Re: CRC in Ada?

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Larry says

<<As for a C implementation being better than an Ada implementation
at single character reads, I imagine there must be another language
characteristics where Ada has some advantage or other.>>

The comparison between C and Ada here is apples-oranges. Text_IO.Get
has far more elaborate semantics than getchar, so you cannot compare
the two. This is a common effect (see another example below). If you
want the simple minded semantics of getchar, then this is trivially
programmed in Ada (the GNAT package GNAT.IO is a much closer approximation
to the C level semantics).

Another interesting case of this is a timing comparison that Bob Klungle
sent me comparing the use of the official log and the log in the
"secret" package Aux. The log in Aux is a direct call to C, the log
in Ada has rather different semantics, as you can see from its coding:

   function Log (X : Float_Type'Base) return Float_Type'Base is
   begin
      if X < 0.0 then
         raise Argument_Error;

      elsif X = 0.0 then
         raise Constraint_Error;

      elsif X = 1.0 then
         return 0.0;
      end if;

      return Float_Type'Base (Aux.Log (Double (X)));
   end Log;

Those extra tests, required by Ada semantics, end up making the use of
the Ada log function very much slower (almost a factor of two in Bob's tests)
which is a bit worrisome -- safety and accuracy are not free :-)