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

with lib;

let

  cfg = config.nix;

in {
  options.nix = {
    registry = mkOption {
      type = types.attrsOf (types.submodule (let
        inputAttrs = types.attrsOf
          (types.oneOf [ types.str types.int types.bool types.package ]);
      in { config, name, ... }: {
        options = {
          from = mkOption {
            type = inputAttrs;
            example = {
              type = "indirect";
              id = "nixpkgs";
            };
            description = "The flake reference to be rewritten.";
          };
          to = mkOption {
            type = inputAttrs;
            example = {
              type = "github";
              owner = "my-org";
              repo = "my-nixpkgs";
            };
            description =
              "The flake reference to which <option>from></option> is to be rewritten.";
          };
          flake = mkOption {
            type = types.nullOr types.attrs;
            default = null;
            example = literalExpression "nixpkgs";
            description = ''
              The flake input to which <option>from></option> is to be rewritten.
            '';
          };
          exact = mkOption {
            type = types.bool;
            default = true;
            description = ''
              Whether the <option>from</option> reference needs to match exactly. If set,
              a <option>from</option> reference like <literal>nixpkgs</literal> does not
              match with a reference like <literal>nixpkgs/nixos-20.03</literal>.
            '';
          };
        };
        config = {
          from = mkDefault {
            type = "indirect";
            id = name;
          };
          to = mkIf (config.flake != null) ({
            type = "path";
            path = config.flake.outPath;
          } // lib.filterAttrs (n: v:
            n == "lastModified" || n == "rev" || n == "revCount" || n
            == "narHash") config.flake);
        };
      }));
      default = { };
      description = ''
        User level flake registry.
      '';
    };
    registryVersion = mkOption {
      type = types.int;
      default = 2;
      internal = true;
      description = "The flake registry format version.";
    };
  };

  config = mkIf (cfg.registry != { }) {
    xdg.configFile."nix/registry.json".text = builtins.toJSON {
      version = cfg.registryVersion;
      flakes =
        mapAttrsToList (n: v: { inherit (v) from to exact; }) cfg.registry;
    };
  };
}