implement week numbers and half of the week-based-year logic for strftime

output for plain week numbers (%U and %W) has been sanity-checked, and
output for the week-based-year week numbers (%V) has been checked
extensively against known-good data for the full non-negative range of
32-bit time_t.

year numbers for week-based years (%g and %G) are not yet implemented.

src/time/strftime.c

@@ -8,6 +8,14 @@
 
 const char *__langinfo(nl_item);
 
+static int is_leap(int y)
+{
+	/* Avoid overflow */
+	if (y>INT_MAX-1900) y -= 2000;
+	y += 1900;
+	return !(y%4) && ((y%100) || !(y%400));
+}
+
 size_t strftime(char *restrict s, size_t n, const char *restrict f, const struct tm *restrict tm)
 {
 	nl_item item;
@@ -116,10 +124,37 @@ do_fmt:
 			fmt = "%d";
 			goto number;
 		case 'U':
-		case 'V':
+			val = (tm->tm_yday + 7 - tm->tm_wday) / 7;
+			fmt = "%02d";
+			goto number;
 		case 'W':
-			// FIXME: week number mess..
-			continue;
+			val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
+			fmt = "%02d";
+			goto number;
+		case 'V':
+			val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
+			/* If 1 Jan is just 1-3 days past Monday,
+			 * the previous week is also in this year. */
+			if ((tm->tm_wday - tm->tm_yday - 2 + 371) % 7 <= 2)
+				val++;
+			if (!val) {
+				val = 52;
+				/* If 31 December of prev year a Thursday,
+				 * or Friday of a leap year, then the
+				 * prev year has 53 weeks. */
+				int dec31 = (tm->tm_wday - tm->tm_yday - 1 + 7) % 7;
+				if (dec31 == 4 || (dec31 == 5 && is_leap(tm->tm_year%400-1)))
+					val++;
+			} else if (val == 53) {
+				/* If 1 January is not a Thursday, and not
+				 * a Wednesday of a leap year, then this
+				 * year has only 52 weeks. */
+				int jan1 = (tm->tm_wday - tm->tm_yday + 371) % 7;
+				if (jan1 != 4 && (jan1 != 3 || !is_leap(tm->tm_year)))
+					val = 1;
+			}
+			fmt = "%02d";
+			goto number;
 		case 'w':
 			val = tm->tm_wday;
 			fmt = "%d";