//! Human-friendly date and recurrence parsing for the CLI (tech-spec §1, §8).
//!
//! `heph-core` is clock-pure (no ambient wall-clock reads); the CLI is a client,
//! so it may read the local clock. Date parsing is split so the logic is
//! deterministically testable: the pure functions take `today` / a year, and the
//! thin wrappers supply `Local::now()`.
//!
//! Recurrence input mirrors how the owner thinks in Todoist (see [[design]]
//! §6.2.1): presets, the common natural-language forms, or a raw RRULE.

use anyhow::{bail, Context, Result};
use chrono::{Datelike, Days, Local, Months, NaiveDate, TimeZone, Weekday};

// ---------------------------------------------------------------------------
// Dates
// ---------------------------------------------------------------------------

/// Parse a human date spec **relative to `today`** into a `NaiveDate`. Accepts:
/// `today`, `tomorrow`/`tom`, `yesterday`; `+Nd`/`+Nw`/`+Nm` (and bare `+N` =
/// days); weekday names (`mon`..`sun`, the soonest such day on/after today);
/// ISO `YYYY-MM-DD`.
pub fn parse_date(input: &str, today: NaiveDate) -> Result<NaiveDate> {
    let s = input.trim().to_lowercase();
    if s.is_empty() {
        bail!("empty date");
    }
    match s.as_str() {
        "today" | "now" => return Ok(today),
        "tomorrow" | "tom" => return Ok(today + Days::new(1)),
        "yesterday" => return Ok(today - Days::new(1)),
        _ => {}
    }
    if let Some(wd) = parse_weekday(&s) {
        return Ok(soonest_weekday(today, wd));
    }
    if let Some(rest) = s.strip_prefix('+') {
        return parse_offset(rest, today);
    }
    NaiveDate::parse_from_str(&s, "%Y-%m-%d").with_context(|| {
        format!("unrecognized date: {input:?} (try today, tomorrow, +3d, fri, or YYYY-MM-DD)")
    })
}

/// `parse_date` against the local `today`.
pub fn parse_date_today(input: &str) -> Result<NaiveDate> {
    parse_date(input, Local::now().date_naive())
}

/// Parse a human date spec to epoch milliseconds at **local midnight** — the
/// form `do_date`/`late_on` are stored in (a date-grained candidacy gate, §7).
pub fn parse_date_ms(input: &str) -> Result<i64> {
    Ok(to_epoch_ms(parse_date_today(input)?))
}

/// Local-midnight epoch ms for a date. Falls back gracefully across a DST gap.
pub fn to_epoch_ms(date: NaiveDate) -> i64 {
    let midnight = date.and_hms_opt(0, 0, 0).expect("00:00:00 is always valid");
    match Local.from_local_datetime(&midnight).earliest() {
        Some(dt) => dt.timestamp_millis(),
        None => midnight.and_utc().timestamp_millis(),
    }
}

/// Compact display of an epoch-ms date: `MM-DD` within the current year,
/// `YYYY-MM-DD` otherwise.
pub fn fmt_date(ms: i64) -> String {
    let date = match Local.timestamp_millis_opt(ms).earliest() {
        Some(dt) => dt.date_naive(),
        None => return ms.to_string(),
    };
    fmt_naive(date, Local::now().date_naive().year())
}

fn fmt_naive(date: NaiveDate, this_year: i32) -> String {
    if date.year() == this_year {
        format!("{:02}-{:02}", date.month(), date.day())
    } else {
        date.format("%Y-%m-%d").to_string()
    }
}

fn parse_offset(rest: &str, today: NaiveDate) -> Result<NaiveDate> {
    let rest = rest.trim();
    let (num, unit) = rest.split_at(
        rest.find(|c: char| !c.is_ascii_digit())
            .unwrap_or(rest.len()),
    );
    let n: u64 = num
        .parse()
        .with_context(|| format!("not a relative date offset: +{rest}"))?;
    match unit.trim() {
        "" | "d" | "day" | "days" => Ok(today + Days::new(n)),
        "w" | "wk" | "week" | "weeks" => Ok(today + Days::new(n * 7)),
        "m" | "mo" | "month" | "months" => Ok(today + Months::new(n as u32)),
        other => bail!("unknown offset unit {other:?} (use d, w, or m)"),
    }
}

/// Map a weekday name (full or common abbreviation) to a `Weekday`. Matches
/// only recognized weekday tokens — not arbitrary prefixes (so "months" is not
/// "mon").
fn parse_weekday(s: &str) -> Option<Weekday> {
    match s {
        "mon" | "monday" => Some(Weekday::Mon),
        "tue" | "tues" | "tuesday" => Some(Weekday::Tue),
        "wed" | "weds" | "wednesday" => Some(Weekday::Wed),
        "thu" | "thur" | "thurs" | "thursday" => Some(Weekday::Thu),
        "fri" | "friday" => Some(Weekday::Fri),
        "sat" | "saturday" => Some(Weekday::Sat),
        "sun" | "sunday" => Some(Weekday::Sun),
        _ => None,
    }
}

/// The soonest date on/after `today` whose weekday is `wd`.
fn soonest_weekday(today: NaiveDate, wd: Weekday) -> NaiveDate {
    let mut d = today;
    for _ in 0..7 {
        if d.weekday() == wd {
            return d;
        }
        d = d + Days::new(1);
    }
    today // unreachable: a weekday occurs within 7 days
}

// ---------------------------------------------------------------------------
// Recurrence
// ---------------------------------------------------------------------------

/// Parse a recurrence spec into an RFC-5545 RRULE. Accepts a raw RRULE
/// (anything containing `FREQ=`), presets (`daily|weekly|monthly|yearly|
/// weekdays`), and the common natural-language forms the owner uses in Todoist
/// (§6.2.1): `every N (day|week|month|year)s`, `every <weekday>`, `every other
/// <weekday|unit>`, `every workday`, `every <Month> <day>`. A trailing
/// `at <time>` is ignored (heph's do-date is date-grained).
pub fn parse_recurrence(spec: &str) -> Result<String> {
    let raw = spec.trim();
    if raw.to_uppercase().contains("FREQ=") {
        return Ok(raw.to_string());
    }

    // Normalize: lowercase, drop a trailing "at <time>" clause.
    let mut s = raw.to_lowercase();
    if let Some(at) = s.find(" at ") {
        s.truncate(at);
    }
    let s = s.trim();

    // Presets.
    match s {
        "daily" | "day" => return Ok("FREQ=DAILY".into()),
        "weekly" | "week" => return Ok("FREQ=WEEKLY".into()),
        "monthly" | "month" => return Ok("FREQ=MONTHLY".into()),
        "yearly" | "annually" | "year" => return Ok("FREQ=YEARLY".into()),
        "weekdays" | "workdays" | "workday" | "weekday" => {
            return Ok("FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR".into())
        }
        _ => {}
    }

    let body = s.strip_prefix("every ").unwrap_or(s).trim();

    // "every other <...>" → INTERVAL=2.
    if let Some(rest) = body.strip_prefix("other ") {
        return interval_form(2, rest.trim());
    }
    // "every workday/weekday".
    if body == "workday" || body == "weekday" || body == "workdays" || body == "weekdays" {
        return Ok("FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR".into());
    }
    // "every <weekday>".
    if let Some(wd) = parse_weekday(body) {
        return Ok(format!("FREQ=WEEKLY;BYDAY={}", byday(wd)));
    }
    // "every <Month> <day>".
    if let Some((m, d)) = parse_month_day(body) {
        return Ok(format!("FREQ=YEARLY;BYMONTH={m};BYMONTHDAY={d}"));
    }
    // "every <Nth>" → monthly on that day of the month ("every 5th").
    if let Some(d) = parse_monthday_ordinal(body) {
        return Ok(format!("FREQ=MONTHLY;BYMONTHDAY={d}"));
    }
    // "every <unit>" / "every N <unit>s".
    let mut it = body.split_whitespace();
    let first = it.next().unwrap_or("");
    if let Ok(n) = first.parse::<u32>() {
        return interval_form(n, it.next().unwrap_or(""));
    }
    interval_form(1, first)
}

/// `FREQ=...[;INTERVAL=n]` for a unit word, or a weekday with an interval.
fn interval_form(n: u32, unit: &str) -> Result<String> {
    if let Some(wd) = parse_weekday(unit) {
        let mut r = format!("FREQ=WEEKLY;BYDAY={}", byday(wd));
        if n != 1 {
            r = format!("FREQ=WEEKLY;INTERVAL={n};BYDAY={}", byday(wd));
        }
        return Ok(r);
    }
    let freq = match unit {
        "day" | "days" => "DAILY",
        "week" | "weeks" => "WEEKLY",
        "month" | "months" => "MONTHLY",
        "year" | "years" => "YEARLY",
        other => bail!(
            "unrecognized recurrence {other:?} (try daily/weekly/monthly/yearly, \
             'every 3 days', 'every fri', or a raw RRULE)"
        ),
    };
    Ok(if n == 1 {
        format!("FREQ={freq}")
    } else {
        format!("FREQ={freq};INTERVAL={n}")
    })
}

fn byday(wd: Weekday) -> &'static str {
    match wd {
        Weekday::Mon => "MO",
        Weekday::Tue => "TU",
        Weekday::Wed => "WE",
        Weekday::Thu => "TH",
        Weekday::Fri => "FR",
        Weekday::Sat => "SA",
        Weekday::Sun => "SU",
    }
}

/// Parse a day-of-month ordinal like "5th", "1st", "22nd", "3rd" → 1..=31.
fn parse_monthday_ordinal(s: &str) -> Option<u32> {
    let digits = s.trim_end_matches(|c: char| c.is_ascii_alphabetic());
    let suffix = &s[digits.len()..];
    if !matches!(suffix, "st" | "nd" | "rd" | "th") {
        return None;
    }
    match digits.parse::<u32>() {
        Ok(d @ 1..=31) => Some(d),
        _ => None,
    }
}

/// Parse "april 15" / "apr 15" (and the reversed "15 april") → (month, day).
fn parse_month_day(s: &str) -> Option<(u32, u32)> {
    let toks: Vec<&str> = s.split_whitespace().collect();
    if toks.len() != 2 {
        return None;
    }
    let month = |t: &str| -> Option<u32> {
        match &t[..t.len().min(3)] {
            "jan" => Some(1),
            "feb" => Some(2),
            "mar" => Some(3),
            "apr" => Some(4),
            "may" => Some(5),
            "jun" => Some(6),
            "jul" => Some(7),
            "aug" => Some(8),
            "sep" => Some(9),
            "oct" => Some(10),
            "nov" => Some(11),
            "dec" => Some(12),
            _ => None,
        }
    };
    let day = |t: &str| {
        t.trim_end_matches(|c: char| !c.is_ascii_digit())
            .parse::<u32>()
            .ok()
    };
    if let (Some(m), Some(d)) = (month(toks[0]), day(toks[1])) {
        return Some((m, d));
    }
    if let (Some(d), Some(m)) = (day(toks[0]), month(toks[1])) {
        return Some((m, d));
    }
    None
}

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

    // 2026-06-02 is a Tuesday.
    fn today() -> NaiveDate {
        NaiveDate::from_ymd_opt(2026, 6, 2).unwrap()
    }

    fn d(y: i32, m: u32, day: u32) -> NaiveDate {
        NaiveDate::from_ymd_opt(y, m, day).unwrap()
    }

    #[test]
    fn parse_date_keywords_and_offsets() {
        let t = today();
        assert_eq!(parse_date("today", t).unwrap(), d(2026, 6, 2));
        assert_eq!(parse_date("tomorrow", t).unwrap(), d(2026, 6, 3));
        assert_eq!(parse_date("yesterday", t).unwrap(), d(2026, 6, 1));
        assert_eq!(parse_date("+3d", t).unwrap(), d(2026, 6, 5));
        assert_eq!(parse_date("+2w", t).unwrap(), d(2026, 6, 16));
        assert_eq!(parse_date("+1m", t).unwrap(), d(2026, 7, 2));
        assert_eq!(parse_date("+5", t).unwrap(), d(2026, 6, 7));
    }

    #[test]
    fn parse_date_weekdays_are_soonest_on_or_after_today() {
        let t = today(); // Tuesday
        assert_eq!(
            parse_date("tue", t).unwrap(),
            d(2026, 6, 2),
            "today if it matches"
        );
        assert_eq!(parse_date("fri", t).unwrap(), d(2026, 6, 5));
        assert_eq!(
            parse_date("mon", t).unwrap(),
            d(2026, 6, 8),
            "wraps to next week"
        );
    }

    #[test]
    fn parse_date_iso_and_errors() {
        assert_eq!(parse_date("2026-12-25", today()).unwrap(), d(2026, 12, 25));
        assert!(parse_date("someday", today()).is_err());
        assert!(parse_date("", today()).is_err());
    }

    #[test]
    fn fmt_naive_elides_the_current_year() {
        assert_eq!(fmt_naive(d(2026, 6, 5), 2026), "06-05");
        assert_eq!(fmt_naive(d(2027, 1, 9), 2026), "2027-01-09");
    }

    #[test]
    fn recurrence_presets_and_raw() {
        assert_eq!(parse_recurrence("daily").unwrap(), "FREQ=DAILY");
        assert_eq!(parse_recurrence("weekly").unwrap(), "FREQ=WEEKLY");
        assert_eq!(parse_recurrence("monthly").unwrap(), "FREQ=MONTHLY");
        assert_eq!(parse_recurrence("yearly").unwrap(), "FREQ=YEARLY");
        assert_eq!(
            parse_recurrence("weekdays").unwrap(),
            "FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR"
        );
        // Raw RRULE passes through untouched.
        assert_eq!(
            parse_recurrence("FREQ=DAILY;INTERVAL=2").unwrap(),
            "FREQ=DAILY;INTERVAL=2"
        );
    }

    #[test]
    fn recurrence_natural_language() {
        assert_eq!(parse_recurrence("every day").unwrap(), "FREQ=DAILY");
        assert_eq!(
            parse_recurrence("every 3 days").unwrap(),
            "FREQ=DAILY;INTERVAL=3"
        );
        assert_eq!(
            parse_recurrence("every 2 weeks").unwrap(),
            "FREQ=WEEKLY;INTERVAL=2"
        );
        assert_eq!(
            parse_recurrence("every 6 months").unwrap(),
            "FREQ=MONTHLY;INTERVAL=6"
        );
        assert_eq!(
            parse_recurrence("every fri").unwrap(),
            "FREQ=WEEKLY;BYDAY=FR"
        );
        assert_eq!(
            parse_recurrence("every other wed").unwrap(),
            "FREQ=WEEKLY;INTERVAL=2;BYDAY=WE"
        );
        assert_eq!(
            parse_recurrence("every other day").unwrap(),
            "FREQ=DAILY;INTERVAL=2"
        );
        assert_eq!(
            parse_recurrence("every workday at 08:00").unwrap(),
            "FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR"
        );
        assert_eq!(
            parse_recurrence("every April 15").unwrap(),
            "FREQ=YEARLY;BYMONTH=4;BYMONTHDAY=15"
        );
        assert_eq!(
            parse_recurrence("every 5th").unwrap(),
            "FREQ=MONTHLY;BYMONTHDAY=5"
        );
        assert_eq!(
            parse_recurrence("every 22nd").unwrap(),
            "FREQ=MONTHLY;BYMONTHDAY=22"
        );
        assert!(parse_recurrence("every blue moon").is_err());
    }
}