Re: Can you help with fixed-point arithmetic?

[magnesium.club.cc.cmu.edu!news.sei.cmu.edu!ae@uunet.uu.net (Arthur Evans]

mfeldman@seas.gwu.edu (Michael Feldman) asked about why his compiler
stated that Constraint_Error would be raised by this statement:
       27    TimeThen := TimeNow + 10 * Duration'(86400.0);

In an earlier message I explained, and I then proposed another way to
accomplish his goal of adding (in effect) days to a value of type
Calendar.Time.  Here's another approach to the latter problem.

Declare a function like this:
	function "+"(Left: in Calendar.Time; Days: in integer)
	    return Calendar.Time;
In the body, call Calendar.Split on Left, and add Days to the DAY
result.  While the result exceeds 365 (or 366 in leap years), subtract
365 (or 366) and add one to the year.  This needs some care; details are
left as an exercise for the student (or, in this case, the professor).
Then put it all back together with Calendar.Time_Of.

If you want to add both days and partial days, give this "+" a third
parameter of type Duration.  Again, detect the possible overflow and
propagate a carry into DAY.  Be careful -- there are subtleties in
detecting the overflow.  Consider
	if SECONDS + Increment > 86400.0 then
Your compiler is not incorrect if it lets Constraint_Error be raised in
evaluating the sum.  It's also not incorrect if it does the comparison
correctly, dealing with an intermediate result (the sum) outside the
range of type Duration.  In the worst case you may have to just do the
sum and detect the Constraint_Error.

If you put this function in a package My_Calendar, it can re-export the
needed parts of package Calendar, so your application would mention only
My_Calendar in a 'with' clause.

Cheers!

Art Evans
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Arthur Evans, Jr, PhD           Ada Consultant
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