shahanshahi 0.2.0

Shahanshahi (Imperial Iranian) civil calendar types and conversions — spec-first, pre-1.0
Documentation 
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//! Solar Hijri leap arithmetic — **Mode A** per [`SPEC.md`](https://github.com/melliran/shahanshahi/blob/main/SPEC.md) (33-year rule).

/// Solar Hijri leap year per the 33-year arithmetic rule (Heydari-Malayeri / Borkowski range).
#[inline]
pub fn is_solar_hijri_leap_arithmetic(y_h: i32) -> bool {
    let rem33 = y_h.rem_euclid(33);
    matches!(rem33, 1 | 5 | 9 | 13 | 17 | 22 | 26 | 30)
}

/// Underlying Solar Hijri year from Shahanshahi year (proleptic offset).
#[inline]
pub fn shahanshahi_to_hijri_shamsi_year(y_s: i32) -> i32 {
    y_s - 1180
}

/// Shahanshahi leap year using Mode A (same solar year as underlying Hijri Shamsi).
#[inline]
pub fn is_shahanshahi_leap_arithmetic(y_s: i32) -> bool {
    is_solar_hijri_leap_arithmetic(shahanshahi_to_hijri_shamsi_year(y_s))
}

/// Days in a Shahanshahi civil month (1925 solar law + Mode A Esfand), **1-based** `month` in `1..=12`.
#[inline]
pub(crate) fn days_in_shahanshahi_month(year: i32, month: u8) -> u8 {
    match month {
        1..=6 => 31,
        7..=11 => 30,
        12 => {
            if is_shahanshahi_leap_arithmetic(year) {
                30
            } else {
                29
            }
        }
        _ => 0,
    }
}

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

    #[test]
    fn spec_window_1354_1357() {
        assert!(is_solar_hijri_leap_arithmetic(1354));
        assert!(!is_solar_hijri_leap_arithmetic(1355));
        assert!(!is_solar_hijri_leap_arithmetic(1356));
        assert!(!is_solar_hijri_leap_arithmetic(1357));
    }

    #[test]
    fn offset_maps_shahanshahi_to_hijri_year() {
        assert_eq!(shahanshahi_to_hijri_shamsi_year(2535), 1355);
        assert_eq!(
            is_shahanshahi_leap_arithmetic(2534),
            is_solar_hijri_leap_arithmetic(1354)
        );
    }
}