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Sep 30, 2025
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Compute LunarChinese reference years with new algorithm #7007

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reference algorithm
robertbastian Sep 30, 2025
ca3b6df
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robertbastian Sep 30, 2025
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109 changes: 83 additions & 26 deletions components/calendar/src/cal/chinese.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -176,8 +176,7 @@ impl Rules for China {
let Some((number, is_leap)) = month_code.parsed() else {
return Err(DateError::UnknownMonthCode(month_code));
};
// Computed via code from
// <https://github.com/unicode-org/icu4x/pull/6910#issuecomment-3303786919>
// Computed by `generate_reference_years`
let extended = match (number, is_leap, day > 29) {
(1, false, false) => 1972,
(1, false, true) => 1970,
Expand Down Expand Up @@ -213,21 +212,17 @@ impl Rules for China {
(8, true, true) => 1718,
(9, false, false) => 1972,
(9, false, true) => 1972,
(9, true, false) => 1832,
(9, true, false) => 2014,
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confirmed: https://www.hko.gov.hk/en/gts/time/calendar/pdf/files/2014e.pdf

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I think in these cases because they were not super far in the past I continued with the backwards-looking algorithm, but the 1900 cutoff seems good.

Comment on lines -216 to +215
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Observation: I see a M09L (闰九月) in the year 2014 in the PMO file and it has 29 days

(9, true, true) => -5738,
(10, false, false) => 1972,
(10, false, true) => 1972,
(10, true, false) => 1870,
(10, true, false) => 1984,
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Observation: I do indeed see a M10L (闰十月) in the year 1984 in the PMO file and it has 29 days

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confirmed: https://www.hko.gov.hk/en/gts/time/calendar/pdf/files/1984e.pdf

(10, true, true) => -4098,
// Dec 31, 1972 is 1972-M11-26, dates after that
// are in the next year
(11, false, false) if day > 26 => 1971,
(11, false, false) => 1972,
(11, false, true) => 1969,
// Specced backwards-looking algorithm produces 1642, but
// 2033 is a better date with a forwards-looking algorithm.
// See <https://github.com/tc39/proposal-intl-era-monthcode/issues/60#issuecomment-3192982095>
// Data from: <https://github.com/unicode-org/icu4x/pull/6910#issuecomment-3303988559>
(11, true, false) => 2033,
(11, true, true) => -2173,
(12, false, false) => 1971,
Expand Down Expand Up @@ -342,13 +337,7 @@ impl Rules for Dangi {
let Some((number, is_leap)) = month_code.parsed() else {
return Err(DateError::UnknownMonthCode(month_code));
};
// Computed via code from
// <https://github.com/unicode-org/icu4x/pull/6910#issuecomment-3303786919>
//
// Some (marked) dates use a forward looking algorithm from
// <https://github.com/tc39/proposal-intl-era-monthcode/issues/60#issuecomment-3192982095>
// to produce a better in-range date.
// Code for that is <https://github.com/unicode-org/icu4x/pull/6910#issuecomment-3303988559>
// Computed by `generate_reference_years`
let extended = match (number, is_leap, day > 29) {
(1, false, false) => 1972,
(1, false, true) => 1970,
Expand All @@ -359,7 +348,7 @@ impl Rules for Dangi {
(2, true, false) => 1947,
(2, true, true) => 1765,
(3, false, false) => 1972,
(3, false, true) => 1968,
(3, false, true) => 1966,
(3, true, false) => 1966,
(3, true, true) => 1955,
(4, false, false) => 1972,
Expand All @@ -381,31 +370,26 @@ impl Rules for Dangi {
(8, false, false) => 1972,
(8, false, true) => 1971,
(8, true, false) => 1957,
// Uses forward-looking algorithm (was: 1718)
(8, true, true) => 2052,
(8, true, true) => 1718,
(9, false, false) => 1972,
(9, false, true) => 1972,
// Uses forward-looking algorithm (was: 1832)
(9, true, false) => 1972,
(9, true, false) => 2014,
(9, true, true) => -5738,
(10, false, false) => 1972,
(10, false, true) => 1972,
// Uses forward-looking algorithm (was: 1870)
(10, true, false) => 1984,
(10, true, true) => -3946,
(10, true, true) => -4098,
// Dec 31, 1972 is 1972-M11-26, dates after that
// are in the next year
(11, false, false) if day > 26 => 1971,
(11, false, false) => 1972,
(11, false, true) => 1969,
// Uses forward-looking algorithm (was: 1851)
(11, true, false) => 2033,
(11, true, true) => -2173,
(12, false, false) => 1971,
(12, false, true) => 1971,
(12, true, false) => 1889,
(12, true, true) => -1182,

(12, true, false) => 1403,
(12, true, true) => -180,
_ => return Err(DateError::UnknownMonthCode(month_code)),
};
Ok(self.year_data(extended))
Expand Down Expand Up @@ -2299,4 +2283,77 @@ mod test {
);
}
}

#[test]
#[ignore]
fn generate_reference_years() {
use crate::Date;
let calendar = crate::cal::LunarChinese::new_china();
let reference_year_end = Date::from_rata_die(
crate::cal::abstract_gregorian::LAST_DAY_OF_REFERENCE_YEAR,
calendar,
);
let year_1900_start = Date::try_new_gregorian(1900, 1, 1)
.unwrap()
.to_calendar(calendar);
let year_2035_end = Date::try_new_gregorian(2035, 12, 31)
.unwrap()
.to_calendar(calendar);
for month in 1..=12 {
for leap in [false, true] {
'outer: for long in [false, true] {
for (start_year, start_month, end_year, end_month, by) in [
(
reference_year_end.extended_year(),
reference_year_end.month().month_number(),
year_1900_start.extended_year(),
year_1900_start.month().month_number(),
-1,
),
(
reference_year_end.extended_year(),
reference_year_end.month().month_number(),
year_2035_end.extended_year(),
year_2035_end.month().month_number(),
1,
),
(
year_1900_start.extended_year(),
year_1900_start.month().month_number(),
-10000,
1,
-1,
),
] {
let mut year = start_year;
while year * by < end_year * by {
if year == start_year && month as i32 * by < start_month as i32 * by
|| year == end_year && month as i32 * by > end_month as i32 * by
{
year += by;
continue;
}
let data = China.year_data(year);
let leap_month = data.leap_month().unwrap_or(15);
let days_in_month = data.days_in_month({
if leap && month + 1 == leap_month {
month + 1
} else {
month + (month + 1 > leap_month) as u8
}
});
if (!long || (days_in_month == 30))
&& (!leap || month + 1 == leap_month)
{
println!("({month}, {leap:?}, {long:?}) => {year},");
continue 'outer;
}
year += by;
}
}
println!("({month}, {leap:?}, {long:?}) => todo!(),")
}
}
}
}
}