{ lib, machineDir, ... }:
let

  allMachineNames = lib.mapAttrsToList (name: _: name) (builtins.readDir machineDir);

  getFactPath = fact: machine:
    "${machineDir}/${machine}/facts/${fact}";

  readFact = fact: machine:
    let
      path = getFactPath fact machine;
    in
    if builtins.pathExists path then
      builtins.readFile path
    else
      null;

  # machine -> factvalue
  readFactFromAllMachines = fact:
    let
      machines = allMachineNames;
      facts = lib.genAttrs machines (readFact fact);
      filteredFacts = lib.filterAttrs (_machine: fact: fact != null) facts;
    in
    filteredFacts;

  # returns an Attrs of machines and it's facts which have all given facts set.
  # machine -> fact -> value
  readFactsFromAllMachines = facts:
    let
      # machine -> fact -> factvalue
      machinesFactsAttrs = lib.genAttrs allMachineNames (machine: lib.genAttrs facts (fact: readFact fact machine));
      # remove all machines which don't have all facts set
      filteredMachineFactAttrs =
        lib.filterAttrs (_machine: values: builtins.all (fact: values.${fact} != null) facts)
          machinesFactsAttrs;
    in
    filteredMachineFactAttrs;



in
{ inherit allMachineNames getFactPath readFact readFactFromAllMachines readFactsFromAllMachines; }