{ config, lib, pkgs, ... }:
let
  cfg = config.hardware.printers;

  ensurePrinter = p: let
    args = lib.cli.toGNUCommandLineShell {} ({
      p = p.name;
      v = p.deviceUri;
      m = p.model;
    } // lib.optionalAttrs (p.location != null) {
      L = p.location;
    } // lib.optionalAttrs (p.description != null) {
      D = p.description;
    } // lib.optionalAttrs (p.ppdOptions != {}) {
      o = lib.mapAttrsToList (name: value: "${name}=${value}") p.ppdOptions;
    });
  in ''
    ${pkgs.cups}/bin/lpadmin ${args} -E
  '';

  ensureDefaultPrinter = name: ''
    ${pkgs.cups}/bin/lpadmin -d '${name}'
  '';

  # "graph but not # or /" can't be implemented as regex alone due to missing lookahead support
  noInvalidChars = str: lib.all (c: c != "#" && c != "/") (lib.stringToCharacters str);
  printerName = (lib.types.addCheck (lib.types.strMatching "[[:graph:]]+") noInvalidChars)
    // { description = "printable string without spaces, # and /"; };


in {
  options = {
    hardware.printers = {
      ensureDefaultPrinter = lib.mkOption {
        type = lib.types.nullOr printerName;
        default = null;
        description = ''
          Ensures the named printer is the default CUPS printer / printer queue.
        '';
      };
      ensurePrinters = lib.mkOption {
        description = ''
          Will regularly ensure that the given CUPS printers are configured as declared here.
          If a printer's options are manually changed afterwards, they will be overwritten eventually.
          This option will never delete any printer, even if removed from this list.
          You can check existing printers with {command}`lpstat -s`
          and remove printers with {command}`lpadmin -x <printer-name>`.
          Printers not listed here can still be manually configured.
        '';
        default = [];
        type = lib.types.listOf (lib.types.submodule {
          options = {
            name = lib.mkOption {
              type = printerName;
              example = "BrotherHL_Workroom";
              description = ''
                Name of the printer / printer queue.
                May contain any printable characters except "/", "#", and space.
              '';
            };
            location = lib.mkOption {
              type = lib.types.nullOr lib.types.str;
              default = null;
              example = "Workroom";
              description = ''
                Optional human-readable location.
              '';
            };
            description = lib.mkOption {
              type = lib.types.nullOr lib.types.str;
              default = null;
              example = "Brother HL-5140";
              description = ''
                Optional human-readable description.
              '';
            };
            deviceUri = lib.mkOption {
              type = lib.types.str;
              example = lib.literalExpression ''
                "ipp://printserver.local/printers/BrotherHL_Workroom"
                "usb://HP/DESKJET%20940C?serial=CN16E6C364BH"
              '';
              description = ''
                How to reach the printer.
                {command}`lpinfo -v` shows a list of supported device URIs and schemes.
              '';
            };
            model = lib.mkOption {
              type = lib.types.str;
              example = lib.literalExpression ''
                "gutenprint.''${lib.versions.majorMinor (lib.getVersion pkgs.gutenprint)}://brother-hl-5140/expert"
              '';
              description = ''
                Location of the ppd driver file for the printer.
                {command}`lpinfo -m` shows a list of supported models.
              '';
            };
            ppdOptions = lib.mkOption {
              type = lib.types.attrsOf lib.types.str;
              example = {
                PageSize = "A4";
                Duplex = "DuplexNoTumble";
              };
              default = {};
              description = ''
                Sets PPD options for the printer.
                {command}`lpoptions [-p printername] -l` shows supported PPD options for the given printer.
              '';
            };
          };
        });
      };
    };
  };

  config = lib.mkIf (cfg.ensurePrinters != [] && config.services.printing.enable) {
    systemd.services.ensure-printers = {
      description = "Ensure NixOS-configured CUPS printers";
      wantedBy = [ "multi-user.target" ];
      wants = [ "cups.service" ];
      after = [ "cups.service" ];

      serviceConfig = {
        Type = "oneshot";
        RemainAfterExit = true;
      };

      script = lib.concatStringsSep "\n" [
        (lib.concatMapStrings ensurePrinter cfg.ensurePrinters)
        (lib.optionalString (cfg.ensureDefaultPrinter != null)
          (ensureDefaultPrinter cfg.ensureDefaultPrinter))
        # Note: if cupsd is "stateless" the service can't be stopped,
        # otherwise the configuration will be wiped on the next start.
        (lib.optionalString (with config.services.printing; startWhenNeeded && !stateless)
          "systemctl stop cups.service")
      ];
    };
  };
}