{
  description = "Lgm's NixOS Flake";

  inputs = {
    # There are many ways to reference flake inputs.
    # The most widely used is `github:owner/name/reference`,
    # which represents the GitHub repository URL + branch/commit-id/tag.

    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    # separate inputs to lock some packages
    # locked - stuff to be updated very unfrequently
    # fresh - mostly desktop apps
    nixpkgs-locked.url = "github:NixOS/nixpkgs/nixos-unstable";
    nixpkgs-fresh.url = "github:NixOS/nixpkgs/nixos-unstable";
    nixpkgs-stable.url = "github:NixOS/nixpkgs/nixos-23.05";
    # nix-index-database.url = "github:nix-community/nix-index-database";
    # nix-index-database.inputs.nixpkgs.follows = "nixpkgs";
    # nur = {
    #   url = "github:nix-community/NUR";
    # };
    home-manager = {
      url = "github:nix-community/home-manager/master";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    nh = {
      url = "github:viperML/nh";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs = inputs@{
    nixpkgs,
    nixpkgs-locked,
    nixpkgs-fresh,
    nixpkgs-stable,
    # nix-index-database,
    # nur,
    home-manager,
    ...
  }: 
  let
    system = "x86_64-linux";
    overlay-locked = final: prev: {
      locked = import nixpkgs-locked {
        inherit system;
        config.allowUnfree = true;
      };
    };
    overlay-fresh = final: prev: {
      fresh = import nixpkgs-fresh {
        inherit system;
        config.allowUnfree = true;
      };
    };
    overlay-stable = final: prev: {
      stable = import nixpkgs-stable {
        inherit system;
        config.allowUnfree = true;
      };
    };
    overlay-vaapiIntel = final: prev: {
      vaapiIntel = prev.vaapiIntel.override { enableHybridCodec = true; };
    };
    # overlay-gl = final: prev: {
    #   libGL = final.stable.libGL;
    #   glfw = final.stable.glfw;
    # };
    my-overlays = [
      overlay-locked
      overlay-fresh
      overlay-stable
      overlay-vaapiIntel
      # overlay-gl
    ];
  in
  {
    nixosConfigurations.lgm-nixos = nixpkgs.lib.nixosSystem rec {
      inherit system;
      specialArgs = {
        secrets = import ./secrets {};
      };
      modules = [
        ({ config, pkgs, ... }: { nixpkgs.overlays = my-overlays; })
        # nur.nixosModules.nur
        inputs.nh.nixosModules.default
        ./configuration.nix
        # nix-index-database.nixosModules.nix-index
        ./mounts.nix

        # make home-manager as a module of nixos
        # so that home-manager configuration will be deployed automatically when executing `nixos-rebuild switch`
        home-manager.nixosModules.home-manager
        {
          home-manager.useGlobalPkgs = true;
          home-manager.useUserPackages = true;

          home-manager.users.lgm = import ./home.nix;

          home-manager.extraSpecialArgs = with specialArgs; {
            inherit secrets;
          };
        }
      ];
    };
  };
}