//! Recurring periods and the `next_occurrence_date` of a template (SPEC §9.5).
//!
//! Pure calendar arithmetic on `YYYY-MM` periods and `YYYY-MM-DD` dates; no clock and no time zone. The occurrence
//! *instant* (the clamped date at 06:00 Europe/Stockholm) is the server's concern; this module only fixes which
//! period and which calendar date.

use std::fmt;
use std::str::FromStr;

use serde::{Deserialize, Serialize};

/// A month, `YYYY-MM`. Orders chronologically.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Period {
    year: u16,
    month: u8,
}

/// Why a recurrence input is malformed.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum RecurrenceError {
    /// A period that is not `YYYY-MM` with month 01 to 12 (years 0001 to 9999).
    PeriodInvalid,
    /// `day_of_month` outside 1 to 31 (SPEC §9.6).
    DayOfMonthOutOfRange,
}

impl fmt::Display for RecurrenceError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(match self {
            Self::PeriodInvalid => "period_invalid",
            Self::DayOfMonthOutOfRange => "day_of_month_out_of_range",
        })
    }
}

impl std::error::Error for RecurrenceError {}

impl Period {
    /// A period, if `month` is 1 to 12 and `year` 1 to 9999.
    pub fn new(year: u16, month: u8) -> Option<Self> {
        ((1..=9999).contains(&year) && (1..=12).contains(&month)).then_some(Self { year, month })
    }

    pub fn year(self) -> u16 {
        self.year
    }

    pub fn month(self) -> u8 {
        self.month
    }

    /// The following month, or `None` after 9999-12.
    pub fn next(self) -> Option<Self> {
        if self.month == 12 {
            Self::new(self.year.checked_add(1)?, 1)
        } else {
            Self::new(self.year, self.month + 1)
        }
    }

    /// Number of days in this month (Gregorian leap years).
    pub fn days(self) -> u8 {
        match self.month {
            1 | 3 | 5 | 7 | 8 | 10 | 12 => 31,
            4 | 6 | 9 | 11 => 30,
            _ if is_leap(self.year) => 29,
            _ => 28,
        }
    }

    /// The occurrence date of `day_of_month` in this month, clamped to its last day (`31` in April is `30`), as
    /// `YYYY-MM-DD`.
    pub fn clamped_date(self, day_of_month: u8) -> Result<String, RecurrenceError> {
        if !(1..=31).contains(&day_of_month) {
            return Err(RecurrenceError::DayOfMonthOutOfRange);
        }
        Ok(format!("{self}-{:02}", day_of_month.min(self.days())))
    }
}

impl fmt::Display for Period {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{:04}-{:02}", self.year, self.month)
    }
}

impl FromStr for Period {
    type Err = RecurrenceError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let bytes = s.as_bytes();
        let digits = |r: std::ops::Range<usize>| bytes[r].iter().all(u8::is_ascii_digit);
        if bytes.len() != 7 || bytes[4] != b'-' || !digits(0..4) || !digits(5..7) {
            return Err(RecurrenceError::PeriodInvalid);
        }
        let year: u16 = s[0..4]
            .parse()
            .map_err(|_| RecurrenceError::PeriodInvalid)?;
        let month: u8 = s[5..7]
            .parse()
            .map_err(|_| RecurrenceError::PeriodInvalid)?;
        Self::new(year, month).ok_or(RecurrenceError::PeriodInvalid)
    }
}

fn is_leap(year: u16) -> bool {
    (year.is_multiple_of(4) && !year.is_multiple_of(100)) || year.is_multiple_of(400)
}

/// The template fields `next_occurrence_date` depends on.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct RecurrenceSchedule {
    pub day_of_month: u8,
    pub start_period: String,
    pub end_period: Option<String>,
    pub last_generated_period: Option<String>,
    pub paused: bool,
    pub deleted: bool,
}

/// The first period that may still be generated: the month after `last_generated_period`, but not before
/// `start_period`; `None` when that is after `end_period` (SPEC §9.5). Paused and deleted are not considered here.
pub fn next_candidate_period(
    start_period: &str,
    end_period: Option<&str>,
    last_generated_period: Option<&str>,
) -> Result<Option<Period>, RecurrenceError> {
    let start: Period = start_period.parse()?;
    let end: Option<Period> = end_period.map(str::parse).transpose()?;
    let after_last: Option<Period> = match last_generated_period {
        Some(last) => match last.parse::<Period>()?.next() {
            Some(p) => Some(p),
            None => return Ok(None),
        },
        None => None,
    };
    let candidate = after_last.map_or(start, |p| p.max(start));
    Ok(match end {
        Some(end) if candidate > end => None,
        _ => Some(candidate),
    })
}

/// `next_occurrence_date` of the RecurringTemplate wire form (SPEC §9.5): the clamped date of the first candidate
/// period after `last_generated_period` that is ≥ `start_period` and ≤ `end_period`; `None` when paused, deleted or
/// past `end_period`.
pub fn next_occurrence_date(
    schedule: &RecurrenceSchedule,
) -> Result<Option<String>, RecurrenceError> {
    if !(1..=31).contains(&schedule.day_of_month) {
        return Err(RecurrenceError::DayOfMonthOutOfRange);
    }
    let candidate = next_candidate_period(
        &schedule.start_period,
        schedule.end_period.as_deref(),
        schedule.last_generated_period.as_deref(),
    )?;
    if schedule.paused || schedule.deleted {
        return Ok(None);
    }
    candidate
        .map(|p| p.clamped_date(schedule.day_of_month))
        .transpose()
}

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

    fn schedule(day: u8, start: &str, end: Option<&str>, last: Option<&str>) -> RecurrenceSchedule {
        RecurrenceSchedule {
            day_of_month: day,
            start_period: start.to_owned(),
            end_period: end.map(str::to_owned),
            last_generated_period: last.map(str::to_owned),
            paused: false,
            deleted: false,
        }
    }

    fn next(s: &RecurrenceSchedule) -> Option<String> {
        next_occurrence_date(s).unwrap()
    }

    #[test]
    fn never_generated_starts_at_start_period() {
        assert_eq!(
            next(&schedule(25, "2026-11", None, None)).as_deref(),
            Some("2026-11-25")
        );
    }

    #[test]
    fn after_last_generated_period() {
        assert_eq!(
            next(&schedule(1, "2026-01", None, Some("2026-10"))).as_deref(),
            Some("2026-11-01")
        );
    }

    #[test]
    fn year_boundary() {
        assert_eq!(
            next(&schedule(15, "2026-01", None, Some("2026-12"))).as_deref(),
            Some("2027-01-15")
        );
    }

    #[test]
    fn start_period_moved_past_last_generated() {
        // Changing start_period never regenerates past periods; a later start wins over last + 1.
        assert_eq!(
            next(&schedule(10, "2027-03", None, Some("2026-10"))).as_deref(),
            Some("2027-03-10")
        );
    }

    #[test]
    fn clamped_to_month_end() {
        assert_eq!(
            next(&schedule(31, "2026-04", None, None)).as_deref(),
            Some("2026-04-30")
        );
        assert_eq!(
            next(&schedule(31, "2027-02", None, None)).as_deref(),
            Some("2027-02-28")
        );
        assert_eq!(
            next(&schedule(30, "2028-02", None, None)).as_deref(),
            Some("2028-02-29")
        );
        assert_eq!(
            next(&schedule(29, "2100-02", None, None)).as_deref(),
            Some("2100-02-28")
        );
        assert_eq!(
            next(&schedule(29, "2000-02", None, None)).as_deref(),
            Some("2000-02-29")
        );
    }

    #[test]
    fn end_period_inclusive_and_past() {
        assert_eq!(
            next(&schedule(5, "2026-01", Some("2026-11"), Some("2026-10"))).as_deref(),
            Some("2026-11-05")
        );
        assert_eq!(
            next(&schedule(5, "2026-01", Some("2026-11"), Some("2026-11"))),
            None
        );
        assert_eq!(next(&schedule(5, "2026-12", Some("2026-11"), None)), None);
    }

    #[test]
    fn paused_or_deleted_is_none() {
        let mut s = schedule(5, "2026-01", None, None);
        s.paused = true;
        assert_eq!(next(&s), None);
        s.paused = false;
        s.deleted = true;
        assert_eq!(next(&s), None);
    }

    #[test]
    fn malformed_input() {
        let bad = |s: RecurrenceSchedule, e| assert_eq!(next_occurrence_date(&s), Err(e));
        bad(
            schedule(0, "2026-01", None, None),
            RecurrenceError::DayOfMonthOutOfRange,
        );
        bad(
            schedule(32, "2026-01", None, None),
            RecurrenceError::DayOfMonthOutOfRange,
        );
        bad(
            schedule(1, "2026-13", None, None),
            RecurrenceError::PeriodInvalid,
        );
        bad(
            schedule(1, "2026-1", None, None),
            RecurrenceError::PeriodInvalid,
        );
        bad(
            schedule(1, "2026-01", Some("x"), None),
            RecurrenceError::PeriodInvalid,
        );
        bad(
            schedule(1, "2026-01", None, Some("2026-00")),
            RecurrenceError::PeriodInvalid,
        );
    }

    #[test]
    fn period_round_trip_and_order() {
        let p: Period = "2026-10".parse().unwrap();
        assert_eq!(p.to_string(), "2026-10");
        assert!(p < "2026-11".parse().unwrap());
        assert!(p > "2025-12".parse().unwrap());
        assert_eq!(p.next().map(|n| n.to_string()).as_deref(), Some("2026-11"));
        assert_eq!("9999-12".parse::<Period>().unwrap().next(), None);
    }
}