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.

diff --git a/src/time/strftime.c b/src/time/strftime.c
index b69a83a4aeec0db37dbc17dc5fa42a2ad4b86df3..ab1c6dcf52029de18a5cb77e08d35d4bef1a03f0 100644 (file)
--- a/src/time/strftime.c
+++ b/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";