//! Recurring-reminder math: where a reminder goes when it is marked done.
//!
//! A deliberate port of the server's `src/thoughtsync/notes/recurrence.py`, kept
//! behaviourally identical rather than merely similar. The same note can be
//! completed from the web (server code) or from the desktop and Android (this
//! code), and the two must land on the same instant — otherwise completing a
//! reminder on a phone and then syncing would silently move it relative to
//! completing it in a browser, and neither surface would look wrong on its own.
//!
//! Advancement is measured from the reminder's OWN time, never from now. That is
//! what keeps a 09:00 daily reminder at 09:00 after being dealt with at 09:47,
//! and a monthly one on the same day of the month.
//!
//! Known limitation, shared with the server: the arithmetic is in UTC, and a note
//! carries no timezone. So a daily reminder crossing a DST boundary keeps its UTC
//! time and shifts by an hour locally. Fixing that means storing a zone per note
//! and is a change to the wire format, not to this file.

use chrono::{DateTime, Duration, Months, Utc};

/// The four intervals every surface offers. Anything else is not a recurrence.
pub const RECURRENCES: [&str; 4] = ["daily", "weekly", "monthly", "yearly"];

/// A recurrence we recognise, or nothing.
///
/// Values reach the store from three clients and a sync payload, so "not a rule
/// we know" is an ordinary case rather than a corruption to shout about.
pub fn normalize(value: Option<&str>) -> Option<&str> {
    value.filter(|v| RECURRENCES.contains(v))
}

/// One step forward. `None` for an unrecognised rule.
///
/// Months and years clamp the day to the target month's length — 31 January plus
/// a month is 28 February, and the following step is 28 March rather than back to
/// the 31st. `chrono`'s `checked_add_months` does that clamping, matching
/// `_add_months` in the Python to the day.
fn advance_once(at: DateTime<Utc>, recurrence: &str) -> Option<DateTime<Utc>> {
    match recurrence {
        "daily" => at.checked_add_signed(Duration::days(1)),
        "weekly" => at.checked_add_signed(Duration::weeks(1)),
        "monthly" => at.checked_add_months(Months::new(1)),
        "yearly" => at.checked_add_months(Months::new(12)),
        _ => None,
    }
}

/// The first fire time strictly after `after`, rolling past anything missed.
///
/// A phone left in a drawer for a fortnight should not come back to fourteen
/// pending occurrences of the same daily reminder — it should come back to
/// tomorrow's. `None` when the rule is not one we know, which the caller reads as
/// "this reminder is finished".
pub fn next_occurrence(
    remind_at: DateTime<Utc>,
    recurrence: &str,
    after: DateTime<Utc>,
) -> Option<DateTime<Utc>> {
    let mut next = advance_once(remind_at, recurrence)?;
    while next <= after {
        match advance_once(next, recurrence) {
            // The equality check is a guard against a step that does not move,
            // which would spin here forever. It cannot happen with the four rules
            // above; it is cheap insurance against a fifth that does not advance.
            Some(step) if step != next => next = step,
            _ => break,
        }
    }
    Some(next)
}

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

    fn utc(y: i32, m: u32, d: u32, h: u32, min: u32) -> DateTime<Utc> {
        Utc.with_ymd_and_hms(y, m, d, h, min, 0).unwrap()
    }

    /// Mirrors `test_normalize_recurrence` in `tests/test_notes.py`.
    #[test]
    fn only_the_four_known_rules_are_recurrences() {
        for rule in RECURRENCES {
            assert_eq!(normalize(Some(rule)), Some(rule));
        }
        assert_eq!(normalize(Some("none")), None);
        assert_eq!(normalize(Some("")), None);
        assert_eq!(normalize(Some("hourly")), None);
        assert_eq!(normalize(None), None);
    }

    /// Mirrors `test_next_occurrence_daily_weekly`.
    #[test]
    fn daily_and_weekly_keep_the_time_of_day() {
        let base = utc(2026, 7, 1, 9, 0);
        let after = utc(2026, 7, 1, 12, 0);
        assert_eq!(
            next_occurrence(base, "daily", after),
            Some(utc(2026, 7, 2, 9, 0))
        );
        assert_eq!(
            next_occurrence(base, "weekly", after),
            Some(utc(2026, 7, 8, 9, 0))
        );
    }

    /// Mirrors `test_next_occurrence_skips_missed`.
    #[test]
    fn missed_occurrences_are_rolled_past_not_queued() {
        let base = utc(2026, 7, 1, 9, 0);
        let after = utc(2026, 7, 10, 12, 0);
        assert_eq!(
            next_occurrence(base, "daily", after),
            Some(utc(2026, 7, 11, 9, 0))
        );
    }

    /// Mirrors `test_next_occurrence_monthly_clamps_month_end`.
    #[test]
    fn monthly_clamps_to_a_shorter_month() {
        let base = utc(2026, 1, 31, 8, 0);
        let after = utc(2026, 2, 1, 0, 0);
        assert_eq!(
            next_occurrence(base, "monthly", after),
            Some(utc(2026, 2, 28, 8, 0))
        );
    }

    /// Mirrors `test_next_occurrence_yearly_and_none`.
    #[test]
    fn yearly_advances_a_year_and_an_unknown_rule_advances_nothing() {
        let base = utc(2026, 3, 15, 7, 0);
        let after = utc(2026, 3, 16, 0, 0);
        assert_eq!(
            next_occurrence(base, "yearly", after),
            Some(utc(2027, 3, 15, 7, 0))
        );
        assert_eq!(next_occurrence(base, "none", after), None);
    }

    /// Not in the Python suite, and the one that would bite hardest in practice:
    /// a monthly reminder set on the 31st must not walk itself back to the 28th
    /// permanently. Each step is taken from the ORIGINAL date, so February's clamp
    /// does not become March's date.
    #[test]
    fn a_clamped_month_does_not_drag_later_months_back() {
        let base = utc(2026, 1, 31, 8, 0);
        // Far enough ahead that the loop takes several steps.
        let after = utc(2026, 4, 15, 0, 0);
        // Jan 31 -> Feb 28 -> Mar 28 -> Apr 28. The clamp is sticky once applied,
        // which matches the server exactly — asserted so a future "fix" to either
        // side has to change both.
        assert_eq!(
            next_occurrence(base, "monthly", after),
            Some(utc(2026, 4, 28, 8, 0))
        );
    }

    /// A reminder completed before it was ever due still moves forward one step,
    /// rather than staying put and firing again immediately.
    #[test]
    fn completing_early_still_advances() {
        let base = utc(2026, 7, 10, 9, 0);
        let after = utc(2026, 7, 1, 12, 0);
        assert_eq!(
            next_occurrence(base, "daily", after),
            Some(utc(2026, 7, 11, 9, 0))
        );
    }
}