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{ lib, config, pkgs, ... }:
with lib; let
  cfg = config.local;
in {
  options.local = with lib.types; {
    snapperSubvols = mkOption {
      type = attrsOf str;
      default = {};
    };

    fs.btrfs = mkOption {
      default = [];

      type = attrsOf (submodule {
        options = {
          device = mkOption {
            type = str;
          };

          subvol = mkOption {
            type = str;
          };

          ssd = mkOption {
            type = bool;
          };

          snapper = mkOption {
            type = nullOr str;
            default = null;
          };
        };
      });
    };
  };

  config = {
    environment.systemPackages = optional (cfg.snapperSubvols != {}) pkgs.local.btclone;

    fileSystems = let
      inherit (cfg) fs;
      btrfs = { device, subvol, ssd, ... }: {
        inherit device;
        fsType = "btrfs";
        options = [ "noatime" "compress=zstd" "subvol=${subvol}" ] ++ optional ssd "ssd";
      };
    in mapAttrs (_: btrfs) cfg.fs.btrfs;

    local.snapperSubvols = let
      snapperEntry = path: opts: { name = opts.snapper; value = path; };
      validEntry = _: opts: opts.snapper != null;
    in mapAttrs' snapperEntry (filterAttrs validEntry cfg.fs.btrfs);

    services.snapper.configs = let
      snapperConfig = _: subvolume: {
        inherit subvolume;

        extraConfig = ''
          # btrfs qgroup for space aware cleanup algorithms
          QGROUP=""

          # fraction of the filesystems space the snapshots may use
          SPACE_LIMIT="0.5"

          # fraction of the filesystems space that should be free
          FREE_LIMIT="0.2"

          # users and groups allowed to work with config
          ALLOW_USERS=""
          ALLOW_GROUPS=""

          # sync users and groups from ALLOW_USERS and ALLOW_GROUPS to .snapshots
          # directory
          SYNC_ACL="no"

          # start comparing pre- and post-snapshot in background after creating
          # post-snapshot
          BACKGROUND_COMPARISON="yes"

          # run daily number cleanup
          NUMBER_CLEANUP="yes"

          # limit for number cleanup
          NUMBER_MIN_AGE="1800"
          NUMBER_LIMIT="100"
          NUMBER_LIMIT_IMPORTANT="10"

          # create hourly snapshots
          TIMELINE_CREATE="yes"

          # cleanup hourly snapshots after some time
          TIMELINE_CLEANUP="yes"

          # limits for timeline cleanup
          TIMELINE_MIN_AGE="1800"
          TIMELINE_LIMIT_HOURLY="24"
          TIMELINE_LIMIT_DAILY="7"
          TIMELINE_LIMIT_WEEKLY="4"
          TIMELINE_LIMIT_MONTHLY="12"
          TIMELINE_LIMIT_YEARLY="10"

          # cleanup empty pre-post-pairs
          EMPTY_PRE_POST_CLEANUP="yes"

          # limits for empty pre-post-pair cleanup
          EMPTY_PRE_POST_MIN_AGE="1800"
        '';
      };
    in mapAttrs snapperConfig cfg.snapperSubvols;
  };
}