add support for repeating entries on the last ???day of the month,
as I have such meetings... eg.

	the last friday, 11am, Boardroom - Departmental catchup

-Steve

Notes:
looks good to me.  an additional patch to use tolowerrune (see isalpharune(2))
also looks warrented.

- erik

Reference: /n/atom/patch/applied/calendar-last
Date: Tue Feb  4 17:10:44 CET 2014
Signed-off-by: steve@quintile.net
Reviewed-by: quanstro

--- /sys/src/cmd/calendar.c	Tue Feb  4 17:06:49 2014
+++ /sys/src/cmd/calendar.c	Tue Feb  4 17:06:52 2014
@@ -137,6 +137,26 @@
 	"saturday"
 };
 
+static	int	dmsize[] =	/* days per month */
+{
+	31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
+};
+static	int	ldmsize[] =	/* leap-year days per month */
+{
+	31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
+};
+
+int
+monthlen(Tm *tm)
+{
+	int y = tm->year+1900;
+
+	if((y%4) == 0 && ((y%100) != 0 || (y%400) == 0))
+		return ldmsize[tm->mday];
+	else
+		return dmsize[tm->mday];
+}
+
 /*
  * Generate two Date structures.  First has month followed by day;
  * second has day followed by month.  Link them into list after
@@ -189,6 +209,16 @@
 	nd->p = regcomp(buf);	
 	nd->next = Base;	
 	Base = nd;
+
+	if(tm->mday + 7 > monthlen(tm)){
+		snprint(buf, sizeof buf, "[\t ]*the[\t ]+last[\t ]+%s", day);
+		if(debug)
+			print("%s\n", buf);
+		nd = emalloc(sizeof(Date));
+		nd->p = regcomp(buf);	
+		nd->next = Base;	
+		Base = nd;
+	}
 
 	snprint(buf, sizeof buf, "^[\t ]*every[ \t]+%s", day);
 	if(debug)