{ 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;

  # Example:
  #
  # readFactFromAllMachines zerotier-ip
  # => {
  #   machineA = "1.2.3.4";
  #   machineB = "5.6.7.8";
  # };
  readFactFromAllMachines = fact:
    let
      machines = allMachineNames;
      facts = lib.genAttrs machines (readFact fact);
      filteredFacts = lib.filterAttrs (_machine: fact: fact != null) facts;
    in
    filteredFacts;

  # all given facts are are set and factvalues are never null.
  #
  # Example:
  #
  # readFactsFromAllMachines [ "zerotier-ip" "syncthing.pub" ]
  # => {
  #   machineA =
  #     {
  #       "zerotier-ip" = "1.2.3.4";
  #       "synching.pub" = "1234";
  #     };
  #   machineB =
  #     {
  #       "zerotier-ip" = "5.6.7.8";
  #       "synching.pub" = "23456719";
  #     };
  # };
  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; }