A file fetcher idea


An idea to generalize access to data that is either on file or in a compressed archive.

A friend and I are addressing a small programming task and it’s still not entirely clear where some data will come from. I mean, they might be in a plain file in the filesystem, or in a ZIP file.

The simplest thing would be to extract the ZIP file in the filesystem and go back to square one, this time with a plain file in the filesystem. But I was not like doing this, because I wanted the compressed stuff to stay compresses for as much as we can.

So I came out with this:

sub file_fetcher (%config) {
   return sub ($type, $filename = undef) {
      return $config{$type} if defined $config{$type};
      state $zh = do {
         die "no zip file\n" unless -r $config{zip};
         my $tmp = Archive::Zip->new;
         die "cannot read zip file '$config{zip}'\n"
            if $tmp->read("$config{zip}") != AZ_OK;
      my $rx = !defined $filename ? qr{(?imxs: . \Q$type\E \z)}
         : !ref $filename ? qr{(?mxs: \A \Q$filename\E \z)}
         :                  $filename;
      for my $member ($zh->members) {
         my $member_name = $member->{fileName} =~ s{\A.*[\\/]}{}rmxs;
         next unless $member_name =~ m{$rx};
         my $stuff = $member->contents;
         return \$stuff;
      die "cannot retrieve requested file\n";

The file_fetcher is a factory function that returns a sub reference that can be later used to retrieve the file’s contents. It takes a %config hash where there should be pairs with keys associated to the specific type of file we want to read, and the value is the path to that file in the filesystem. One of them is zip, pointing to the ZIP archive with the other files.

The returned sub takes a file $type and an optional $filename to do its magic.

If there is indeed that $type in the general %config hash, then it is used. This allows using files that are inside the filesystem, passed directly in the hash.

Otherwise, the ZIP file is searched. Here I’m probably being a bit crude - all file names of members of the ZIP file are stripped of everything and we’re keeping the base name only (in Unix, most probably).

The specification of what to find depends on the optional $filename parameter and, again, $type. The gist of it is that presence of a $filename triggers its use, otherwise the $type is used as a fallback to find the file by extension.

To use it we can do:

my %config = (
    zip => '/path/to/zip',  # contains bar.txt
    foo => '/some/galook.foo'.
my $fetcher = file_fetcher(%config);
my $foo = $fetcher->('foo');            # from filesystem
my $bar = $fetcher->(bar => 'bar.txt'); # from the ZIP archive

What do we get back? Something that *can be opened`, i.e. either a file name or a reference to a scalar. Which, by the way, can also be used straight away.

I’m still not sure it’s the right way to do it… but at least we are covered independently of whether the specific files appears in the filesystem or in an archive.

Thoughts? In the meantime… stay safe!

Comments? Octodon, , GitHub, Reddit, or drop me a line!