// 日本祝日计算函数。返回 YYYY-MM-DD → 节假日名称的映射。
// weekday(date) 的周日为 0；date(year, month, day) 与 add_days(date, n) 生成日期。
// 规则适用 1955–2099 年；未公布年份的春秋分日为近似计算。
fn monday(year, month, nth) {
    let first = date(year, month, 1);
    1 + (8 - weekday(first)) % 7 + 7 * (nth - 1)
}

fn japanese_holidays(year) {
    let h = #{};
    if year < 1955 || year > 2099 { return h; }
    h[date(year, 1, 1)] = "元日";
    h[date(year, 1, if year >= 2000 { monday(year, 1, 2) } else { 15 })] = "成人の日";
    if year >= 1967 { h[date(year, 2, 11)] = "建国記念の日"; }
    if year >= 2020 { h[date(year, 2, 23)] = "天皇誕生日"; }
    if year >= 1989 && year <= 2018 { h[date(year, 12, 23)] = "天皇誕生日"; }
    h[date(year, 4, 29)] = if year >= 2007 { "昭和の日" }
        else if year >= 1989 { "みどりの日" } else { "天皇誕生日" };

    // 春分・秋分近似式。官报公布前的将来日期仅为推算。
    let spring_base = if year < 1980 { 20.8357 } else { 20.8431 };
    let autumn_base = if year < 1980 { 23.2588 } else { 23.2488 };
    let leap_base = if year < 1980 { 1983 } else { 1980 };
    let shift = 0.242194 * (year - 1980) - (year - leap_base) / 4;
    h[date(year, 3, floor(spring_base + shift))] = "春分の日";
    h[date(year, 9, floor(autumn_base + shift))] = "秋分の日";
    h[date(year, 5, 3)] = "憲法記念日";
    if year >= 2007 { h[date(year, 5, 4)] = "みどりの日"; }
    h[date(year, 5, 5)] = "こどもの日";
    h[date(year, 11, 3)] = "文化の日";
    h[date(year, 11, 23)] = "勤労感謝の日";

    if year >= 1966 {
        h[date(year, 9, if year >= 2003 { monday(year, 9, 3) } else { 15 })] = "敬老の日";
        let sport = if year == 2020 { date(year, 7, 24) }
            else if year == 2021 { date(year, 7, 23) }
            else { date(year, 10, if year >= 2000 { monday(year, 10, 2) } else { 10 }) };
        h[sport] = if year >= 2020 { "スポーツの日" }
            else if year == 2019 { "体育の日（スポーツの日）" } else { "体育の日" };
    }
    if year >= 1996 {
        let sea = if year == 2020 { date(year, 7, 23) }
            else if year == 2021 { date(year, 7, 22) }
            else { date(year, 7, if year >= 2003 { monday(year, 7, 3) } else { 20 }) };
        h[sea] = "海の日";
    }
    if year >= 2016 {
        h[date(year, 8, if year == 2020 { 10 } else if year == 2021 { 8 } else { 11 })] = "山の日";
    }
    // 即位・成婚等の特例。
    if year == 1959 { h[date(year, 4, 10)] = "結婚の儀"; }
    if year == 1989 { h[date(year, 2, 24)] = "大喪の礼"; }
    if year == 1990 { h[date(year, 11, 12)] = "即位礼正殿の儀"; }
    if year == 1993 { h[date(year, 6, 9)] = "結婚の儀"; }
    if year == 2019 {
        h[date(year, 5, 1)] = "休日（祝日扱い）";
        h[date(year, 10, 22)] = "休日（祝日扱い）";
    }
    let national = h;
    let first = date(year, 1, 1);
    // 祝日に挟まれた日：1986 年から適用、日曜日は対象外。
    if year >= 1986 {
        for day in 0..days_in_year(year) {
            let d = add_days(first, day);
            if weekday(d) != 0 && !h.contains(d)
                && national.contains(add_days(d, -1)) && national.contains(add_days(d, 1)) {
                h[d] = "休日";
            }
        }
    }
    // 振替休日：1973 年導入、2007 年以降は次の祝日でない日まで繰り越す。
    for d in national.keys() {
        if d >= "1973-04-12" && weekday(d) == 0 {
            let substitute = add_days(d, 1);
            if year >= 2007 {
                while h.contains(substitute) { substitute = add_days(substitute, 1); }
            }
            if !h.contains(substitute) { h[substitute] = "休日"; }
        }
    }
    h
}