config/pkgs/compress-drv/default.nix

/*
  compressDrv compresses files in a given derivation.

  Inputs:

  - formats :: [String]

      List of file extensions to compress.

      Example: ["txt" "svg" "xml"]

  - compressors :: {String -> String}

      Map a desired extension (e.g. `gz`) to a compress program.

      The compressor program that will be executed to get the `COMPRESSOR`
      extension. The program should have a single " {}", which will be the
      replaced with the target filename.

      Compressor must:
      - read symlinks (thus --force is needed to gzip, zstd, xz).
      - keep the original file in place (--keep).

      Example:

        {
          xz = "${xz}/bin/xz --force --keep {}";
        }

   See compressDrvWeb, which is a wrapper on top of compressDrv, for broader
   use examples.
*/
{
  lib,
  xorg,
  runCommand,
}:
drv:
{ formats, compressors }:
let
  validProg =
    ext: prog:
    let
      matches = (builtins.length (builtins.split "\\{}" prog) - 1) / 2;
    in
    lib.assertMsg (
      matches == 1
    ) "compressor ${ext} needs to have exactly one '{}', found ${builtins.toString matches}";
  mkCmd =
    ext: prog:
    assert validProg ext prog;
    ''
      find -L $out -type f -regextype posix-extended -iregex '.*\.(${formatsPipe})' -print0 \
        | xargs -0 -P$NIX_BUILD_CORES -I{} ${prog}
    '';
  formatsPipe = builtins.concatStringsSep "|" formats;
in
runCommand "${drv.name}-compressed" { } ''
  mkdir $out
  (cd $out; ${xorg.lndir}/bin/lndir ${drv})

  ${lib.concatStringsSep "\n\n" (lib.mapAttrsToList mkCmd compressors)}
''