Add from weekdate functions.

src/libiso8601/calc.c

@@ -127,7 +127,7 @@ void iso8601_to_cal(int* year, int* month, int* day, const struct iso8601_date*
     mc = iso8601_isleap(*year) ? _days_in_month_leap : _days_in_month_common;
     *month = 1;
     while(ndays >= mc->days) {
-        ++*month;
+        *month += 1;
         ndays -= mc->days;
         ++mc;
     }
@@ -149,6 +149,97 @@ void iso8601_to_ord(int* year, int* oday, const struct iso8601_date* date)
 
 
 
+static int _weekday_of_year(int year)
+{
+    int w = 6;
+
+    /* Algorithm notes:
+     *  - 0 = sun, 1 = mon, ..., 6 = sat
+     *  - 0000-001 is a Saturday, day 6
+     *  - every year we pass gives us one additional day (364 is divisible by 7)
+     *  - but of course every leap year we pass gives us a further day
+     *  - so for every 400 years, we add 497 (400 common years, 97 leap years); 497 % 7 = 0
+     */
+    year %= 400;
+    if(year < 0) year += 400; // end up with between 0-399 years left
+    w += year; // excluding leap years, we increase by 1 day a year
+    w += (year + 3) / 4; // there is one leap year for every four years
+    w -= (year - 1) / 100; // but one less for every century over 0
+    w %= 7;
+
+    return w;
+}
+
+
+
+void iso8601_to_week(int* year, int* week, int* wday, const struct iso8601_date* date)
+{
+    int ndays, w, has53 = 0;
+    div_t d;
+
+    // perform year and weekday calculation
+    _to_year(year, &ndays, date);
+    w = _weekday_of_year(*year);
+
+    // find out what day jan 1 was; from there, we can find the ISO week and year number
+    switch(w) {
+    case 4: // W01 starts XXXY-12-28
+        w += 7;
+        has53 = 1; // years starting Thursday have 53 weeks
+        break;
+
+    case 5: // W01 starts XXXZ-01-03
+    case 6: // W01 starts XXXZ-01-02
+        break;
+
+    case 0: // W01 starts XXXZ-01-01
+    case 1: // W01 starts XXXY-12-31
+    case 2: // W01 starts XXXY-12-30
+        w += 7; // for week calculation
+        break;
+
+    case 3: // W01 starts XXXY-12-29
+        w += 7;
+        if(iso8601_isleap(*year)) has53 = 1; // leap years starting Wednesday have 53 weeks
+    }
+
+    // now we simply add the number of days elapsed since the start of the year, and % 7
+    w += ndays;
+    d = div(w, 7); // w can never be 0
+
+    // do Sunday correction
+    if(!d.rem) {
+        d.rem = 7;
+        --d.quot;
+    }
+
+    *wday = d.rem;
+    if(d.quot) {
+        if(d.quot == 53 && !has53) {
+            d.quot = 1;
+            *year += 1;
+        }
+        *week = d.quot;
+    } else {
+        *year -= 1;
+        switch(_weekday_of_year(*year)) {
+        case 3:
+            *week = iso8601_isleap(*year) ? 53 : 52;
+            break;
+
+        case 4:
+            *week = 53;
+            break;
+
+        default:
+            *week = 52;
+            break;
+        }
+    }
+}
+
+
+
 int _from_year(struct iso8601_date* date, int year)
 {
     div_t qr;

src/libiso8601/functions.h

@@ -20,6 +20,8 @@ void iso8601_to_time_t(time_t* t, const struct iso8601_date* date);
 int iso8601_isleap(int year);
 void iso8601_to_cal(int* year, int* month, int* day, const struct iso8601_date* date);
 int iso8601_from_cal(struct iso8601_date* date, int year, int month, int day);
+void iso8601_to_ord(int* year, int* oday, const struct iso8601_date* date);
+void iso8601_to_week(int* year, int* week, int* wday, const struct iso8601_date* date);
 
 /* print.c */
 void iso8601_print(char* str, int amt, const struct iso8601_date* date, 

src/libiso8601/print.c

@@ -19,7 +19,7 @@ static const struct iso8601_details _default_details = {
 void iso8601_print(char* str, int amt, const struct iso8601_date* date, 
     const struct iso8601_details* details)
 {
-    int y, m, d, ret, extended;
+    int y, m, d, ret = 0, extended;
     struct iso8601_date dttz;
     double frac;
 
@@ -41,13 +41,11 @@ void iso8601_print(char* str, int amt, const struct iso8601_date* date,
         return;
 
     case iso8601_prec_month:
-        iso8601_to_cal(&y, &m, &d, &dttz);
         if(y < 0) snprintf(str, amt, "%05d-%02d", y, m);
         else snprintf(str, amt, extended ? "%04d-%02d" : "%04d%02d", y, m);
         return;
 
     case iso8601_prec_day:
-        iso8601_to_cal(&y, &m, &d, &dttz);
         if(y < 0) ret = snprintf(str, amt, "%05d-%02d-%02d", y, m, d);
         else ret = snprintf(str, amt, extended ? "%04d-%02d-%02d" : "%04d%02d%02d", y, m, d);
         break;
@@ -58,9 +56,19 @@ void iso8601_print(char* str, int amt, const struct iso8601_date* date,
         else ret = snprintf(str, amt, extended ? "%04d-%03d" : "%04d%03d", y, d);
         break;
 
-    default:
+    case iso8601_prec_week:
-        fprintf(stderr, "<<< NOT IMPLEMENTED >>>\n");
+        iso8601_to_week(&y, &m, &d, &dttz);
-        abort();
+        extended = y < 0 || y > 9999 || details->extended; // ISO year is different
+        if(y < 0) snprintf(str, amt, "%05d-W%02d", y, m);
+        else snprintf(str, amt, extended ? "%04d-W%02d" : "%04dW%02d", y, m);
+        return;
+
+    case iso8601_prec_wday:
+        iso8601_to_week(&y, &m, &d, &dttz);
+        extended = y < 0 || y > 9999 || details->extended; // ISO year is different
+        if(y < 0) ret = snprintf(str, amt, "%05d-W%02d-%d", y, m, d);
+        else ret = snprintf(str, amt, extended ? "%04d-W%02d-%d" : "%04dW%02d%d", y, m, d);
+        break;
     }
 
     if(ret < 1 || ret >= amt) return;

src/tests/calconv.c

@@ -123,6 +123,43 @@ int try_neg_year(int year)
 
 
 
+int try_weeks(void)
+{
+    // starting position
+    int weekcount = 52, weekday = 5, isoyear = -1;
+    int y, wk, wd, ret = 0;
+    struct iso8601_date dt;
+    
+    for(dt.day = 0; dt.day < MAX_YEAR_COUNT * 366; ++dt.day) {
+        iso8601_to_week(&y, &wk, &wd, &dt);
+        if(wd == 1) {
+            if(weekday != 7) goto discont;
+            ++weekcount;
+            if(wk == 1) {
+                if(weekcount != 53 && weekcount != 54) goto discont;
+                weekcount = 1;
+                ++isoyear;
+            }
+        }
+        if(wk != weekcount || y != isoyear) goto discont;
+
+        weekday = wd;
+        continue;
+
+    discont:
+        printf("[weekdiscont     ] %04d-W%02d-%d ==> %04d-W%02d-%d (%d)\n",
+            isoyear, weekcount, weekday, y, wk, wd, dt.day);
+        isoyear = y;
+        weekcount = wk;
+        weekday = wd;
+        if(++ret > 20) return ret;
+    }
+
+    return ret;
+}
+
+
+
 int main(int argc, char* argv[])
 {
     int ret = 0, year;
@@ -142,6 +179,9 @@ int main(int argc, char* argv[])
         if(ret > 20) return ret;
     }
 
+    ret += try_weeks();
+    if(ret > 20) return ret;
+
     return ret;
 }