let
  inherit (builtins)
    readDir
    attrNames
    match
    concatLists
    any
    substring
    stringLength
    elem
    ;

  hasPrefix = prefix: str:
    substring 0 (stringLength prefix) str == prefix;

  matchesAny = patterns: name:
    any (pattern: match pattern name != null) patterns;

in
{
  path                          # dir to walk
, include ? [ ".*\\.nix" ]      # regexes; file kept if it matches ANY
, exclude ? [ "flake.nix" "flake.lock" ]  # exact names to always skip
, ignoreUnderscore ? true       # skip files/dirs starting with "_"
}:

let
  validName = name:
    !(ignoreUnderscore && hasPrefix "_" name)
    && !(elem name exclude);

  walk = dir:
    let
      entries = readDir dir;
      names = attrNames entries;
    in
    concatLists (map (name:
      let
        type = entries.${name};
        p = dir + "/${name}";
      in
      if !(validName name) then
        [ ]
      else if type == "directory" then
        walk p
      else if type == "regular" && matchesAny include name then
        [ p ]
      else
        [ ]
    ) names);

in
walk path