[GAP Forum] How to write a compressed file

[Frank Lübeck]

On Tue, Apr 14, 2020 at 08:23:51PM +0100, Raphael C wrote:
> I am making a large DFA and would like to compress it before writing
> it to a file. I tried the following:
> 
>     dfa := RemovedSinkStates(NFAtoDFA(nfa));
>     output := OutputTextFile("test.gz", false);
>     AppendTo(output, dfa);
>     CloseStream(output);
> 
> But the file test.gz is not compressed.
> 
> I was attempting to follow the instructions from
> https://www.gap-system.org/Manuals/doc/ref/chap10.html .
> 
> How should this be done?
> 
> Raphael

Dear Raphael, dear Forum,

Indeed, the documentation (or implementation) of InputTextFile and
OutputTextFile seems wrong. Either the documentation or the implementation
should be fixed.

For writing data into compressed files I usually use the IO package, see

?IO: Using filters when reading or writing files sequentially

in the GAP help system. This can also be used for other types of filter
programs, not just gzip. 

I'm also sometimes reading GAP code (usually large amounts of data) from
compressed files. This works as follows.

The command 

   Read("guck.g");

just reads the file as GAP code if it exists. If the file does not exist,
then GAP checks if there is a file "guck.g.gz". If yes, GAP transparently
decompresses the file and reads the result. If not, an error is raised.

The same behaviour can be observed with
   
   inp := InputTextFile("guck.g");
   Read(inp);

I cannot find a documentation of this behaviour. It should probably be
added.

Best regards,
   Frank

-- 
///  Dr. Frank Lübeck, Lehrstuhl D für Mathematik, Pontdriesch 14/16,
\\\                    52062 Aachen, Germany
///  E-mail: Frank.Luebeck at Math.RWTH-Aachen.De
\\\  WWW:    http://www.math.rwth-aachen.de/~Frank.Luebeck/