# NixOS module — system-wide install. Headroom itself is a user-scope
# daemon (it talks to the user's PipeWire session), so this module's
# job is narrow:
#
#   1. Make the `headroom` binary available on every login's PATH.
#   2. Drop the systemd user unit into the system-wide location so a
#      user can `systemctl --user enable --now headroom` without first
#      having to use Home Manager.
#   3. Ensure the standard audio stack (PipeWire + WirePlumber) is
#      enabled, since headroom can't function without them.
#
# For per-user defaults — activeProfile, shipped-profile install,
# RT-priority tuning — use the Home Manager module
# (`homeModules.default`) instead. The two compose.
self:
{
  config,
  lib,
  pkgs,
  ...
}:

let
  inherit (lib)
    mkEnableOption
    mkOption
    mkIf
    types
    literalExpression
    ;

  cfg = config.programs.headroom;
in
{
  options.programs.headroom = {
    enable = mkEnableOption "Headroom — automatic loudness and per-app volume control for PipeWire";

    package = mkOption {
      type = types.package;
      default = self.packages.${pkgs.stdenv.hostPlatform.system}.headroom;
      defaultText = literalExpression "headroom.packages.\${pkgs.system}.headroom";
      description = ''
        The headroom package to install system-wide.
      '';
    };
  };

  config = mkIf cfg.enable {
    # Binary + manpages (when we have them) on the global PATH.
    environment.systemPackages = [ cfg.package ];

    # Make the shipped systemd user unit discoverable by `systemctl
    # --user`. Setting `packages` here is the canonical NixOS way to
    # install user-scope unit files from a package — it materialises
    # `/etc/systemd/user/headroom.service` pointing at the package's
    # `lib/systemd/user/headroom.service`.
    systemd.packages = [ cfg.package ];

    # Headroom requires PipeWire; refuse to evaluate the module if
    # the user enabled headroom but not pipewire, with a pointer
    # rather than a confusing runtime failure.
    assertions = [
      {
        assertion = config.services.pipewire.enable;
        message = ''
          programs.headroom.enable requires services.pipewire.enable = true;
          headroom is a PipeWire-only daemon.
        '';
      }
    ];
  };
}