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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Saturated Math
References: <3ED7437E.5060607@noplace.com>
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Date: 2003-05-30T20:33:49+00:00
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>I don't really care if the computation itself is done with infinite
>precision.  What I care about is the *result* being limited to within
>the bounds of the actuator.  So what I need is saturation on assignment
  In that case, a Limit function seems more straightforward:
    subtype Color_Brightness is integer range 0 .. 255;
    function Limit(V : Integer) return Color_Brightness is
    begin
      if V < Color_Brightness'first then return Color_Brightness'first;
      elsif V > Color_Brightness'last then return Color_Brightness'last;
      else return V;
      end if;
    end Limit;
    X,Y,Z : Color_Brightness;
    ...
    X := Limit(X+Y-Z);
Granted it's a little extra writing, but with inlining and a compiler
that doesn't generate an unnecessary range check, it should be nearly
costless at run time.  It gets around non-associative arithmetic
questions.  If you forget the function call, ie, X :=X+Y-Z; then
you raise an exception instead of saturating, so at least you know
there's a problem.
  In MMX the saturating arithmetic is tied up with SIMD vector operations,
so the hardware doesn't help for simple variables.