Data commit

Task/Doomsday-rule/XPL0/doomsday-rule.xpl0

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+\Finds the day of the week of a date using John Conway's Doomsday rule.
+\Returns the day of the week (Sunday = 0, Monday = 1,...) for the date
+\ specified by CCYY, MM and DD.
+function DOW ( CCYY, MM, DD );
+integer CCYY, MM, DD;
+integer Doomsday, AnchorDay, LeapYear, Dooms;
+begin
+Doomsday  := rem(( \Tuesday \2
+                 + 5 * rem( CCYY/4 )
+                 + 4 * rem( CCYY/100 )
+                 + 6 * rem( CCYY/400 )
+                 ) / 7);
+LeapYear  := rem(CCYY/4) = 0 and ( rem(CCYY/100) # 0 or rem(CCYY/ 400) = 0 );
+Dooms     := [[4, 1, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5],
+              [3, 7, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5]];
+AnchorDay := Dooms(if LeapYear then 0 else 1, MM-1);
+return rem( ( Doomsday + ( DD - AnchorDay ) + 7 ) / 7);
+end; \DOW
+
+\Prints a test date and its day of the week
+procedure TestDOW ( CCYY, MM, DD );
+integer CCYY, MM, DD;
+integer DayName;
+begin
+DayName := ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday",
+            "Saturday"];
+IntOut(0, CCYY);
+ChOut(0, ^-);  IntOut(0, MM/10);  IntOut(0, rem(0));
+ChOut(0, ^-);  IntOut(0, DD/10);  IntOut(0, rem(0));
+Text(0, ": "); Text(0, DayName( DOW( CCYY, MM, DD ) ));
+CrLf(0);
+end;
+
+begin   \task test cases
+TestDOW( 1800,  1,  6 );
+TestDOW( 1875,  3, 29 );
+TestDOW( 1915, 12,  7 );
+TestDOW( 1970, 12, 23 );
+TestDOW( 2043,  5, 14 );
+TestDOW( 2077,  2, 12 );
+TestDOW( 2101,  4,  2 );
+TestDOW( 2022,  6, 19 )
+end