{ config, lib, ... }:
let
  cfg = config.router;
  ipv4 = (import ./lib.nix { inherit lib; }).ipv4;

  lansByName = cfg.lans;
  lans = lib.attrValues lansByName;

  validIp = s: ipv4.parse s != null;

  # Assertions about a single LAN; skipped (reported once) when its addresses
  # do not even parse, so we never feed null into the integer helpers.
  perLan =
    name: lan:
    let
      where = "router.lans.${name}";
      addrOk = validIp lan.ipv4.address;
      startOk = validIp lan.dhcp.pool.start;
      endOk = validIp lan.dhcp.pool.end;
      addr = ipv4.parse lan.ipv4.address;
      start = ipv4.parse lan.dhcp.pool.start;
      end = ipv4.parse lan.dhcp.pool.end;
      net = {
        address = addr;
        inherit (lan.ipv4) prefixLength;
      };
      parsed = addrOk && startOk && endOk;
    in
    [
      {
        assertion = addrOk;
        message = "router: ${where}.ipv4.address `${lan.ipv4.address}` is not a valid IPv4 address.";
      }
      {
        # IFNAMSIZ is 16 including the terminating NUL.
        assertion = lib.stringLength lan.device <= 15;
        message = "router: ${where}: device name `${lan.device}` is longer than 15 characters.";
      }
    ]
    ++ lib.optionals lan.dhcp.enable (
      [
        {
          assertion = startOk && endOk;
          message = "router: ${where}.dhcp.pool.start/end must be valid IPv4 addresses.";
        }
      ]
      ++ lib.optionals parsed [
        {
          assertion = start <= end;
          message = "router: ${where}.dhcp.pool starts after it ends.";
        }
        {
          assertion = ipv4.inSubnet net start && ipv4.inSubnet net end;
          message = "router: ${where}.dhcp.pool is not inside ${lan.ipv4.subnet}.";
        }
        {
          assertion = !(start <= addr && addr <= end);
          message = "router: ${where}.dhcp.pool contains the router's own address ${lan.ipv4.address}.";
        }
      ]
    );

  wgs = lib.attrValues cfg.wireguard;
  zoneNames = lib.unique (map (l: l.zone) lans ++ map (w: w.zone) wgs);

  isV6 = lib.hasInfix ":";
  v4Cidr = s: lib.hasInfix "/" s && ipv4.parseNet s != null;
  v6Cidr =
    s:
    let
      m = builtins.match "([0-9a-fA-F:.]+)/([0-9]{1,3})" s;
    in
    isV6 s && m != null && lib.toInt (builtins.elemAt m 1) <= 128;
  validCidr = s: v4Cidr s || v6Cidr s;

  lanNets = map (l: {
    address = ipv4.parse l.ipv4.address;
    inherit (l.ipv4) prefixLength;
  }) (lib.filter (l: validIp l.ipv4.address) lans);

  # IPv4 networks a tunnel brings into its zone (own addresses and what peers may send).
  wgV4Nets =
    w:
    lib.filter (n: n != null) (
      map ipv4.parseNet (
        lib.filter (n: !isV6 n && v4Cidr n) (
          w.addresses ++ lib.concatMap (p: p.allowedIPs) (lib.attrValues w.peers)
        )
      )
    );
  zoneV4Nets =
    zone:
    map (l: {
      address = ipv4.parse l.ipv4.address;
      inherit (l.ipv4) prefixLength;
    }) (lib.filter (l: l.zone == zone && validIp l.ipv4.address) lans)
    ++ lib.concatMap wgV4Nets (lib.filter (w: w.zone == zone) wgs);

  portOk =
    p:
    if builtins.isInt p then
      p >= 1 && p <= 65535
    else
      let
        m = builtins.match "([0-9]+)-([0-9]+)" p;
        lo = lib.toInt (builtins.elemAt m 0);
        hi = lib.toInt (builtins.elemAt m 1);
      in
      lo >= 1 && hi <= 65535 && lo < hi;

  ruleAssertions =
    r:
    let
      where = "router.firewall.rules (${r.from} -> ${r.to})";
      fromWan = r.from == "wan";
      v6Dest = r.destination != null && isV6 r.destination;
      dest = if r.destination == null || v6Dest then null else ipv4.parseNet r.destination;
      inZone =
        dest != null
        && lib.any (n: dest.prefixLength >= n.prefixLength && ipv4.inSubnet n dest.address) (
          zoneV4Nets r.to
        );
      singleHost = dest != null && dest.prefixLength == 32;
      plainPorts = lib.all builtins.isInt r.ports;
      internetMember = lib.any (l: l.internet) lans || lib.any (w: w.internet) wgs;
    in
    [
      {
        assertion = fromWan || lib.elem r.from zoneNames;
        message = "router: ${where}: unknown source zone `${r.from}` (zones: wan, ${lib.concatStringsSep ", " zoneNames}).";
      }
      {
        assertion = lib.elem r.to zoneNames;
        message =
          if r.to == "wan" then
            "router: ${where}: `wan` cannot be a destination; use `internet = true` on the LAN or tunnel."
          else
            "router: ${where}: unknown destination zone `${r.to}` (zones: ${lib.concatStringsSep ", " zoneNames}).";
      }
      {
        assertion = r.from != r.to;
        message = "router: ${where}: source and destination are the same zone; traffic within a zone is already allowed.";
      }
      {
        assertion = r.ports == [ ] || r.protocol == "tcp" || r.protocol == "udp";
        message = "router: ${where}: ports are only valid with protocol tcp or udp.";
      }
      {
        assertion = lib.all portOk r.ports;
        message = "router: ${where}: a port or port range is out of range (1-65535, ranges need low < high).";
      }
      {
        assertion = lib.all validCidr (
          map (
            x:
            if lib.hasInfix "/" x then
              x
            else if isV6 x then
              "${x}/128"
            else
              "${x}/32"
          ) r.sources
        );
        message = "router: ${where}: every source must be an IPv4/IPv6 address or CIDR.";
      }
    ]
    # Rules from the outside must not be able to expose a whole zone by accident.
    ++ lib.optionals fromWan [
      {
        assertion = r.destination != null;
        message = "router: ${where}: rules from `wan` must name a destination host or network.";
      }
      {
        assertion = r.protocol != "any";
        message = "router: ${where}: rules from `wan` must name a protocol (tcp, udp or icmp).";
      }
      {
        assertion = r.protocol == "icmp" || r.protocol == "any" || r.ports != [ ];
        message = "router: ${where}: rules from `wan` must list the ports they open.";
      }
    ]
    ++ lib.optionals (r.destination != null) [
      {
        assertion = v6Dest || dest != null;
        message = "router: ${where}: destination `${r.destination}` is not a valid IPv4 address or CIDR.";
      }
      {
        assertion = v6Dest || dest == null || !(lib.elem r.to zoneNames) || inZone;
        message = "router: ${where}: destination `${r.destination}` is not inside any network of zone `${r.to}`.";
      }
      {
        assertion = v6Dest || dest == null || ipv4.networkOf dest.address dest.prefixLength == dest.address;
        message = "router: ${where}: destination `${r.destination}` has host bits set.";
      }
      {
        assertion =
          !v6Dest
          || v6Cidr (if lib.hasInfix "/" r.destination then r.destination else "${r.destination}/128");
        message = "router: ${where}: destination `${r.destination}` is not a valid IPv6 address or CIDR.";
      }
    ]
    ++ lib.optionals r.dnat [
      {
        assertion = fromWan;
        message = "router: ${where}: dnat is only valid for rules from `wan`.";
      }
      {
        assertion = singleHost;
        message = "router: ${where}: dnat needs a single IPv4 host as destination.";
      }
      {
        assertion = r.protocol == "tcp" || r.protocol == "udp";
        message = "router: ${where}: dnat needs protocol tcp or udp.";
      }
      {
        assertion = lib.all (x: !isV6 x) r.sources;
        message = "router: ${where}: dnat is IPv4-only, so its sources must be IPv4.";
      }
    ]
    ++ lib.optionals (r.toPort != null) [
      {
        assertion = r.dnat && lib.length r.ports == 1 && plainPorts;
        message = "router: ${where}: toPort needs dnat and exactly one (non-range) port.";
      }
    ]
    ++ lib.optionals r.hairpin [
      {
        assertion = r.dnat;
        message = "router: ${where}: hairpin only applies to dnat rules.";
      }
      {
        assertion = internetMember;
        message = "router: ${where}: hairpin needs at least one LAN or tunnel with internet = true.";
      }
    ];

  overlapsLan = n: lib.any (l: ipv4.overlaps n l) lanNets;

  wgAssertions =
    name: w:
    let
      where = "router.wireguard.${name}";
      peers = lib.attrValues w.peers;
      allowed = lib.concatMap (p: p.allowedIPs) peers;
      v4Allowed = lib.filter (n: v4Cidr n) allowed;
      v4Own = lib.filter v4Cidr w.addresses;
      asNet = n: ipv4.parseNet n;
    in
    [
      {
        assertion = w.peers != { };
        message = "router: ${where} has no peers.";
      }
      {
        assertion = lib.stringLength w.interface <= 15;
        message = "router: ${where}: device name `${w.interface}` is longer than 15 characters.";
      }
      {
        assertion = lib.all validCidr w.addresses;
        message = "router: ${where}.addresses must be IPv4/IPv6 addresses in CIDR notation.";
      }
      {
        assertion = lib.all validCidr allowed;
        message = "router: ${where}: every peer allowedIPs entry must be an IPv4/IPv6 CIDR.";
      }
      {
        assertion = !(lib.any (n: lib.hasSuffix "/0" n) allowed);
        message = "router: ${where}: default-route tunnels (allowedIPs ending in /0) are not supported; list the destinations instead.";
      }
      {
        assertion = lib.length allowed == lib.length (lib.unique allowed);
        message = "router: ${where}: an allowedIPs entry appears more than once (WireGuard would give it to one peer only).";
      }
      {
        assertion = lib.all (n: !(overlapsLan (asNet n))) (
          lib.filter (n: !lib.hasSuffix "/0" n) (v4Own ++ v4Allowed)
        );
        message = "router: ${where}: a tunnel address or allowedIPs entry overlaps one of the LAN subnets.";
      }
      {
        assertion = lib.all (p: builtins.match "[A-Za-z0-9+/]{43}=" p.publicKey != null) peers;
        message = "router: ${where}: a peer publicKey is not a valid WireGuard public key.";
      }
      {
        assertion = lib.all (
          p: p.endpoint == null || builtins.match ".+:[0-9]{1,5}" p.endpoint != null
        ) peers;
        message = "router: ${where}: a peer endpoint must look like host:port or [ipv6]:port.";
      }
    ];

  overlapPairs = lib.concatLists (
    lib.imap0 (
      i: a:
      map (b: {
        inherit a b;
      }) (lib.drop (i + 1) (lib.attrNames lansByName))
    ) (lib.attrNames lansByName)
  );
in
{
  config = lib.mkIf cfg.enable {
    assertions = [
      {
        assertion = lans != [ ];
        message = "router: at least one LAN must be defined.";
      }
      {
        assertion = lib.any (l: l.allowManagement) lans;
        message = "router: no LAN has allowManagement = true; you would be locked out of the router.";
      }
      {
        assertion = !(lib.elem cfg.wan.interface (map (l: l.interface) lans));
        message = "router: the WAN interface `${cfg.wan.interface}` is also used by a LAN.";
      }
      {
        assertion = cfg.wan.ipv4.type != "pppoe" || cfg.wan.ipv4.pppoe.username != "";
        message = "router: router.wan.ipv4.pppoe.username must be set for a PPPoE uplink.";
      }
      {
        assertion = cfg.wan.ipv4.type != "pppoe" || cfg.wan.ipv4.pppoe.passwordFile != null;
        message = "router: router.wan.ipv4.pppoe.passwordFile must be set for a PPPoE uplink.";
      }
      {
        assertion = lib.length (lib.filter (l: l.ipv6.pd) lans) <= 1;
        message = "router: more than one LAN sets ipv6.pd, but the delegated /64 cannot be split.";
      }
      {
        assertion = !(lib.any (l: l.ipv6.pd) lans) || cfg.wan.ipv6.enable;
        message = "router: a LAN sets ipv6.pd but router.wan.ipv6.enable is false.";
      }
      {
        assertion =
          let
            devices = map (l: l.device) lans;
          in
          lib.length devices == lib.length (lib.unique devices);
        message = "router: two LANs share the same device (same interface and VLAN).";
      }
    ]
    ++ lib.concatLists (lib.mapAttrsToList perLan lansByName)
    ++ lib.concatMap ruleAssertions cfg.firewall.rules
    ++ [
      {
        assertion = !(lib.elem "wan" zoneNames);
        message = "router: `wan` is a reserved zone name; give the LAN or tunnel a different `zone`.";
      }
    ]
    ++ lib.concatLists (lib.mapAttrsToList wgAssertions cfg.wireguard)
    ++ [
      {
        assertion =
          let
            ports = lib.filter (p: p != null) (map (w: w.listenPort) wgs);
          in
          lib.length ports == lib.length (lib.unique ports);
        message = "router: two WireGuard tunnels listen on the same UDP port.";
      }
      {
        assertion =
          let
            own = map (w: w.interface) wgs;
            others = map (l: l.device) lans ++ [ cfg.wan.interface ];
          in
          lib.length own == lib.length (lib.unique own) && !(lib.any (d: lib.elem d others) own);
        message = "router: a WireGuard device name clashes with a LAN device or the WAN interface.";
      }
    ]
    ++ map (
      { a, b }:
      let
        la = lansByName.${a};
        lb = lansByName.${b};
      in
      {
        # Only comparable when both addresses parse; the per-LAN assertion reports the rest.
        assertion =
          !(validIp la.ipv4.address && validIp lb.ipv4.address)
          || !(ipv4.overlaps
            {
              address = ipv4.parse la.ipv4.address;
              inherit (la.ipv4) prefixLength;
            }
            {
              address = ipv4.parse lb.ipv4.address;
              inherit (lb.ipv4) prefixLength;
            }
          );
        message = "router: LANs `${a}` (${la.ipv4.subnet}) and `${b}` (${lb.ipv4.subnet}) overlap.";
      }
    ) overlapPairs;
  };
}