{
  description = "bunshin - Smol Mail server (Rust)";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";

    # RNS carrier sources (RNS.md sec 8): the build sandbox has no network,
    # so every microReticulum FetchContent dependency is vendored as its own
    # flake input and handed over via RNS_<DEP>_SOURCE_DIR. microReticulum
    # is pinned at the commit RNS.md sec 5 verified against.
    microReticulum = {
      url = "github:attermann/microReticulum/40fa628809d57140180c1c833559ab96fec992c1";
      flake = false;
    };
    rns-arduinojson = {
      url = "github:bblanchon/ArduinoJson/ed69feadb95182dc53ac978975d310122060a767";
      flake = false;
    };
    rns-msgpack = {
      url = "github:hideakitai/MsgPack/1f552c31b940d6e9063ee17a4b3fa10c47b27169";
      flake = false;
    };
    rns-arxcontainer = {
      url = "github:hideakitai/ArxContainer/d6affcd0bc83219b863c20abf7c269214db8db2a";
      flake = false;
    };
    rns-arxtypetraits = {
      url = "github:hideakitai/ArxTypeTraits/702de9cc59c7e047cdc169ae3547718b289d2c02";
      flake = false;
    };
    rns-debuglog = {
      url = "github:hideakitai/DebugLog/b581f7dde6c276c5df684e2328f406d9754d2f46";
      flake = false;
    };
    rns-crypto = {
      url = "github:attermann/Crypto/984dc891330986c302a86c4e312d4f5abcc28359";
      flake = false;
    };
    rns-microstore = {
      url = "github:attermann/microStore/0f28567fe00ab8ab14624a34c2e9e0a000a44c46";
      flake = false;
    };
  };

  outputs = { self, nixpkgs, flake-utils, microReticulum, rns-arduinojson
            , rns-msgpack, rns-arxcontainer, rns-arxtypetraits, rns-debuglog
            , rns-crypto, rns-microstore }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs { inherit system; };
        inherit (pkgs) lib;

        # build.rs consumes these directly (RNS.md sec 7.1); with every
        # dependency vendored, the cmake configure step never fetches.
        rnsSourceEnv = {
          MICRORETICULUM_SOURCE_DIR = "${microReticulum}";
          RNS_ARDUINOJSON_SOURCE_DIR = "${rns-arduinojson}";
          RNS_MSGPACK_SOURCE_DIR = "${rns-msgpack}";
          RNS_ARXCONTAINER_SOURCE_DIR = "${rns-arxcontainer}";
          RNS_ARXTYPETRAITS_SOURCE_DIR = "${rns-arxtypetraits}";
          RNS_DEBUGLOG_SOURCE_DIR = "${rns-debuglog}";
          RNS_CRYPTO_SOURCE_DIR = "${rns-crypto}";
          RNS_MICROSTORE_SOURCE_DIR = "${rns-microstore}";
        };

        bunshin = { rns ? false }: pkgs.rustPlatform.buildRustPackage {
          pname = "bunshin";
          version = "0.1.0";
          src = ./.;
          cargoLock.lockFile = ./Cargo.lock;
          nativeBuildInputs = [ pkgs.pkg-config ]
            ++ lib.optionals rns [ pkgs.cmake pkgs.ninja pkgs.gcc ];
          buildInputs = [ pkgs.sqlite ];
          buildFeatures = lib.optionals rns [ "rns" ];
          env = lib.optionalAttrs rns rnsSourceEnv;
          # ninja's setup hook claims buildPhase before the cargo hooks
          # can, which would run ninja against no build.ninja instead of
          # cargo; ninja here is only for build.rs to invoke through cmake.
          dontUseNinjaBuild = rns;
          dontUseNinjaCheck = rns;
          dontUseNinjaInstall = rns;
        };
      in
      {
        packages.default = bunshin { };
        packages.rns = bunshin { rns = true; };

        apps.default = flake-utils.lib.mkApp {
          drv = bunshin { };
          name = "bunshin";
        };

        devShells.default = pkgs.mkShell {
          packages = with pkgs; [ cargo rustc rustfmt clippy pkg-config gcc sqlite cmake ninja ];
          env = rnsSourceEnv;
        };
      }) // {
        nixosModules.default = { config, lib, pkgs, ... }:
          let
            cfg = config.services.bunshin;
            inherit (lib) mkEnableOption mkOption mkIf types;
          in
          {
            options.services.bunshin = {
              enable = mkEnableOption "the bunshin Smol Mail server";

              package = mkOption {
                type = types.package;
                default = self.packages.${pkgs.system}.default;
                description = ''
                  bunshin package to run. Use `packages.rns` when the RNS
                  carrier is enabled: the default package is built without it.
                '';
              };

              host = mkOption {
                type = types.str;
                default = "0.0.0.0";
                description = "Address to listen on.";
              };

              port = mkOption {
                type = types.port;
                default = 1961;
                description = "TCP port to listen on.";
              };

              keyFile = mkOption {
                type = types.path;
                description = ''
                  Path to the server's static Noise X25519 private key
                  (32 raw bytes, generated with `bunshin keygen`). Provisioned
                  out of band; this module does not generate it.
                '';
              };

              dataDir = mkOption {
                type = types.path;
                default = "/var/lib/bunshin";
                description = "Directory holding mail.db.";
              };

              maxEnvelope = mkOption {
                type = types.ints.positive;
                default = 786432;
                description = "Maximum accepted envelope size, in bytes.";
              };

              quota = mkOption {
                type = types.ints.positive;
                default = 67108864;
                description = "Per-mailbox main-tier storage quota, in bytes.";
              };

              requestsQuota = mkOption {
                type = types.ints.positive;
                default = 2097152;
                description = "Per-mailbox requests-tier storage quota, in bytes.";
              };

              retentionDays = mkOption {
                type = types.ints.positive;
                default = 30;
                description = "Days a main-tier message is retained before being purged.";
              };

              requestsRetentionDays = mkOption {
                type = types.ints.positive;
                default = 7;
                description = "Days a requests-tier message is retained before being purged.";
              };

              maxTokens = mkOption {
                type = types.ints.positive;
                default = 1024;
                description = "Maximum accept tokens a mailbox may hold.";
              };

              rateConnections = mkOption {
                type = types.ints.positive;
                default = 120;
                description = "Max accepted connections per minute, per source IP.";
              };

              rateSends = mkOption {
                type = types.ints.positive;
                default = 60;
                description = "Max SEND operations per minute, per source IP.";
              };

              rateTokens = mkOption {
                type = types.ints.positive;
                default = 30;
                description = "Max SEND operations per minute, per accept token.";
              };

              inviteToken = mkOption {
                type = types.nullOr types.str;
                default = null;
                description = ''
                  Registration invite token. Null means open registration.
                  Prefer `inviteTokenFile` to avoid storing the token in the
                  world-readable Nix store.
                '';
              };

              inviteTokenFile = mkOption {
                type = types.nullOr types.path;
                default = null;
                description = ''
                  Path to a file (readable by the service via LoadCredential)
                  containing the registration invite token.
                '';
              };

              openFirewall = mkOption {
                type = types.bool;
                default = false;
                description = "Open the configured TCP port in the firewall.";
              };

              verbose = mkOption {
                type = types.bool;
                default = false;
                description = ''
                  Enable debug-level logging (--verbose): per-op timing,
                  session summaries and RNS link events on both carriers.
                '';
              };

              rns = {
                enable = mkEnableOption "the RNS carrier alongside TCP (needs an rns-built package)";

                keyFile = mkOption {
                  type = types.path;
                  description = ''
                    Path to the server's Reticulum identity (64 raw bytes,
                    x25519 || ed25519 private halves, generated with
                    `bunshin rns-keygen`). RNS writes the private key
                    unencrypted, so protect the file like any long-term key.
                  '';
                };

                maxEnvelope = mkOption {
                  type = types.ints.positive;
                  default = 32768;
                  description = "Maximum accepted envelope size over RNS, in bytes (RNS.md sec 3).";
                };

                fetchBudget = mkOption {
                  type = types.ints.positive;
                  default = 32768;
                  description = "Bytes one FETCH response may total over RNS.";
                };

                maxLinks = mkOption {
                  type = types.ints.positive;
                  default = 100;
                  description = "Maximum concurrent Reticulum links; further links are refused.";
                };

                rateLinkRequests = mkOption {
                  type = types.ints.positive;
                  default = 60;
                  description = "Max requests per minute, per link.";
                };

                rateLinkBytes = mkOption {
                  type = types.ints.positive;
                  default = 1048576;
                  description = "Max request bytes per minute, per link.";
                };

                udpListenPort = mkOption {
                  type = types.port;
                  default = 4242;
                  description = ''
                    UDP port the Reticulum interface listens on. RNS reaches
                    the mesh through a configured interface rather than a
                    listening TCP port; only this UDP port needs a firewall
                    opening.
                  '';
                };

                udpForward = mkOption {
                  type = types.nullOr types.str;
                  default = null;
                  description = ''
                    Optional host[:port] the interface forwards every packet
                    to. Without it, replies go to the source address of the
                    last datagram received, which suits a listen-only
                    point-to-point setup.
                  '';
                };
              };
            };

            config = mkIf cfg.enable {
              assertions = [
                {
                  assertion = !(cfg.inviteToken != null && cfg.inviteTokenFile != null);
                  message = "services.bunshin: set only one of inviteToken or inviteTokenFile.";
                }
              ];

              systemd.services.bunshin = {
                description = "bunshin Smol Mail server";
                wantedBy = [ "multi-user.target" ];
                after = [ "network.target" ];

                serviceConfig = {
                  ExecStart = pkgs.writeShellScript "bunshin-serve" ''
                    set -euo pipefail
                    args=(
                      serve
                      --key ${cfg.keyFile}
                      --db ${cfg.dataDir}/mail.db
                      --host ${cfg.host}
                      --port ${toString cfg.port}
                      --max-envelope ${toString cfg.maxEnvelope}
                      --quota ${toString cfg.quota}
                      --requests-quota ${toString cfg.requestsQuota}
                      --retention-days ${toString cfg.retentionDays}
                      --requests-retention-days ${toString cfg.requestsRetentionDays}
                      --max-tokens ${toString cfg.maxTokens}
                      --rate-connections ${toString cfg.rateConnections}
                      --rate-sends ${toString cfg.rateSends}
                      --rate-tokens ${toString cfg.rateTokens}
                    )
                    ${lib.optionalString cfg.rns.enable ''
                      args+=(
                        --rns
                        --rns-key ${cfg.rns.keyFile}
                        --rns-max-envelope ${toString cfg.rns.maxEnvelope}
                        --rns-fetch-budget ${toString cfg.rns.fetchBudget}
                        --rns-max-links ${toString cfg.rns.maxLinks}
                        --rns-rate-link-requests ${toString cfg.rns.rateLinkRequests}
                        --rns-rate-link-bytes ${toString cfg.rns.rateLinkBytes}
                        --rns-udp 0.0.0.0:${toString cfg.rns.udpListenPort}
                      )
                    ''}
                    ${lib.optionalString (cfg.rns.enable && cfg.rns.udpForward != null)
                      ''args+=(--rns-udp-forward ${lib.escapeShellArg cfg.rns.udpForward})''}
                    ${lib.optionalString cfg.verbose
                      ''args+=(--verbose)''}
                    ${lib.optionalString (cfg.inviteToken != null)
                      ''args+=(--invite-token ${lib.escapeShellArg cfg.inviteToken})''}
                    ${lib.optionalString (cfg.inviteTokenFile != null)
                      ''args+=(--invite-token "$(cat "$CREDENTIALS_DIRECTORY/invite-token")")''}
                    exec ${cfg.package}/bin/bunshin "''${args[@]}"
                  '';
                  DynamicUser = true;
                  StateDirectory = "bunshin";
                  StateDirectoryMode = "0700";
                  Restart = "on-failure";
                } // lib.optionalAttrs (cfg.inviteTokenFile != null) {
                  LoadCredential = "invite-token:${cfg.inviteTokenFile}";
                };
              };

              # RNS needs no TCP port; only its UDP interface may be opened.
              networking.firewall.allowedUDPPorts =
                mkIf (cfg.rns.enable && cfg.openFirewall) [ cfg.rns.udpListenPort ];
              networking.firewall.allowedTCPPorts = mkIf cfg.openFirewall [ cfg.port ];
            };
          };
      };
}