assert_eq!(days_from_civil(2026, 9, 1), 20_697);
    }

    #[test]
    fn dates_format_as_iso_and_as_a_heading() {
        assert_eq!(iso(2026, 9, 1), "2026-09-01");
        assert_eq!(long(2026, 9, 1), "Tuesday 1 September 2026");
        assert_eq!(long(1970, 1, 1), "Thursday 1 January 1970");
        assert_eq!(long(2024, 2, 29), "Thursday 29 February 2024");
    }

    #[test]
    fn shifting_crosses_month_and_year_ends() {
        assert_eq!(shift(2026, 9, 1, -1), (2026, 8, 31));
        assert_eq!(shift(2026, 12, 31, 1), (2027, 1, 1));
        assert_eq!(shift(2024, 2, 28, 1), (2024, 2, 29));
    }

    #[test]
    fn the_offset_reads_date_z_and_the_fixed_tz_spellings() {
        assert_eq!(offset_from_z("+0100"), Some(3600));
        assert_eq!(offset_from_z("-0730"), Some(-27_000));
        assert_eq!(offset_from_z("0100"), None);
        assert_eq!(offset_from_z("+1"), None);
        assert_eq!(offset_from_tz("UTC"), Some(0));
        assert_eq!(offset_from_tz("EST5"), Some(-18_000));
        assert_eq!(offset_from_tz("UTC-3"), Some(10_800));
        assert_eq!(offset_from_tz("GMT+02:00"), Some(-7200));
        // a named zone needs the database, so it is left to `date`
        assert_eq!(offset_from_tz("Europe/London"), None);
        assert_eq!(offset_from_tz("America/New_York"), None);
        assert_eq!(offset_from_tz(""), None);
    }

    #[test]
    fn today_is_a_real_date() {
        let (y, m, d) = today();
        assert!(y >= 2026);
        assert!((1..=12).contains(&m));
        assert!((1..=31).contains(&d));
    }
}