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Date: Sat, 28 Aug 2010 18:45:16 -0400
From: "Peter C. Chapin" <chapinp@acm.org>
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Subject: Another question about fixed point types.
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Date: 2010-08-28T18:45:16-04:00
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Hello!

Consider this type definition:

   type Julian_Day is delta 0.001 range 2_415_385.5 .. 2_489_665.0;

Now consider this function that is intended to return the (approximate)
interval between two Julian days in seconds:

   function Interval_Between
      (Left : Julian_Day; Right : Julian_Day) return Duration is
      Result : Duration;
   begin
      if Left > Right then
         Result := 86_400.0 * (Left - Right);
      else
         Result := 86_400.0 * (Right - Left);
      end if;
      return Result;
   end Interval_Between;

The difference 'Left - Right' is definitely not in the range of
Julian_Day. It is likely to be a small value and could even be zero. I
understand that the computation of 'Left - Right' will be done in
Julian_Day's base type but I'm unclear what that means to me.

It is my understanding that the compiler is allowed to choose a base
type that can only deal with the small-ish range of values between the
limits of Julian_Day. That range does not include zero. However, the
code above compiles and runs fine using GNAT GPL 2010. My test program
includes a case where Left and Right are the same so the difference is
zero. Is this code reliable or am I just seeing the effect of GNAT's
particular, implementation-specific choices?

Peter

P.S. The code above is a simplification of the real code, but I believe
it illustrates the question I have.