{ lib }:

dir:

let
  inherit (builtins)
    readDir
    attrNames
    filter
    match
    pathExists;

  inherit (lib)
    concatLists
    hasPrefix;

  entries = readDir dir;

  names = attrNames entries;

  validName = name:
    !(hasPrefix "_" name);

  recurse = name:
    let
      type = entries.${name};
      path = dir + "/${name}";
    in
      if !validName name then
        [ ]

      else if type == "directory" then
        importTree path

      else if
        type == "regular"
        && name != "default.nix"
        && match ".*\\.nix" name != null
      then
        [ path ]

      else
        [ ];

  importTree = d:
    let
      dEntries = readDir d;
      dNames = attrNames dEntries;
    in
      concatLists (map (name:
        let
          type = dEntries.${name};
          path = d + "/${name}";
        in
          if !validName name then
            [ ]

          else if type == "directory" then
            importTree path

          else if
            type == "regular"
            && name != "import-tree.nix"
            && name != "flake.nix"
            && name != "flake.lock"
            && match ".*\\.nix" name != null
          then
            [ path ]

          else
            [ ]
      ) dNames);

in
importTree dir