[m-rev.] Re: for review: new calendar standard library module

[Ian MacLarty]

On Mon, Feb 02, 2009 at 01:26:02PM +1100, Ian MacLarty wrote:
> Here is a new version of the calendar module for review.
> 

After a chat with Julien I've committed this with the following
additional interdiff:

diff -u library/calendar.m library/calendar.m
--- library/calendar.m	2 Feb 2009 03:01:07 -0000
+++ library/calendar.m	4 Feb 2009 22:56:33 -0000
@@ -117,6 +117,14 @@
     %
 :- pred current_utc_time(date::out, io::di, io::uo) is det.
 
+    % Calculate the Julian day number for a date on the Gregorian calendar.
+    %
+:- func julian_day_number(date) = int.
+
+    % Returns 1970/01/01 00:00:00.
+    %
+:- func unix_epoch = date.
+
     % A period of time measured in years, months, days, hours, minutes,
     % seconds and microseconds.  Internally a duration is represented
     % using only months, days, seconds and microseconds components.
@@ -817,10 +825,8 @@
             Hour1 = DateB ^ dt_hour - !.Borrow,
             Hour2 = DateA ^ dt_hour,
             subtract_ints_with_borrow(24, Hour1, Hour2, Hours, !:Borrow),
-            JDN1 = julian_day(DateB ^ dt_year, DateB ^ dt_month,
-                DateB ^ dt_day),
-            JDN2 = julian_day(DateA ^ dt_year, DateA ^ dt_month,
-                DateA ^ dt_day),
+            JDN1 = julian_day_number(DateB),
+            JDN2 = julian_day_number(DateA),
             Days = JDN1 - !.Borrow - JDN2,
             Duration = init_duration(0, 0, Days, Hours, Minutes, Seconds,
                 MicroSeconds)
@@ -929,13 +935,11 @@
 %
 
 day_of_week(Date) = DayOfWeek :-
-    JDN = julian_day(Date ^ dt_year, Date ^ dt_month, Date ^ dt_day),
+    JDN = julian_day_number(Date),
     Mod = JDN mod 7,
     DayOfWeek = det_day_of_week_from_mod(Mod).
 
-:- func julian_day(int, int, int) = int.
-
-julian_day(Year, Month, Day) = JDN :-
+julian_day_number(date(Year, Month, Day, _, _, _, _)) = JDN :-
     A = (14 - Month) div 12,
     Y = Year + 4800 - A,
     M = Month + 12 * A - 3,
@@ -1087,6 +1091,8 @@
 day_of_week_num(saturday, 5).
 day_of_week_num(sunday, 6).
 
+unix_epoch = date(1970, 1, 1, 0, 0, 0, 0).
+
 %-----------------------------------------------------------------------------%
 :- end_module calendar.
 %-----------------------------------------------------------------------------%
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