{ config, pkgs, lib, ... }:
let
  inherit (lib)
    concatMapStringsSep
    concatStringsSep
    filterAttrs
    flatten
    isAttrs
    isString
    literalExpression
    mapAttrs'
    mapAttrsToList
    mkIf
    mkOption
    optionalString
    partition
    typeOf
    types
    ;

  cfg = config.services.btrbk;
  sshEnabled = cfg.sshAccess != [ ];
  serviceEnabled = cfg.instances != { };
  attr2Lines = attr:
    let
      pairs = mapAttrsToList (name: value: { inherit name value; }) attr;
      isSubsection = value:
        if isAttrs value then true
        else if isString value then false
        else throw "invalid type in btrbk config ${typeOf value}";
      sortedPairs = partition (x: isSubsection x.value) pairs;
    in
    flatten (
      # non subsections go first
      (
        map (pair: [ "${pair.name} ${pair.value}" ]) sortedPairs.wrong
      )
      ++ # subsections go last
      (
        map
          (
            pair:
            mapAttrsToList
              (
                childname: value:
                  [ "${pair.name} ${childname}" ] ++ (map (x: " " + x) (attr2Lines value))
              )
              pair.value
          )
          sortedPairs.right
      )
    )
  ;
  addDefaults = settings: { backend = "btrfs-progs-sudo"; } // settings;
  mkConfigFile = settings: concatStringsSep "\n" (attr2Lines (addDefaults settings));
  mkTestedConfigFile = name: settings:
    let
      configFile = pkgs.writeText "btrbk-${name}.conf" (mkConfigFile settings);
    in
    pkgs.runCommand "btrbk-${name}-tested.conf" { } ''
      mkdir foo
      cp ${configFile} $out
      if (set +o pipefail; ${pkgs.btrbk}/bin/btrbk -c $out ls foo 2>&1 | grep $out);
      then
      echo btrbk configuration is invalid
      cat $out
      exit 1
      fi;
    '';
in
{
  meta.maintainers = with lib.maintainers; [ oxalica ];

  options = {
    services.btrbk = {
      extraPackages = mkOption {
        description = lib.mdDoc "Extra packages for btrbk, like compression utilities for `stream_compress`";
        type = types.listOf types.package;
        default = [ ];
        example = literalExpression "[ pkgs.xz ]";
      };
      niceness = mkOption {
        description = lib.mdDoc "Niceness for local instances of btrbk. Also applies to remote ones connecting via ssh when positive.";
        type = types.ints.between (-20) 19;
        default = 10;
      };
      ioSchedulingClass = mkOption {
        description = lib.mdDoc "IO scheduling class for btrbk (see ionice(1) for a quick description). Applies to local instances, and remote ones connecting by ssh if set to idle.";
        type = types.enum [ "idle" "best-effort" "realtime" ];
        default = "best-effort";
      };
      instances = mkOption {
        description = lib.mdDoc "Set of btrbk instances. The instance named `btrbk` is the default one.";
        type = with types;
          attrsOf (
            submodule {
              options = {
                onCalendar = mkOption {
                  type = types.nullOr types.str;
                  default = "daily";
                  description = lib.mdDoc ''
                    How often this btrbk instance is started. See systemd.time(7) for more information about the format.
                    Setting it to null disables the timer, thus this instance can only be started manually.
                  '';
                };
                settings = mkOption {
                  type = let t = types.attrsOf (types.either types.str (t // { description = "instances of this type recursively"; })); in t;
                  default = { };
                  example = {
                    snapshot_preserve_min = "2d";
                    snapshot_preserve = "14d";
                    volume = {
                      "/mnt/btr_pool" = {
                        target = "/mnt/btr_backup/mylaptop";
                        subvolume = {
                          "rootfs" = { };
                          "home" = { snapshot_create = "always"; };
                        };
                      };
                    };
                  };
                  description = lib.mdDoc "configuration options for btrbk. Nested attrsets translate to subsections.";
                };
              };
            }
          );
        default = { };
      };
      sshAccess = mkOption {
        description = lib.mdDoc "SSH keys that should be able to make or push snapshots on this system remotely with btrbk";
        type = with types; listOf (
          submodule {
            options = {
              key = mkOption {
                type = str;
                description = lib.mdDoc "SSH public key allowed to login as user `btrbk` to run remote backups.";
              };
              roles = mkOption {
                type = listOf (enum [ "info" "source" "target" "delete" "snapshot" "send" "receive" ]);
                example = [ "source" "info" "send" ];
                description = lib.mdDoc "What actions can be performed with this SSH key. See ssh_filter_btrbk(1) for details";
              };
            };
          }
        );
        default = [ ];
      };
    };

  };
  config = mkIf (sshEnabled || serviceEnabled) {
    environment.systemPackages = [ pkgs.btrbk ] ++ cfg.extraPackages;
    security.sudo.extraRules = [
      {
        users = [ "btrbk" ];
        commands = [
          { command = "${pkgs.btrfs-progs}/bin/btrfs"; options = [ "NOPASSWD" ]; }
          { command = "${pkgs.coreutils}/bin/mkdir"; options = [ "NOPASSWD" ]; }
          { command = "${pkgs.coreutils}/bin/readlink"; options = [ "NOPASSWD" ]; }
          # for ssh, they are not the same than the one hard coded in ${pkgs.btrbk}
          { command = "/run/current-system/bin/btrfs"; options = [ "NOPASSWD" ]; }
          { command = "/run/current-system/sw/bin/mkdir"; options = [ "NOPASSWD" ]; }
          { command = "/run/current-system/sw/bin/readlink"; options = [ "NOPASSWD" ]; }
        ];
      }
    ];
    users.users.btrbk = {
      isSystemUser = true;
      # ssh needs a home directory
      home = "/var/lib/btrbk";
      createHome = true;
      shell = "${pkgs.bash}/bin/bash";
      group = "btrbk";
      openssh.authorizedKeys.keys = map
        (
          v:
          let
            options = concatMapStringsSep " " (x: "--" + x) v.roles;
            ioniceClass = {
              "idle" = 3;
              "best-effort" = 2;
              "realtime" = 1;
            }.${cfg.ioSchedulingClass};
          in
          ''command="${pkgs.util-linux}/bin/ionice -t -c ${toString ioniceClass} ${optionalString (cfg.niceness >= 1) "${pkgs.coreutils}/bin/nice -n ${toString cfg.niceness}"} ${pkgs.btrbk}/share/btrbk/scripts/ssh_filter_btrbk.sh --sudo ${options}" ${v.key}''
        )
        cfg.sshAccess;
    };
    users.groups.btrbk = { };
    systemd.tmpfiles.rules = [
      "d /var/lib/btrbk 0750 btrbk btrbk"
      "d /var/lib/btrbk/.ssh 0700 btrbk btrbk"
      "f /var/lib/btrbk/.ssh/config 0700 btrbk btrbk - StrictHostKeyChecking=accept-new"
    ];
    environment.etc = mapAttrs'
      (
        name: instance: {
          name = "btrbk/${name}.conf";
          value.source = mkTestedConfigFile name instance.settings;
        }
      )
      cfg.instances;
    systemd.services = mapAttrs'
      (
        name: _: {
          name = "btrbk-${name}";
          value = {
            description = "Takes BTRFS snapshots and maintains retention policies.";
            unitConfig.Documentation = "man:btrbk(1)";
            path = [ "/run/wrappers" ] ++ cfg.extraPackages;
            serviceConfig = {
              User = "btrbk";
              Group = "btrbk";
              Type = "oneshot";
              ExecStart = "${pkgs.btrbk}/bin/btrbk -c /etc/btrbk/${name}.conf run";
              Nice = cfg.niceness;
              IOSchedulingClass = cfg.ioSchedulingClass;
              StateDirectory = "btrbk";
            };
          };
        }
      )
      cfg.instances;

    systemd.timers = mapAttrs'
      (
        name: instance: {
          name = "btrbk-${name}";
          value = {
            description = "Timer to take BTRFS snapshots and maintain retention policies.";
            wantedBy = [ "timers.target" ];
            timerConfig = {
              OnCalendar = instance.onCalendar;
              AccuracySec = "10min";
              Persistent = true;
            };
          };
        }
      )
      (filterAttrs (name: instance: instance.onCalendar != null)
        cfg.instances);
  };

}