Compute LunarChinese reference years with new algorithm (#7007)

Author: robertbastian

components/calendar/src/cal/chinese.rs

@@ -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,
@@ -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,
             (9, true, true) => -5738,
             (10, false, false) => 1972,
             (10, false, true) => 1972,
-            (10, true, false) => 1870,
+            (10, true, false) => 1984,
             (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,
@@ -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,
@@ -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,
@@ -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))
@@ -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!(),")
+                }
+            }
+        }
+    }
 }