{ config, lib, ... }:

# TODO: make a remote-build user on nixos boxes, instead of giving access to pbsds
# TODO: https://exozy.me/quickstart
# TODO: https://github.com/winterqt/darwin-build-box

let
  inherit (builtins) map fromTOML readFile elem attrNames attrValues;
  inherit (lib) mkIf;

  known-hosts' = lib.importTOML ../hosts/known-hosts.toml; # TODO: eww
  known-hosts = lib.pipe known-hosts' [
    (lib.flip lib.removeAttrs ["__default__"])
    (lib.mapAttrs (name: host:
      lib.recursiveUpdate (known-hosts'."__default__" or {}) host
    ))
  ];
  hostNames = attrNames known-hosts;
  thisHost = known-hosts.${config.networking.fqdn};
  thisHostIsBuilder = thisHost.buildMachine.maxJobs > 0;
  thisHostIsHopHost = builtins.elem config.networking.fqdn (lib.forEach (attrValues known-hosts) (host: host.ssh.proxyJump or null));
  thisHostIsConsumer = thisHost.ssh ? userPublicKey;

  mkRemoteConfig = fqdn: let
    thatHost = known-hosts.${fqdn};
    thatJump = known-hosts.${thatHost.ssh.proxyJump};
    buildMachine = thatHost.buildMachine // {
      hostName = fqdn;
      sshUser = thatHost.ssh.listenUser;
    };
    thatHostIsBuilder = thatHost.buildMachine.maxJobs > 0;
    thatHostIsConsumer = thatHost.ssh ? userPublicKey && thisHostIsBuilder;
    thatHostIsThis = fqdn == config.networking.fqdn;
  in mkIf (!thatHostIsThis) ( lib.mkMerge [
    # out
    (lib.mkIf (thisHostIsConsumer && thatHostIsBuilder) {

      nix.buildMachines = [ buildMachine ];

    })
    # out or jump
    (lib.mkIf (thisHostIsConsumer && thatHost.ssh ? listenPublicKey) {
      programs.ssh.knownHosts.${fqdn}.publicKey = thatHost.ssh.listenPublicKey;
      # TODO: use nix.buildMachines.*.publicHostKey ?

      # timeouts are great when remote is unresponsive. nix doesn't care, lix is way and tests each remote only once
      programs.ssh.extraConfig = ''
        Host ${fqdn}
          ConnectTimeout 3
          Port ${builtins.toString thatHost.ssh.listenPort}
          ${lib.optionalString (thatHost.ssh ? proxyJump) ''
            ProxyJump ${thatJump.ssh.listenUser}@${thatHost.ssh.proxyJump}:${builtins.toString thatJump.ssh.listenPort}
          ''}
          ${lib.optionalString (thatHost.ssh ? userPrivateKey) ''
            IdentityFile ${thatHost.ssh.userPrivateKey}
          ''}
      '';

      sops.secrets = lib.mkIf (lib.hasPrefix "/run/secrets/" (thatHost.ssh.userPrivateKey or "")) {
        "${lib.removePrefix "/run/secrets/" thatHost.ssh.userPrivateKey}" = {};
      };
    })
    # in
    (mkIf ((thisHostIsBuilder || thisHostIsHopHost) && thatHostIsConsumer) {
      users.users.${thisHost.ssh.listenUser} = {
        isSystemUser = lib.mkDefault (!config.users.users.${thisHost.ssh.listenUser}.isNormalUser);
        openssh.authorizedKeys.keys = [ thatHost.ssh.userPublicKey ];
        group = lib.mkOptionDefault "nogroup";
      };
    })
    (mkIf (thisHostIsBuilder && thatHostIsConsumer) {
      nix.settings.allowed-users = [ thisHost.ssh.listenUser ];
      nix.settings.trusted-users = [ thisHost.ssh.listenUser ];
    })
  ]);

in {

  nix.distributedBuilds = true;

  # TODO: Allow setting speedFactor for local builds, as local is currently fixed to 0
  # https://github.com/NixOS/nix/issues/2457

  # useful when the builder has a faster internet connection than i do
  nix.settings.builders-use-substitutes = true;

  imports = lib.forEach hostNames mkRemoteConfig;

}