Properly compress files which are already partially compressed

[marcospb19]

There is currently no way to compress an file.tar into file.tar.gz by only adding the .gz part, ouch will detect two extensions, tar and gz, repeating the .tar process for that file that was already in that format.

So basically, ouch compress file.tar.gz file.tar.gz.xz generates a file of format .tar.gz.tar.gz.xz, instead of .tar.gz.xz.

[marcospb19]

I have already thought about a solution for this, here is some pseudocode:

Assume ouch was called like this:

ouch compress file.tar.gz file.tar.gz.xz

We correctly check if it's compressing only one file, as this check additional check does not make sense when creating archives.

Then we can run the function to detect the extensions of both files.

input : [Tar, Gz]
output: [Tar, Gz, Xz]

Now we check if the input extensions are a sublist of the output extensions (and at the start).

let input_extensions_are_included_in_output_extensions =
    input_extensions.len() < output_extensions.len()
        && input_extensions.iter().zip(&output_extensions).all(|(a, b)| a == b);

In this case, instead of passing the output_extensions normally to the compress function, we drop the first N elements.

// Remove the first N elements efficiently
let drain_iter = output_extensions.drain(..input_extensions.len());
drop(drain_iter); // [Tar, Gz, Xz] -> [Xz]

Now use the output_extensions and only compress file with the remaning Xz.

[marcospb19]

Note: this should not care if the filenames are different.

file.tar -> output.tar.gz still should skip the .tar compression part.