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

with lib;

let
  cfg = config.services.btrfs-snapshots;

  snapshotMount = str: "${cfg.mountPoint}/${cfg.mountPrefix}${str}";
in {
  options = {

    services.btrfs-snapshots = {
      enable = mkEnableOption "a systemd unit for btrfs snapshots";   

      mountPoint = mkOption {
        readOnly = true;
        type = types.path;
        default = "/mnt";
      };

      mountPrefix = mkOption {
        readOnly = true;
        type = types.str;
        default = "snapshot-";
      };

      readOnly = mkOption {
        type = types.bool;
        default = true;
      };

      persist = mkOption {
        type = types.bool;
        default = false;
      };
    };

  };

  
  config = mkIf cfg.enable {

    systemd.services."btrfs-snapshot@" = {
      enable = true;

      unitConfig = {
        AssertPathIsDirectory = cfg.mountPoint;
        StopWhenUnneeded = !cfg.persist;
      };

      serviceConfig = with pkgs; {
        Type = "oneshot";
        ExecStartPre = "-${btrfs-progs}/bin/btrfs subvolume delete -c ${snapshotMount "%i"}";
        ExecStart = "${btrfs-progs}/bin/btrfs subvolume snapshot ${optionalString cfg.readOnly "-r"} %f ${snapshotMount "%i"}";
        RemainAfterExit = true;
        ExecStop = "${btrfs-progs}/bin/btrfs subvolume delete -c ${snapshotMount "%i"}";
      };
    };

  };
}