ICU-22991 Clean up gregorian calculation

unicode-org · Jan 8, 2025 · e700e4a · e700e4a
1 parent 4d6f08d
commit e700e4a
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Showing 2 changed files with 29 additions and 30 deletions.
diff --git a/icu4c/source/i18n/gregoimp.cpp b/icu4c/source/i18n/gregoimp.cpp
@@ -119,49 +119,35 @@ int64_t Grego::fieldsToDay(int32_t year, int32_t month, int32_t dom) {
 
 void Grego::dayToFields(int32_t day, int32_t& year, int8_t& month,
                         int8_t& dom, int8_t& dow, int16_t& doy, UErrorCode& status) {
+    year = dayToYear(day, doy, status);
+    UBool isLeap = isLeapYear(year);
 
-    if (U_FAILURE(status)) return;
     // Convert from 1970 CE epoch to 1 CE epoch (Gregorian calendar)
     if (uprv_add32_overflow(day, JULIAN_1970_CE - JULIAN_1_CE, &day)) {
         status = U_ILLEGAL_ARGUMENT_ERROR;
         return;
     }
 
-    // Convert from the day number to the multiple radix
-    // representation.  We use 400-year, 100-year, and 4-year cycles.
-    // For example, the 4-year cycle has 4 years + 1 leap day; giving
-    // 1461 == 365*4 + 1 days.
-    int32_t doy32;
-    int32_t n400 = ClockMath::floorDivide(day, 146097, &doy32); // 400-year cycle length
-    int32_t n100 = ClockMath::floorDivide(doy32, 36524, &doy32); // 100-year cycle length
-    int32_t n4   = ClockMath::floorDivide(doy32, 1461, &doy32); // 4-year cycle length
-    int32_t n1   = ClockMath::floorDivide(doy32, 365, &doy32);
-    year = 400*n400 + 100*n100 + 4*n4 + n1;
-    if (n100 == 4 || n1 == 4) {
-        doy = 365; // Dec 31 at end of 4- or 400-year cycle
-    } else {
-        doy = doy32;
-        ++year;
-    }
-
-    UBool isLeap = isLeapYear(year);
-
     // Gregorian day zero is a Monday.
     dow = (day + 1) % 7;
     dow += (dow < 0) ? (UCAL_SUNDAY + 7) : UCAL_SUNDAY;
 
     // Common Julian/Gregorian calculation
     int32_t correction = 0;
     int32_t march1 = isLeap ? 60 : 59; // zero-based DOY for March 1
-    if (doy >= march1) {
+    if (doy > march1) {
         correction = isLeap ? 1 : 2;
     }
-    month = (12 * (doy + correction) + 6) / 367; // zero-based month
-    dom = doy - DAYS_BEFORE[month + (isLeap ? 12 : 0)] + 1; // one-based DOM
-    doy++; // one-based doy
+    month = (12 * (doy - 1 + correction) + 6) / 367; // zero-based month
+    dom = doy - DAYS_BEFORE[month + (isLeap ? 12 : 0)]; // one-based DOM
 }
 
 int32_t Grego::dayToYear(int32_t day, UErrorCode& status) {
+    int16_t unusedDOY;
+    return dayToYear(day, unusedDOY, status);
+}
+
+int32_t Grego::dayToYear(int32_t day, int16_t& doy, UErrorCode& status) {
     if (U_FAILURE(status)) return 0;
     // Convert from 1970 CE epoch to 1 CE epoch (Gregorian calendar)
     if (uprv_add32_overflow(day, JULIAN_1970_CE - JULIAN_1_CE, &day)) {
@@ -173,17 +159,22 @@ int32_t Grego::dayToYear(int32_t day, UErrorCode& status) {
     // representation.  We use 400-year, 100-year, and 4-year cycles.
     // For example, the 4-year cycle has 4 years + 1 leap day; giving
     // 1461 == 365*4 + 1 days.
-    int32_t doy;
-    int32_t n400 = ClockMath::floorDivide(day, 146097, &doy); // 400-year cycle length
-    int32_t n100 = ClockMath::floorDivide(doy, 36524, &doy); // 100-year cycle length
-    int32_t n4   = ClockMath::floorDivide(doy, 1461, &doy); // 4-year cycle length
-    int32_t n1   = ClockMath::floorDivide(doy, 365, &doy);
+    int32_t doy32;
+    int32_t n400 = ClockMath::floorDivide(day, 146097, &doy32); // 400-year cycle length
+    int32_t n100 = ClockMath::floorDivide(doy32, 36524, &doy32); // 100-year cycle length
+    int32_t n4   = ClockMath::floorDivide(doy32, 1461, &doy32); // 4-year cycle length
+    int32_t n1   = ClockMath::floorDivide(doy32, 365, &doy32);
     int32_t year = 400*n400 + 100*n100 + 4*n4 + n1;
-    if (!(n100 == 4 || n1 == 4)) {
+    if (n100 == 4 || n1 == 4) {
+        doy = 365; // Dec 31 at end of 4- or 400-year cycle
+    } else {
+        doy = doy32;
         ++year;
     }
+    doy++; // one-based doy
     return year;
 }
+
 void Grego::dayToFields(int32_t day, int32_t& year, int8_t& month,
                         int8_t& dom, int8_t& dow, UErrorCode& status) {
     int16_t unusedDOY;

diff --git a/icu4c/source/i18n/gregoimp.h b/icu4c/source/i18n/gregoimp.h
@@ -246,6 +246,14 @@ class Grego {
      * @return year.
      */
     static int32_t dayToYear(int32_t day, UErrorCode& status);
+    /**
+     * Convert a 1970-epoch day number to proleptic Gregorian year.
+     * @param day 1970-epoch day
+     * @param doy output parameter to receive day-of-year (1-based)
+     * @param status error code.
+     * @return year.
+     */
+    static int32_t dayToYear(int32_t day, int16_t& doy, UErrorCode& status);
 
     /**
      * Convert a 1970-epoch milliseconds to proleptic Gregorian year,