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

with lib;

let
  cfg = config.nix.gc;
  darwinIntervals =
    [ "hourly" "daily" "weekly" "monthly" "semiannually" "annually" ];

  mkCalendarInterval = frequency:
    let
      freq = {
        "hourly" = [{ Minute = 0; }];
        "daily" = [{
          Hour = 0;
          Minute = 0;
        }];
        "weekly" = [{
          Weekday = 1;
          Hour = 0;
          Minute = 0;
        }];
        "monthly" = [{
          Day = 1;
          Hour = 0;
          Minute = 0;
        }];
        "semiannually" = [
          {
            Month = 1;
            Day = 1;
            Hour = 0;
            Minute = 0;
          }
          {
            Month = 7;
            Day = 1;
            Hour = 0;
            Minute = 0;
          }
        ];
        "annually" = [{
          Month = 1;
          Day = 1;
          Hour = 0;
          Minute = 0;
        }];
      };
    in freq.${frequency};

  nixPackage = if config.nix.enable && config.nix.package != null then
    config.nix.package
  else
    pkgs.nix;
in {
  meta.maintainers = [ maintainers.shivaraj-bh ];

  options = {
    nix.gc = {
      automatic = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Automatically run the garbage collector at a specific time.

          Note: This will only garbage collect the current user's profiles.
        '';
      };

      frequency = mkOption {
        type = types.str;
        default = "weekly";
        example = "03:15";
        description = ''
          When to run the Nix garbage collector.

          On Linux this is a string as defined by {manpage}`systemd.time(7)`.

          On Darwin it must be one of: ${toString darwinIntervals}, which are
          implemented as defined in the manual page above.
        '';
      };

      options = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = "--max-freed $((64 * 1024**3))";
        description = ''
          Options given to {file}`nix-collect-garbage` when the
          garbage collector is run automatically.
        '';
      };

      persistent = mkOption {
        type = types.bool;
        default = true;
        example = false;
        description = ''
          If true, the time when the service unit was last triggered is
          stored on disk. When the timer is activated, the service unit is
          triggered immediately if it would have been triggered at least once
          during the time when the timer was inactive.
        '';
      };
    };
  };

  config = lib.mkIf cfg.automatic (mkMerge [
    (mkIf pkgs.stdenv.isLinux {
      systemd.user.services.nix-gc = {
        Unit = { Description = "Nix Garbage Collector"; };
        Service = {
          ExecStart = "${nixPackage}/bin/nix-collect-garbage ${
              lib.optionalString (cfg.options != null) cfg.options
            }";
        };
      };
      systemd.user.timers.nix-gc = {
        Unit = { Description = "Nix Garbage Collector"; };
        Timer = {
          OnCalendar = "${cfg.frequency}";
          Persistent = cfg.persistent;
          Unit = "nix-gc.service";
        };
        Install = { WantedBy = [ "timers.target" ]; };
      };
    })

    (mkIf pkgs.stdenv.isDarwin {
      assertions = [{
        assertion = elem cfg.frequency darwinIntervals;
        message = "On Darwin nix.gc.frequency must be one of: ${
            toString darwinIntervals
          }.";
      }];

      launchd.agents.nix-gc = {
        enable = true;
        config = {
          ProgramArguments = [ "${nixPackage}/bin/nix-collect-garbage" ]
            ++ lib.optional (cfg.options != null) cfg.options;
          StartCalendarInterval = mkCalendarInterval cfg.frequency;
        };
      };
    })
  ]);
}