Re: CRC in Ada?

[Michael & Amy Hartsough <Hartsough@worldnet.att.net>]

Robert Dewar wrote:
> 
> <<It took my Ada CRC32 18 minutes to calculate the checksum for a 65K file! It too
> k
> the C++ CRC32 27 seconds to checksum the same file! Timings were performed using
> the stopwatch function on my Timex. ;^) Why the difference? File I/O. Turns out
> all but about 8-9 seconds of those 18 minutes were spent reading from the hard d
> isk!>>
> 
> It would be interesting to see how you coded this up, surely you did not
> use Text_IO for the I/O (as I say, I have almost never seen people use

I instantiated Sequential I/O with an unsigned 8-bit type.

> One other thing here is that I generally find that when people recode a
> piece of code written in language X into language Y, they often do a much
> worse job than if they wrote a new program from scratch in Y. It is much
> too easy to copy inappropriate approaches and design from the other
> implementation. It would be an interesting experiment to code up something
> like the CRC efficiently in Ada, which of course can be done, and then
> have it translated to C++. For example, if one used packed arrays in Ada,
> you can often get some very efficient code that does not map nicely into
> C++ at all. The proper approach for recoding this in C++ is to redesign
> the data structures, but it would be a path of lesser resistance to emulate
> the packed stuff inefficiently in C.

In this particular case I don't think you'd see a great deal of difference.
There's some anding and xoring, a table look up and the 8-bit shift right and
that's about it (the code's at work, I'm at home now). So you're basically
about as close to machine code as you can get with a HOL.

If you don't include the file access time, the Ada and C++ execution times were
nearly identical.

Later,
	Michael