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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: Time_Span divide question
Date: 1997/05/16
Message-ID: <337C5D12.6230@gsfc.nasa.gov>#1/1
X-Deja-AN: 241905751
References: <5lg555$rba3@castor.cca.rockwell.com> <337BD41B.2FE1@mail.us.net>
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Reply-To: Stephen.Leake@gsfc.nasa.gov
Newsgroups: comp.lang.ada
Date: 1997-05-16T00:00:00+00:00
List-Id: <comp.lang.ada>


Mark D. McKinney wrote:
> 
> >  D1 := 0.495;
> >  D2 := 0.500;
> >  Put_Line ("Duration divide result: " & Integer'Image(Integer(D1/D2)));
>    D1 / D2 = 0.990
>    Integer(D1/D2) = 1
> >
> >  TS1 := To_Time_Span (D1);
> >  TS2 := To_Time_Span (D2);
> >  Put_Line ("Time_Span divide result: " & Integer'Image(TS1/TS2));
> 
> To_Time_Span(D1) = Integer(D1) = 0
> To_Time_Span(D2) = Integer(D2) = 1

Um, no. Time_Span is an integer type, but it is scaled, so that 1 is one
Time_Unit, which is a small fraction of seconds. In GNAT, Time_Unit is
one nanosecond. So To_Time_Span (foo) is NOT the same as Integer (foo).

> TS1 / TS2 = 0
> 
> Yes they should be different.
> 
> Careful where rounding occurs.

Here's a compilable test:

with Text_IO; use Text_IO;
with Ada.Real_Time; use Ada.Real_Time;
with Unchecked_Conversion;
with Interfaces;
procedure Test
is
   D1, D2 : Duration;
   TS1, TS2 : Time_Span;

   function To_Int_64 is new Unchecked_Conversion
      (Source => Time_Span,
       Target => Interfaces.Integer_64);
begin
   D1 := 0.495;
   D2 := 0.500;
   Put_Line ("Duration divide result: " &
Integer'Image(Integer(D1/D2)));

   TS1 := To_Time_Span (D1);
   TS2 := To_Time_Span (D2);
   Put_Line (" Int_64 (TS1) => " & Interfaces.Integer_64'Image
(To_Int_64 (TS1)));
   Put_Line (" Int_64 (TS2) => " & Interfaces.Integer_64'Image
(To_Int_64 (TS2)));

   Put_Line ("Time_Span divide result: " & Integer'Image(TS1/TS2));

end Test;

Note that your compiler may have a different representation for
Time_Span.

Running the code on GNAT 3.09, under Windows 95 on a Pentium, I get:

Duration divide result:  0
 Int_64 (TS1) =>  495000000
 Int_64 (TS2) =>  500000000
Time_Span divide result:  0

Integer division truncates, in order to satisfy the rules in RM 4.5.5 (1
.. 5). Fixed point division goes up or down; see RM 4.5.5(21)
-- 
- Stephe