Deal with strptime() on OS X and *BSD (fix #1415)

strptime() on OS X and *BSDs (reputedly) does not set tm_wday and
tm_yday unless corresponding %U and %j format specifiers were used.
That can be... surprising when one parsed year, month, and day anyways.
Glibc's strptime() conveniently sets tm_wday and tm_yday in those cases,
but OS X's does not, ignoring them completely.

This commit makes jq compute those where possible, though the day of
week computation may be wrong for dates before 1900-03-01 or after
2099-12-31.

src/builtin.c

@@ -1221,6 +1221,63 @@ static time_t my_mktime(struct tm *tm) {
 #endif
 }
 
+/* Compute and set tm_wday */
+static void set_tm_wday(struct tm *tm) {
+  /*
+   * https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week#Gauss.27s_algorithm
+   * https://cs.uwaterloo.ca/~alopez-o/math-faq/node73.html
+   *
+   * Tested with dates from 1900-01-01 through 2100-01-01.  This
+   * algorithm produces the wrong day-of-the-week number for dates in
+   * the range 1900-01-01..1900-02-28, and for 2100-01-01..2100-02-28.
+   * Since this is only needed on OS X and *BSD, we might just document
+   * this.
+   */
+  int century = (1900 + tm->tm_year) / 100;
+  int year = (1900 + tm->tm_year) % 100;
+  if (tm->tm_mon < 2)
+    year--;
+  /*
+   * The month value in the wday computation below is shifted so that
+   * March is 1, April is 2, .., January is 11, and February is 12.
+   */
+  int mon = tm->tm_mon - 1;
+  if (mon < 1)
+    mon += 12;
+  int wday =
+    (tm->tm_mday + (int)floor((2.6 * mon - 0.2)) + year + (int)floor(year / 4.0) + (int)floor(century / 4.0) - 2 * century) % 7;
+  if (wday < 0)
+    wday += 7;
+#if 0
+  /* See commentary above */
+  assert(wday == tm->tm_wday || tm->tm_wday == 8);
+#endif
+  tm->tm_wday = wday;
+}
+/*
+ * Compute and set tm_yday.
+ *
+ */
+static void set_tm_yday(struct tm *tm) {
+  static const int d[] = {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334};
+  int mon = tm->tm_mon;
+  int year = 1900 + tm->tm_year;
+  int leap_day = 0;
+  if (tm->tm_mon > 1 &&
+      ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)))
+    leap_day = 1;
+
+  /* Bound check index into d[] */
+  if (mon < 0)
+    mon = -mon;
+  if (mon > 11)
+    mon %= 12;
+
+  int yday = d[mon] + leap_day + tm->tm_mday - 1;
+  assert(yday == tm->tm_yday || tm->tm_yday == 367);
+  tm->tm_yday = yday;
+}
+
 #ifdef HAVE_STRPTIME
 static jv f_strptime(jq_state *jq, jv a, jv b) {
   if (jv_get_kind(a) != JV_KIND_STRING || jv_get_kind(b) != JV_KIND_STRING)
@@ -1241,10 +1298,19 @@ static jv f_strptime(jq_state *jq, jv a, jv b) {
     return e;
   }
   jv_free(b);
-  if ((tm.tm_wday == 8 || tm.tm_yday == 367) && my_timegm(&tm) == (time_t)-2) {
-    jv_free(a);
-    return jv_invalid_with_msg(jv_string("strptime/1 not supported on this platform"));
-  }
+  /*
+   * This is OS X or some *BSD whose strptime() is just not that
+   * helpful!
+   *
+   * We don't know that the format string did involve parsing a
+   * year, or a month (if tm->tm_mon == 0).  But with our invalid
+   * day-of-week and day-of-year sentinel checks above, the worst
+   * this can do is produce garbage.
+   */
+  if (tm.tm_wday == 8 && tm.tm_mday != 0 && tm.tm_mon >= 0 && tm.tm_mon <= 11)
+    set_tm_wday(&tm);
+  if (tm.tm_yday == 367 && tm.tm_mday != 0 && tm.tm_mon >= 0 && tm.tm_mon <= 11)
+    set_tm_yday(&tm);
   jv r = tm2jv(&tm);
   if (*end != '\0')
     r = jv_array_append(r, jv_string(end));