{ lib, self }:
let
  inherit (lib)
    filterAttrs
    hasSuffix
    mapAttrs
    mapAttrs'
    mkOption
    nameValuePair
    removeSuffix
    types
    ;

  registryName = fileName: removeSuffix ".nix" fileName;

  registryOption = registry: mkOption {
    type = types.attrsOf (types.attrsOf registry.type);
    default = { };
    description = ''
      Lazily collected fleet-visible registry entries, nested by host and entry name.
    '';
  };

  definitions = mapAttrs' (
    name: type:
    let
      path = self + "/registry/${name}";
    in
    nameValuePair (registryName name) (import path { inherit lib; })
  ) (filterAttrs (name: type: type == "regular" && hasSuffix ".nix" name) (builtins.readDir (self + "/registry")));
  join = registry: host: entry:
    filterAttrs (_name: value: value != null) (
      mapAttrs (registryName: _registry: (registry.${registryName}.${host} or { }).${entry} or null) definitions
    );

  collectData =
    { hosts }:
    mapAttrs (
      registryName: _registry:
      mapAttrs (hostName: host: host.config.registry.${registryName}.${hostName} or { }) hosts
    ) definitions;

  withOperations = registry:
    registry
    // {
      join = host: entry: join registry host entry;
    };
in
{
  inherit definitions;

  module = {
    options.registry = mapAttrs (_name: registry: registryOption registry) definitions;
  };

  collect = args: withOperations (collectData args);
}