Add from-week-date conversion functions

src/libiso8601/calc.c

@@ -16,6 +16,8 @@
  *  of the next day).
  */
 
+#define DAYS_IN_400_YEARS (146097)
+
 
 
 int iso8601_isleap(int year)
@@ -74,14 +76,14 @@ static void _to_year(int* year, int* days_left, const struct iso8601_date* date)
     div_t qr;
     int ndays = date->day;
 
-    // Each 400 years have 97 leap days, giving 365*400+97 = 146097 days in 400 years
+    // Each 400 years have 97 leap days, giving 365*400+97 = DAYS_IN_400_YEARS days in 400 years
-    qr = div(ndays, 146097);
+    qr = div(ndays, DAYS_IN_400_YEARS);
     *year = qr.quot * 400;
     ndays = qr.rem;
 
     // ensure that we always end up with between 0 and 146096 days remaining
     if(ndays < 0) {
-        ndays += 146097;
+        ndays += DAYS_IN_400_YEARS;
         *year -= 400;
     }
 
@@ -250,14 +252,14 @@ int _from_year(struct iso8601_date* date, int year)
         return -1;
     }
 
-    // Each 400 years have 97 leap days, giving 365*400+97 = 146097 days in 400 years
+    // Each 400 years have 97 leap days, giving 365*400+97 = DAYS_IN_400_YEARS days in 400 years
     qr = div(year, 400);
-    date->day = qr.quot * 146097;
+    date->day = qr.quot * DAYS_IN_400_YEARS;
     year = qr.rem;
 
     // ensure we have between 0 and 399 years
     if(year < 0) {
-        date->day -= 146097;
+        date->day -= DAYS_IN_400_YEARS;
         year += 400;
     }
 
@@ -316,6 +318,28 @@ int iso8601_from_ord(struct iso8601_date* date, int year, int oday)
 
 int iso8601_from_week(struct iso8601_date* date, int isoyear, int week, int wday)
 {
+    int day1off;
+
+    _from_year(date, isoyear);
+
+    // 400-year cycle; ensure we're between 0-400 years
+    isoyear %= 400;
+    if(isoyear < 0) isoyear += 400;
+
+    /* Algorithm notes:
+     *  We now compute the offset between the start day of the ISO year and the start day of the 
+     *  real year. Year 0000 starts on a Saturday, meaning the ISO year 0000 starts on 0000-003
+     *  (offset of +2 days). Each year reduces the offset by one day, and each leap year reduces it
+     *  by a further day; the offset wraps from -3 to +3.
+     */
+    day1off = 2 - isoyear; // reduce offset by 1 for each year
+    day1off += (isoyear - 1) / 100; // cancel out 1 leap year for each century past the first
+    day1off -= (isoyear + 3) / 4; // 1 leap year every 4 years
+    day1off %= 7;
+    if(day1off < -3) day1off += 7;
+
+    // now simply add in the day offset and days/weeks elapsed
+    date->day += day1off + (week - 1) * 7 + wday - 1;
 }
 
 

src/tests/calconv.c

@@ -128,7 +128,7 @@ int try_weeks(void)
     // starting position
     int weekcount = 52, weekday = 5, isoyear = -1;
     int y, wk, wd, ret = 0;
-    struct iso8601_date dt;
+    struct iso8601_date dt, dt2;
     
     for(dt.day = 0; dt.day < MAX_YEAR_COUNT * 366; ++dt.day) {
         iso8601_to_week(&y, &wk, &wd, &dt);
@@ -141,9 +141,17 @@ int try_weeks(void)
                 ++isoyear;
             }
         }
-        if(wk != weekcount || y != isoyear) goto discont;
 
+        if(wk != weekcount || y != isoyear) goto discont;
         weekday = wd;
+
+        iso8601_from_week(&dt2, isoyear, weekcount, weekday);
+        if(dt2.day != dt.day) {
+            printf("[weekmismatch    ] %04d-W%02d-%d: should be %d, got %d\n", 
+                isoyear, weekcount, weekday, dt.day, dt2.day);
+            if(++ret > 20) return ret;
+        }
+
         continue;
 
     discont: