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

with lib;
let
  hostname = config.instance.hostname;
  has-attrs = set: length (attrNames set) > 0;
  host-keypairs =
    if (hasAttr hostname config.fudo.secrets.files.host-ssh-keypairs) then
      config.fudo.secrets.files.host-ssh-keypairs.${hostname}
    else [];


  sshfp-filename = host: keypair: "ssh-${host}-${keypair.key-type}.sshfp-record";

  dns-sshfp-records = host: keypair: let
    filename = sshfp-filename host keypair;
  in pkgs.stdenv.mkDerivation {
    name = "${host}-sshfp-record";

    phases = [ "installPhase" ];

    buildInputs = with pkgs; [ openssh ];

    installPhase = ''
      mkdir $out
      ssh-keygen -r REMOVEME -f "${keypair.public-key}" | sed 's/^REMOVEME IN SSHFP //' > $out/${filename}
    '';
  };

  read-lines = filename: splitString "\n" (fileContents filename);

  host-cfg = config.fudo.hosts.${hostname};

in {
  config = {
    fudo = {
      secrets.host-secrets.${hostname} = listToAttrs
        (map
          (keypair: nameValuePair "host-${keypair.key-type}-private-key" {
            source-file = keypair.private-key;
            target-file = "/var/run/ssh/private/host-${keypair.key-type}-private-key";
            user = "root";
          })
          host-keypairs);

      hosts = mapAttrs (hostname: keypairs: {
        ssh-pubkeys = map (keypair: keypair.public-key) keypairs;
        ssh-fingerprints = concatMap (keypair:
          let
            fingerprint-derivation = dns-sshfp-records hostname keypair;
            filename = sshfp-filename hostname keypair;
          in read-lines "${fingerprint-derivation}/${filename}") keypairs;
      }) config.fudo.secrets.files.host-ssh-keypairs;
    };

    services.openssh.hostKeys = map (keypair: {
      path = "/var/run/ssh/private/host-${keypair.key-type}-private-key";
      type = keypair.key-type;
    }) host-keypairs;

    programs.ssh.knownHosts = let

      keyed-hosts =
        filterAttrs (h: o: o.ssh-pubkeys != [])
          config.fudo.hosts;

      crossProduct = f: list0: list1:
        concatMap (el0: map (el1: f el0 el1) list1) list0;

      all-hostnames = hostname: opts:
        [ hostname ] ++
        (crossProduct (host: domain: "${host}.${domain}")
          ([ hostname ] ++ opts.aliases)
          ([ opts.domain ] ++ opts.extra-domains));

    in mapAttrs (hostname: hostOpts: {
      publicKeyFile = builtins.head hostOpts.ssh-pubkeys;
      hostNames = all-hostnames hostname host-cfg;
    }) keyed-hosts;
  };
}