Fix ISO weekday calculations in negative years

CHANGELOG.md

@@ -10,6 +10,7 @@
   - `icu_calendar`
     - New `DateTime::local_unix_epoch()` convenience constructor (https://github.com/unicode-org/icu4x/pull/4479)
     - Improved approximation for Persian calendrical calculations (https://github.com/unicode-org/icu4x/issues/4713)
+    - Fix weekday calculations in negative ISO years (https://github.com/unicode-org/icu4x/pull/4894)
   - `icu_datetime`
     - `FormattedDateTime` and `FormattedZonedDateTime` now implement `Clone` and `Copy` (https://github.com/unicode-org/icu4x/pull/4476)
   - `icu_locid`

components/calendar/src/hebrew.rs

@@ -470,7 +470,7 @@ impl<A: AsCalendar<Calendar = Hebrew>> DateTime<A> {
 mod tests {
 
     use super::*;
-    use crate::types::MonthCode;
+    use crate::types::{Era, MonthCode};
     use calendrical_calculations::hebrew_keviyah::*;
 
     // Sentinel value for Adar I
@@ -587,4 +587,17 @@ mod tests {
         let yi = YearInfo::compute_for(88369);
         assert_eq!(yi.keviyah.year_length(), 383);
     }
+
+    #[test]
+    fn test_weekdays() {
+        // https://github.com/unicode-org/icu4x/issues/4893
+        let cal = Hebrew::new();
+        let era = "am".parse::<Era>().unwrap();
+        let month_code = "M01".parse::<MonthCode>().unwrap();
+        let dt = cal.date_from_codes(era, 3760, month_code, 1).unwrap();
+
+        // Should be Saturday per:
+        // https://www.hebcal.com/converter?hd=1&hm=Tishrei&hy=3760&h2g=1
+        assert_eq!(6, cal.day_of_week(&dt) as usize);
+    }
 }

components/calendar/src/iso.rs

@@ -135,7 +135,9 @@ impl Calendar for Iso {
 
         // The days of the week are the same every 400 years
         // so we normalize to the nearest multiple of 400
-        let years_since_400 = date.0.year % 400;
+        let years_since_400 = date.0.year.rem_euclid(400);
+        debug_assert!(years_since_400 >= 0); // rem_euclid returns positive numbers
+        let years_since_400 = years_since_400 as u32;
         let leap_years_since_400 = years_since_400 / 4 - years_since_400 / 100;
         // The number of days to the current year
         // Can never cause an overflow because years_since_400 has a maximum value of 399.
@@ -169,7 +171,7 @@ impl Calendar for Iso {
             }
         };
         let january_1_2000 = 5; // Saturday
-        let day_offset = (january_1_2000 + year_offset + month_offset + date.0.day as i32) % 7;
+        let day_offset = (january_1_2000 + year_offset + month_offset + date.0.day as u32) % 7;
 
         // We calculated in a zero-indexed fashion, but ISO specifies one-indexed
         types::IsoWeekday::from((day_offset + 1) as usize)
@@ -832,4 +834,49 @@ mod test {
         check(-1439, 1969, 12, 31, 0, 1);
         check(-2879, 1969, 12, 30, 0, 1);
     }
+
+    #[test]
+    fn test_continuity_near_year_zero() {
+        // https://github.com/unicode-org/icu4x/issues/4893
+        const ONE_DAY_DURATION: DateDuration<Iso> = DateDuration {
+            years: 0,
+            months: 0,
+            weeks: 0,
+            days: 1,
+            marker: core::marker::PhantomData,
+        };
+
+        let mut iso_date = Date::try_new_iso_date(-10, 1, 1).unwrap();
+        let mut rata_die = iso_date.to_fixed();
+        let mut weekday = iso_date.day_of_week();
+
+        for _ in 0..(366 * 20) {
+            let next_iso_date = iso_date.added(ONE_DAY_DURATION);
+            let next_rata_die = next_iso_date.to_fixed();
+            assert_eq!(
+                next_rata_die,
+                rata_die + 1,
+                "{iso_date:?}..{next_iso_date:?}"
+            );
+            let next_weekday = next_iso_date.day_of_week();
+            assert_eq!(
+                (next_weekday as usize) % 7,
+                (weekday as usize + 1) % 7,
+                "{iso_date:?}..{next_iso_date:?}"
+            );
+            if iso_date.month().code.parsed().unwrap().0 == 2 && iso_date.day_of_month().0 == 28 {
+                assert_eq!(next_iso_date.is_in_leap_year(), iso_date.is_in_leap_year());
+                if iso_date.is_in_leap_year() {
+                    assert_eq!(next_iso_date.month().code.parsed().unwrap().0, 2);
+                    assert_eq!(next_iso_date.day_of_month().0, 29);
+                } else {
+                    assert_eq!(next_iso_date.month().code.parsed().unwrap().0, 3);
+                    assert_eq!(next_iso_date.day_of_month().0, 1);
+                }
+            }
+            iso_date = next_iso_date;
+            rata_die = next_rata_die;
+            weekday = next_weekday;
+        }
+    }
 }