{ lib, config, pkgs, ... }:

let
  templateSubmodule = {...}: {
    options = {
      enable = lib.mkEnableOption "this template";

      target = lib.mkOption {
        description = "Path in the container";
        type = lib.types.path;
      };
      template = lib.mkOption {
        description = ".tpl file for rendering the target";
        type = lib.types.path;
      };
      when = lib.mkOption {
        description = "Events which trigger a rewrite (create, copy)";
        type = lib.types.listOf (lib.types.str);
      };
      properties = lib.mkOption {
        description = "Additional properties";
        type = lib.types.attrs;
        default = {};
      };
    };
  };

  toYAML = name: data: pkgs.writeText name (lib.generators.toYAML {} data);

  cfg = config.virtualisation.lxc;
  templates = if cfg.templates != {} then let
    list = lib.mapAttrsToList (name: value: { inherit name; } // value)
      (lib.filterAttrs (name: value: value.enable) cfg.templates);
  in
    {
      files = map (tpl: {
        source = tpl.template;
        target = "/templates/${tpl.name}.tpl";
      }) list;
      properties = lib.listToAttrs (map (tpl: lib.nameValuePair tpl.target {
        when = tpl.when;
        template = "${tpl.name}.tpl";
        properties = tpl.properties;
      }) list);
    }
  else { files = []; properties = {}; };

in {
  options = {
    virtualisation.lxc = {
      templates = lib.mkOption {
        description = "Templates for LXD";
        type = lib.types.attrsOf (lib.types.submodule templateSubmodule);
        default = {};
        example = lib.literalExpression ''
          {
            # create /etc/hostname on container creation
            "hostname" = {
              enable = true;
              target = "/etc/hostname";
              template = builtins.writeFile "hostname.tpl" "{{ container.name }}";
              when = [ "create" ];
            };
            # create /etc/nixos/hostname.nix with a configuration for keeping the hostname applied
            "hostname-nix" = {
              enable = true;
              target = "/etc/nixos/hostname.nix";
              template = builtins.writeFile "hostname-nix.tpl" "{ ... }: { networking.hostName = "{{ container.name }}"; }";
              # copy keeps the file updated when the container is changed
              when = [ "create" "copy" ];
            };
            # copy allow the user to specify a custom configuration.nix
            "configuration-nix" = {
              enable = true;
              target = "/etc/nixos/configuration.nix";
              template = builtins.writeFile "configuration-nix" "{{ config_get(\"user.user-data\", properties.default) }}";
              when = [ "create" ];
            };
          };
        '';
      };
    };
  };

  config = {
    system.build.metadata = pkgs.callPackage ../../lib/make-system-tarball.nix {
      contents = [
        {
          source = toYAML "metadata.yaml" {
            architecture = builtins.elemAt (builtins.match "^([a-z0-9_]+).+" (toString pkgs.system)) 0;
            creation_date = 1;
            properties = {
              description = "${config.system.nixos.distroName} ${config.system.nixos.codeName} ${config.system.nixos.label} ${pkgs.system}";
              os = "${config.system.nixos.distroId}";
              release = "${config.system.nixos.codeName}";
            };
            templates = templates.properties;
          };
          target = "/metadata.yaml";
        }
      ] ++ templates.files;
    };
  };
}