[GAP Forum] How to write a compressed file

[Raphael C]

Thank you very much. I have managed to get the IO package to work
using xz compression.

Best wishes,
Raphael

On Wed, 15 Apr 2020 at 12:38, Frank Lübeck
<frank.luebeck at math.rwth-aachen.de> wrote:
>
> 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/