// Ported from filius-ipad ios/FiliusPadTests/TopologyNetworkRuntimeEngineTests.swift:
// the event queue, the virtual clock, Java-compatible randomness, restart,
// Ethernet/switch/ARP/IPv4/ICMP, routing errors, the firewall, packet loss
// and trace retention, UDP and TCP sockets, DHCP. The remaining Swift tests
// (DNS, RIP, NAT, remote link, packet viewer) follow with their protocol
// slices.

import { describe, expect, it } from 'vitest';
import {
  decodeDhcpMessage,
  DHCP_DYNAMIC_LEASE_LIFETIME_MILLISECONDS,
  deviceConfiguration,
  dhcpServerConfiguration,
  encodeDhcpMessage,
  EMPTY_TOPOLOGY_SNAPSHOT,
  ETHERNET_BROADCAST_MAC_ADDRESS,
  firewallConfiguration,
  firewallRule,
  DIRECTLY_CONNECTED_SOURCE_MARKER,
  forwardingClone,
  icmpMessage,
  interfaceKey,
  JavaCompatibleRandom,
  NetworkRuntimeEngine,
  portSnapshot,
  stableMacAddress,
  switchConfiguration,
  TCP_FLAG,
  tcpFlagDescription,
  topologySnapshot,
  traceRetentionPolicy,
  UNSPECIFIED_MAC_ADDRESS,
  type DeviceConfiguration,
  type DhcpServerConfiguration,
  type EthernetFrame,
  type EthernetPayload,
  type FirewallConfiguration,
  type IPv4Packet,
  type NodeSnapshot,
  type RuntimeOutput,
  type ScheduledEventKind,
} from '../src/index.js';
import { device, link, pc, startedEngine, uuid } from './fixtures.js';

const marker = (label: string): ScheduledEventKind => ({ kind: 'parityMarker', label });

function firedKinds(outputs: RuntimeOutput[]): ScheduledEventKind[] {
  return outputs.flatMap((output) => (output.kind.type === 'fired' ? [output.kind.event.kind] : []));
}

function routedTopology() {
  const sourceNodeId = uuid(0x1451);
  const sourcePortId = uuid(0x1461);
  const routerNodeId = uuid(0x1452);
  const routerPortA = uuid(0x1462);
  const routerPortB = uuid(0x1463);
  const targetNodeId = uuid(0x1453);
  const targetPortId = uuid(0x1464);
  const router: NodeSnapshot = {
    id: routerNodeId,
    kind: 'router',
    ports: [portSnapshot(routerPortA, 'rt1'), portSnapshot(routerPortB, 'rt2')],
  };
  const snapshot = topologySnapshot({
    nodes: [pc(sourceNodeId, sourcePortId), router, pc(targetNodeId, targetPortId)],
    links: [
      link(sourceNodeId, sourcePortId, routerNodeId, routerPortA),
      link(routerNodeId, routerPortB, targetNodeId, targetPortId),
    ],
    deviceConfigurations: new Map([
      [sourceNodeId, device('10.0.0.2', { gateway: '10.0.0.1' })],
      [routerNodeId, device('10.0.0.1')],
      [targetNodeId, device('20.0.0.2', { gateway: '20.0.0.1' })],
    ]),
    interfaceConfigurations: new Map([
      [interfaceKey(routerNodeId, routerPortA), { ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }],
      [interfaceKey(routerNodeId, routerPortB), { ipAddress: '20.0.0.1', subnetMask: '255.255.255.0' }],
    ]),
  });
  const engine = new NetworkRuntimeEngine(14);
  engine.handle({ type: 'start', snapshot, seed: 14, initialTimeMilliseconds: 0 });
  return { engine, sourceNodeId, routerNodeId };
}

function firewallEngine(configuration: FirewallConfiguration) {
  const routerId = uuid(0x1802);
  const portId = uuid(0x1812);
  const engine = startedEngine(
    {
      nodes: [{ id: routerId, kind: 'router', ports: [portSnapshot(portId, 'rt1')] }],
      links: [],
      interfaceConfigurations: new Map([
        [interfaceKey(routerId, portId), { ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }],
      ]),
      firewallConfigurationsByNodeId: new Map([[routerId, configuration]]),
    },
    18,
  );
  return { engine, routerId };
}

function tcpPacket(identity: number, destinationPort: number, flags: number): IPv4Packet {
  return {
    identity,
    senderIPAddress: '203.0.113.10',
    receiverIPAddress: '10.0.0.1',
    timeToLive: 64,
    protocol: 'tcp',
    payload: {
      kind: 'tcp',
      segment: {
        sourcePort: 50_000,
        destinationPort,
        sequenceNumber: identity,
        acknowledgementNumber: 0,
        flags,
        payload: new Uint8Array(),
      },
    },
  };
}

describe('event queue and virtual clock', () => {
  it('orders events by deadline, then by insertion sequence', () => {
    const engine = new NetworkRuntimeEngine(42);
    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 42, initialTimeMilliseconds: 0 });
    engine.handle({ type: 'schedule', deadlineMilliseconds: 100, kind: marker('first-at-100') });
    engine.handle({ type: 'schedule', deadlineMilliseconds: 50, kind: marker('at-50') });
    engine.handle({ type: 'schedule', deadlineMilliseconds: 100, kind: marker('second-at-100') });

    const outputs = engine.handle({ type: 'advance', toMilliseconds: 100 });

    expect(firedKinds(outputs)).toEqual([marker('at-50'), marker('first-at-100'), marker('second-at-100')]);
    expect(engine.state.currentTimeMilliseconds).toBe(100);
    expect(engine.state.pendingEvents).toEqual([]);
  });

  it('rejects a past time without changing the clock', () => {
    const engine = new NetworkRuntimeEngine(7);
    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 7, initialTimeMilliseconds: 10 });
    engine.handle({ type: 'advance', toMilliseconds: 25 });

    const outputs = engine.handle({ type: 'advance', toMilliseconds: 24 });

    expect(outputs.map((output) => output.kind)).toEqual([{ type: 'advanceRejectedPastTime' }]);
    expect(engine.state.currentTimeMilliseconds).toBe(25);
  });

  it('rejects a past deadline without using up a sequence number', () => {
    const engine = new NetworkRuntimeEngine(8);
    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 8, initialTimeMilliseconds: 25 });

    const outputs = engine.handle({ type: 'schedule', deadlineMilliseconds: 24, kind: marker('past') });

    expect(outputs.map((output) => output.kind)).toEqual([
      { type: 'scheduleRejectedPastTime', deadlineMilliseconds: 24 },
    ]);
    expect(engine.state.pendingEvents).toEqual([]);
    expect(engine.state.nextEventSequenceNumber).toBe(0);
    expect(engine.state.currentTimeMilliseconds).toBe(25);
  });

  it('replays a named scenario straight to a stable state', () => {
    const finalState = NetworkRuntimeEngine.run({
      name: 'equal-deadline-order',
      seed: 1,
      advanceToMilliseconds: 1_000,
      scheduledEvents: [
        { deadlineMilliseconds: 500, kind: marker('a') },
        { deadlineMilliseconds: 500, kind: marker('b') },
      ],
    });

    expect(finalState.currentTimeMilliseconds).toBe(1_000);
    expect(finalState.pendingEvents).toEqual([]);
    expect(finalState.seed).toBe(1n);
  });

  it('clears transient state on restart and numbers packets from one again', () => {
    const nodeId = uuid(0x12);
    const engine = new NetworkRuntimeEngine(12);
    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 12, initialTimeMilliseconds: 0 });
    engine.handle({ type: 'schedule', deadlineMilliseconds: 40, kind: marker('stale') });
    expect(engine.allocateFrameIdentity()).toBe(1);
    expect(engine.allocatePacketIdentity()).toBe(1);
    engine.recordTrace({ packetIdentity: 1, nodeId, direction: 'local', layer: 'application', operation: 'created' });
    engine.handle({ type: 'stop' });

    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 12, initialTimeMilliseconds: 0 });

    const state = engine.state;
    expect(state.pendingEvents).toEqual([]);
    expect(state.packetTraces).toEqual([]);
    expect(state.arpCachesByNodeId.size).toBe(0);
    expect(state.switchForwardingTablesByNodeId.size).toBe(0);
    expect(state.switchSeenFrameIdentitiesByNodeId.size).toBe(0);
    expect(state.deliveredIPv4PacketsByNodeId.size).toBe(0);
    expect(state.icmpObservationsByNodeId.size).toBe(0);
    expect(state.firewallDecisions).toEqual([]);
    expect(engine.allocateFrameIdentity()).toBe(1);
    expect(engine.allocatePacketIdentity()).toBe(1);
  });

  it('forgets UDP sockets on stop, binds none while stopped, and numbers them from one again', () => {
    const nodeId = uuid(0x13);
    const portId = uuid(0x14);
    const input = { nodes: [pc(nodeId, portId)], links: [], deviceConfigurations: new Map([[nodeId, device('10.2.0.1')]]) };
    const engine = startedEngine(input);
    expect(engine.bindUdpSocket(nodeId, { localPort: 7 })).toBe(uuid(1));
    expect(engine.bindUdpSocket(nodeId)).toBe(uuid(2));

    engine.handle({ type: 'stop' });
    expect(engine.state.socketsById.size).toBe(0);
    expect(engine.state.udpReceiveQueuesBySocketId.size).toBe(0);
    expect(engine.bindUdpSocket(nodeId, { localPort: 7 })).toBeUndefined();

    engine.handle({ type: 'start', snapshot: topologySnapshot(input), seed: 14, initialTimeMilliseconds: 0 });
    expect(engine.bindUdpSocket(nodeId, { localPort: 7 })).toBe(uuid(1));
  });

  it('forgets DHCP leases and client state on stop', () => {
    const [serverId, serverPortId, clientId, clientPortId] = [0x31, 0x32, 0x33, 0x34].map(uuid) as [string, string, string, string];
    const engine = startedEngine({
      nodes: [pc(serverId, serverPortId), pc(clientId, clientPortId)],
      links: [link(serverId, serverPortId, clientId, clientPortId)],
      deviceConfigurations: new Map([
        [serverId, device('10.4.0.1')],
        [clientId, device('0.0.0.0')],
      ]),
      dhcpClientConfigurationsByNodeId: new Map([[clientId, { isEnabled: true }]]),
      dhcpServerConfigurationsByNodeId: new Map([
        [serverId, dhcpServerConfiguration({ isActive: true, lowerBoundIPAddress: '10.4.0.20', upperBoundIPAddress: '10.4.0.29' })],
      ]),
    });
    engine.handle({ type: 'advance', toMilliseconds: 5_000 });
    expect(engine.state.dhcpLeasesByIPAddress.size).toBe(1);

    engine.handle({ type: 'stop' });
    const state = engine.state;
    expect([
      state.dhcpOffersByIPAddress.size,
      state.dhcpLeasesByIPAddress.size,
      state.dhcpBlacklistByServerNodeId.size,
      state.dhcpLastOfferedAddressByServerNodeId.size,
      state.dhcpServerSocketIdsByNodeId.size,
      state.dhcpClientStatusesByNodeId.size,
      state.dhcpClientContextsByNodeId.size,
    ]).toEqual([0, 0, 0, 0, 0, 0, 0]);
  });

  it('forgets TCP sessions and accepted sockets on stop', () => {
    const [clientId, clientPortId, serverId, serverPortId] = [0x21, 0x22, 0x23, 0x24].map(uuid) as [string, string, string, string];
    const engine = startedEngine({
      nodes: [pc(clientId, clientPortId), pc(serverId, serverPortId)],
      links: [link(clientId, clientPortId, serverId, serverPortId)],
      deviceConfigurations: new Map([
        [clientId, device('10.3.0.1')],
        [serverId, device('10.3.0.2')],
      ]),
    });
    engine.openTcpServerSocket(serverId, 80);
    expect(engine.connectTcp(engine.openTcpClientSocket(clientId, '10.3.0.2', 80)!)).toBe(true);
    expect(engine.state.tcpSessionsById.size).toBe(3);

    engine.handle({ type: 'stop' });
    expect(engine.state.tcpSessionsById.size).toBe(0);
    expect(engine.state.tcpAcceptedSocketIdsByListenerId.size).toBe(0);
  });
});

describe('UDP sockets', () => {
  const text = (payload: Uint8Array | undefined) => new TextDecoder().decode(payload ?? new Uint8Array());

  it('binds, delivers and releases a port', () => {
    const [sourceNodeId, sourcePortId, targetNodeId, targetPortId] = [0x15b1, 0x15b2, 0x15b3, 0x15b4].map(uuid) as [string, string, string, string];
    const engine = startedEngine({
      nodes: [pc(sourceNodeId, sourcePortId), pc(targetNodeId, targetPortId)],
      links: [link(sourceNodeId, sourcePortId, targetNodeId, targetPortId)],
      deviceConfigurations: new Map([
        [sourceNodeId, device('192.168.5.10')],
        [targetNodeId, device('192.168.5.20')],
      ]),
    });
    const automatic = engine.bindUdpSocket(targetNodeId);
    const automaticPort = automatic === undefined ? undefined : engine.state.socketsById.get(automatic)?.localPort;
    expect(automaticPort).toBeGreaterThanOrEqual(49_152);
    expect(automaticPort).toBeLessThan(65_535);

    const receiver = engine.bindUdpSocket(targetNodeId, { localPort: 9000 });
    const sender = engine.bindUdpSocket(sourceNodeId, { localPort: 8000, remoteIPAddress: '192.168.5.20', remotePort: 9000 });
    expect(receiver).toBeDefined();
    expect(sender).toBeDefined();
    expect(engine.bindUdpSocket(targetNodeId, { localPort: 9000 })).toBeUndefined();

    engine.sendUdp(sender!, new TextEncoder().encode('hello'));
    const received = engine.receiveUdp(receiver!);
    expect(text(received?.datagram.payload)).toBe('hello');
    expect(received?.senderIPAddress).toBe('192.168.5.10');

    engine.closeSocket(receiver!);
    expect(engine.bindUdpSocket(targetNodeId, { localPort: 9000 })).toBeDefined();
  });

  it('draws ephemeral ports without disturbing the seeded RIP jitter', () => {
    const nodeId = uuid(0x15f1);
    const input = { nodes: [pc(nodeId, uuid(0x15f2))], links: [], deviceConfigurations: new Map([[nodeId, device('192.168.10.10')]]) };
    const socketEngine = startedEngine(input);
    const ripOnlyEngine = startedEngine(input);

    expect(socketEngine.bindUdpSocket(nodeId)).toBeDefined();
    expect(socketEngine.nextRipJitterMilliseconds()).toBe(ripOnlyEngine.nextRipJitterMilliseconds());
  });

  it('broadcasts on every interface, and the receiver replies to its last peer', () => {
    const [routerId, routerPortA, routerPortB, pcAId, pcAPort, pcBId, pcBPort] = [0x15e1, 0x15e2, 0x15e3, 0x15e4, 0x15e5, 0x15e6, 0x15e7].map(
      uuid,
    ) as [string, string, string, string, string, string, string];
    const engine = startedEngine(
      {
        nodes: [
          { id: routerId, kind: 'router', ports: [portSnapshot(routerPortA, 'rt1'), portSnapshot(routerPortB, 'rt2')] },
          pc(pcAId, pcAPort),
          pc(pcBId, pcBPort),
        ],
        links: [link(routerId, routerPortA, pcAId, pcAPort), link(routerId, routerPortB, pcBId, pcBPort)],
        deviceConfigurations: new Map([
          [pcAId, device('10.0.0.2')],
          [pcBId, device('20.0.0.2')],
        ]),
        interfaceConfigurations: new Map([
          [interfaceKey(routerId, routerPortA), { ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }],
          [interfaceKey(routerId, routerPortB), { ipAddress: '20.0.0.1', subnetMask: '255.255.255.0' }],
        ]),
      },
      15,
    );
    const sender = engine.bindUdpSocket(routerId, { localPort: 8000 })!;
    const receiverA = engine.bindUdpSocket(pcAId, { localPort: 9000 })!;
    const receiverB = engine.bindUdpSocket(pcBId, { localPort: 9000 })!;

    engine.sendUdp(sender, new TextEncoder().encode('broadcast'), { ipAddress: '255.255.255.255', port: 9000 });

    expect(engine.receiveUdp(receiverA)?.senderIPAddress).toBe('10.0.0.1');
    expect(engine.receiveUdp(receiverB)?.senderIPAddress).toBe('20.0.0.1');
    expect(engine.state.socketsById.get(receiverA)?.remoteIPAddress).toBe('10.0.0.1');
    expect(engine.state.socketsById.get(receiverA)?.remotePort).toBe(8000);

    engine.sendUdp(receiverA, new TextEncoder().encode('reply'));
    expect(text(engine.receiveUdp(sender)?.datagram.payload)).toBe('reply');
  });

  it('advances the virtual clock for a receive timeout and returns nothing', () => {
    const nodeId = uuid(0x15d1);
    const engine = startedEngine({ nodes: [pc(nodeId, uuid(0x15d2))], links: [], deviceConfigurations: new Map([[nodeId, device('10.1.0.1')]]) });
    const socketId = engine.bindUdpSocket(nodeId, { localPort: 9999 })!;

    expect(engine.receiveUdp(socketId, 250)).toBeUndefined();
    expect(engine.state.currentTimeMilliseconds).toBe(250);
  });
});

describe('Java-compatible randomness', () => {
  it('draws the same RIP jitter as java.util.Random', () => {
    const random = new JavaCompatibleRandom(42);

    expect(Array.from({ length: 5 }, () => random.nextRipJitterMilliseconds())).toEqual([
      32_475, 25_746, 32_032, 25_679, 28_287,
    ]);
  });

  it('matches java.util.Random.nextInt for bounded and power-of-two ranges', () => {
    // Measured on OpenJDK 21 (29.09.2026): new java.util.Random(42),
    // nextInt(100) three times, then nextInt(16) three times.
    const random = new JavaCompatibleRandom(42);

    expect([random.nextInt(100), random.nextInt(100), random.nextInt(100)]).toEqual([30, 63, 48]);
    expect([random.nextInt(16), random.nextInt(16), random.nextInt(16)]).toEqual([0, 4, 15]);
  });

  it('matches java.util.Random.nextDouble', () => {
    // Measured on OpenJDK 21 (30.09.2026): nextDouble twice, then
    // Math.round(nextDouble() * 253f) as the DHCP client draws a link-local address.
    const cases: [number, number, number, number][] = [
      [42, 0.7275636800328681, 0.6832234717598454, 78],
      [0, 0.730967787376657, 0.24053641567148587, 161],
      [-7, 0.2691218093260761, 0.6731811902798038, 132],
    ];
    for (const [seed, first, second, rounded] of cases) {
      const random = new JavaCompatibleRandom(seed);
      expect([random.nextDouble(), random.nextDouble(), Math.round(random.nextDouble() * 253)]).toEqual([first, second, rounded]);
    }
  });
});

describe('IPv4 packets', () => {
  it('keeps the identity and lowers the TTL when forwarding', () => {
    const packet: IPv4Packet = {
      identity: 99,
      senderIPAddress: '192.168.0.10',
      receiverIPAddress: '192.168.1.20',
      timeToLive: 2,
      protocol: 'icmp',
      payload: { kind: 'icmp', message: icmpMessage('echoRequest', { identifier: 3, sequenceNumber: 4 }) },
    };

    const forwarded = forwardingClone(packet);
    const expired = forwardingClone(forwarded);

    expect(forwarded).toMatchObject({ identity: 99, timeToLive: 1 });
    expect(expired).toMatchObject({ identity: 99, timeToLive: 0 });
  });
});

describe('Ethernet, ARP and ICMP', () => {
  it('fills both ARP caches with one request and reply over a direct cable', () => {
    const sourceNodeId = uuid(0x1401);
    const sourcePortId = uuid(0x1411);
    const targetNodeId = uuid(0x1402);
    const targetPortId = uuid(0x1412);
    const engine = startedEngine({
      nodes: [pc(sourceNodeId, sourcePortId), pc(targetNodeId, targetPortId)],
      links: [link(sourceNodeId, sourcePortId, targetNodeId, targetPortId)],
      deviceConfigurations: new Map([
        [sourceNodeId, device('192.168.0.10')],
        [targetNodeId, device('192.168.0.20')],
      ]),
    });

    const resolved = engine.resolveMacAddress(sourceNodeId, '192.168.0.20');

    expect(resolved).toBe(stableMacAddress(targetPortId));
    expect(engine.state.arpCachesByNodeId.get(sourceNodeId)?.get('192.168.0.20')?.macAddress).toBe(resolved);
    expect(engine.state.arpCachesByNodeId.get(targetNodeId)?.get('192.168.0.10')?.macAddress).toBe(
      stableMacAddress(sourcePortId),
    );
  });

  it('pings across a switch with Ethernet, ARP and ICMP packets', () => {
    const sourceNodeId = uuid(0x1421);
    const sourcePortId = uuid(0x1431);
    const switchNodeId = uuid(0x1422);
    const switchPortA = uuid(0x1432);
    const switchPortB = uuid(0x1433);
    const targetNodeId = uuid(0x1423);
    const targetPortId = uuid(0x1434);
    const switchNode: NodeSnapshot = {
      id: switchNodeId,
      kind: 'networkSwitch',
      ports: [portSnapshot(switchPortA, 'sw1'), portSnapshot(switchPortB, 'sw2')],
    };
    const engine = startedEngine({
      nodes: [pc(sourceNodeId, sourcePortId), switchNode, pc(targetNodeId, targetPortId)],
      links: [
        link(sourceNodeId, sourcePortId, switchNodeId, switchPortA),
        link(switchNodeId, switchPortB, targetNodeId, targetPortId),
      ],
      deviceConfigurations: new Map([
        [sourceNodeId, device('192.168.0.10')],
        [targetNodeId, device('192.168.0.20')],
      ]),
    });

    const result = engine.sendIcmpEcho(sourceNodeId, '192.168.0.20', { identifier: 14, sequenceNumber: 1 });

    expect(result.type).toBe('delivered');
    expect(engine.state.icmpObservationsByNodeId.get(sourceNodeId)?.at(-1)?.message.kind).toBe('echoReply');
    expect(engine.state.switchForwardingTablesByNodeId.get(switchNodeId)?.size).toBe(2);
    expect(engine.state.packetTraces.some((trace) => trace.detail === 'ARP request for 192.168.0.20')).toBe(true);
    expect(
      engine.state.packetTraces.some((trace) => trace.packetIdentity !== undefined && trace.layer === 'network'),
    ).toBe(true);
  });

  it('answers an expired TTL at the router with time exceeded and the original packet', () => {
    const topology = routedTopology();

    const result = topology.engine.sendIcmpEcho(topology.sourceNodeId, '20.0.0.2', {
      timeToLive: 1,
      identifier: 140,
      sequenceNumber: 1,
    });

    expect(result).toMatchObject({ type: 'icmpError', kind: 'timeExceeded' });
    const observation = topology.engine.state.icmpObservationsByNodeId.get(topology.sourceNodeId)?.at(-1);
    expect(observation?.message.kind).toBe('timeExceeded');
    expect(observation?.message.embeddedOriginalPacket?.timeToLive).toBe(1);
    expect(observation?.message.embeddedOriginalPacket?.receiverIPAddress).toBe('20.0.0.2');
  });

  it('keeps the packet identity and lowers the TTL when a router forwards', () => {
    const topology = routedTopology();

    const result = topology.engine.sendIcmpEcho(topology.sourceNodeId, '20.0.0.2', {
      timeToLive: 8,
      identifier: 141,
      sequenceNumber: 1,
    });

    expect(result.type).toBe('delivered');
    const forwardingTrace = topology.engine.state.packetTraces.find(
      (trace) =>
        trace.nodeId === topology.routerNodeId && trace.operation === 'forwarded' && trace.detail === 'TTL decremented',
    );
    expect(forwardingTrace?.packetIdentity).toBeDefined();
    expect(forwardingTrace?.beforeHeaders.find((field) => field.name === 'ttl')?.value).toBe('8');
    expect(forwardingTrace?.afterHeaders.find((field) => field.name === 'ttl')?.value).toBe('7');
  });

  it('reports network unreachable when the router has no route', () => {
    const topology = routedTopology();
    const snapshot = topology.engine.state.topologySnapshot;
    const sourceToRouter = snapshot.links.find(
      (candidate) => candidate.sourceNodeId === topology.sourceNodeId || candidate.targetNodeId === topology.sourceNodeId,
    );
    const engine = new NetworkRuntimeEngine(14);
    engine.handle({
      type: 'start',
      snapshot: topologySnapshot({
        nodes: snapshot.nodes,
        links: sourceToRouter === undefined ? [] : [sourceToRouter],
        deviceConfigurations: snapshot.deviceConfigurations,
        interfaceConfigurations: snapshot.interfaceConfigurations,
        manualRoutesByNodeId: snapshot.manualRoutesByNodeId,
      }),
      seed: 14,
      initialTimeMilliseconds: 0,
    });

    const result = engine.sendIcmpEcho(topology.sourceNodeId, '30.0.0.2', { identifier: 142, sequenceNumber: 1 });

    expect(result).toMatchObject({ type: 'icmpError', kind: 'destinationNetworkUnreachable' });
  });

  it('reports host unreachable when a connected target never answers ARP', () => {
    const sourceNodeId = uuid(0x1481);
    const sourcePortId = uuid(0x1491);
    const engine = startedEngine({
      nodes: [pc(sourceNodeId, sourcePortId)],
      links: [],
      deviceConfigurations: new Map([[sourceNodeId, device('192.168.50.10')]]),
    });

    const result = engine.sendIcmpEcho(sourceNodeId, '192.168.50.99', { identifier: 143, sequenceNumber: 1 });

    expect(result).toMatchObject({ type: 'icmpError', kind: 'destinationHostUnreachable' });
  });
});

describe('switch', () => {
  it('learns, floods, forwards, filters and ages its address table', () => {
    const hostAId = uuid(0x1a01);
    const hostAPortId = uuid(0x1a11);
    const hostBId = uuid(0x1a02);
    const hostBPortId = uuid(0x1a12);
    const hostCId = uuid(0x1a03);
    const hostCPortId = uuid(0x1a13);
    const switchId = uuid(0x1a04);
    const switchPorts = [1, 2, 3].map((index) => portSnapshot(uuid(0x1a20 + index), `sw${index}`));
    const engine = startedEngine(
      {
        nodes: [
          pc(hostAId, hostAPortId),
          pc(hostBId, hostBPortId),
          pc(hostCId, hostCPortId),
          { id: switchId, kind: 'networkSwitch', ports: switchPorts },
        ],
        links: [
          link(hostAId, hostAPortId, switchId, switchPorts[0]!.id),
          link(hostBId, hostBPortId, switchId, switchPorts[1]!.id),
          link(hostCId, hostCPortId, switchId, switchPorts[2]!.id),
        ],
        switchConfigurationsByNodeId: new Map([[switchId, switchConfiguration('Classroom', 1_000)]]),
      },
      20,
    );
    const macA = stableMacAddress(hostAPortId);
    const macB = stableMacAddress(hostBPortId);
    const payload: EthernetPayload = {
      kind: 'arp',
      packet: {
        operation: 'request',
        senderMacAddress: macA,
        senderIPAddress: '192.0.2.1',
        targetMacAddress: UNSPECIFIED_MAC_ADDRESS,
        targetIPAddress: '192.0.2.200',
      },
    };
    const frame = (sourceMacAddress: string, destinationMacAddress: string): EthernetFrame => ({
      identity: engine.allocateFrameIdentity(),
      sourceMacAddress,
      destinationMacAddress,
      payload,
    });
    const switchTraces = (detail: string) =>
      engine.state.packetTraces.filter((trace) => trace.nodeId === switchId && trace.detail === detail);

    engine.sendEthernetFrame(hostAId, hostAPortId, frame(macA, '02:00:00:00:00:FE'));
    expect(switchTraces('switch unknown unicast flooding')).toHaveLength(2);

    engine.sendEthernetFrame(hostBId, hostBPortId, frame(macB, macA));
    expect(switchTraces('switch learned unicast forwarding')).toHaveLength(1);

    engine.sendEthernetFrame(hostAId, hostAPortId, frame(macA, macA));
    expect(switchTraces('switch filtered destination on incoming port').length).toBeGreaterThan(0);
    expect(engine.switchSatEntries(switchId)).toHaveLength(2);

    const retention = 1_000;
    const updateTimes = [...engine.state.switchForwardingUpdatedAtMillisecondsByNodeId.get(switchId)!.values()];
    const expirationDeadlines = updateTimes.map((time) => time + retention);
    const firstExpiration = Math.min(...expirationDeadlines);
    const lastExpiration = Math.max(...expirationDeadlines);

    engine.handle({ type: 'advance', toMilliseconds: firstExpiration - 1 });
    expect(engine.switchSatEntries(switchId)).toHaveLength(2);
    engine.handle({ type: 'advance', toMilliseconds: lastExpiration });
    expect(engine.switchSatEntries(switchId)).toEqual([]);
  });
});

describe('firewall', () => {
  it('accepts when inactive, and an active ICMP drop bypasses the ordered rules', () => {
    const routerId = uuid(0x1801);
    const portId = uuid(0x1811);
    const router: NodeSnapshot = { id: routerId, kind: 'router', ports: [portSnapshot(portId, 'rt1')] };
    const packet: IPv4Packet = {
      identity: 18,
      senderIPAddress: '203.0.113.10',
      receiverIPAddress: '10.0.0.1',
      timeToLive: 64,
      protocol: 'icmp',
      payload: { kind: 'icmp', message: icmpMessage('echoRequest') },
    };
    const interfaces = new Map([
      [interfaceKey(routerId, portId), { ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }],
    ]);

    const inactive = startedEngine(
      {
        nodes: [router],
        links: [],
        interfaceConfigurations: interfaces,
        firewallConfigurationsByNodeId: new Map([[routerId, firewallConfiguration()]]),
      },
      18,
    );
    expect(inactive.evaluateFirewall(packet, routerId).accepted).toBe(true);

    const active = startedEngine(
      {
        nodes: [router],
        links: [],
        interfaceConfigurations: interfaces,
        firewallConfigurationsByNodeId: new Map([
          [
            routerId,
            firewallConfiguration({
              isActive: true,
              defaultPolicy: 'accept',
              dropIcmp: true,
              rules: [firewallRule({ protocol: 'icmp', action: 'accept' })],
            }),
          ],
        ]),
      },
      18,
    );
    expect(active.evaluateFirewall(packet, routerId).accepted).toBe(false);
    expect(active.state.firewallDecisions.at(-1)?.ruleIndex).toBeUndefined();
    expect(active.state.packetTraces.at(-1)?.operation).toBe('dropped');
  });

  it('lets the first matching TCP rule win, and established traffic bypasses SYN-only rules', () => {
    const { engine, routerId } = firewallEngine(
      firewallConfiguration({
        isActive: true,
        defaultPolicy: 'drop',
        filterSynSegmentsOnly: true,
        rules: [
          firewallRule({ port: 443, protocol: 'tcp', action: 'accept' }),
          firewallRule({ port: 443, protocol: 'tcp', action: 'drop' }),
        ],
      }),
    );

    const synDecision = engine.evaluateFirewall(tcpPacket(181, 443, TCP_FLAG.synchronize), routerId);
    expect(synDecision.accepted).toBe(true);
    expect(synDecision.ruleIndex).toBe(0);
    expect(engine.evaluateFirewall(tcpPacket(182, 22, TCP_FLAG.acknowledgement), routerId).accepted).toBe(true);
  });

  it('matches a UDP response by its explicit source port and the direct-network marker', () => {
    const { engine, routerId } = firewallEngine(
      firewallConfiguration({
        isActive: true,
        defaultPolicy: 'drop',
        rules: [
          firewallRule({
            sourceIPAddress: DIRECTLY_CONNECTED_SOURCE_MARKER,
            destinationIPAddress: '8.8.8.8',
            destinationSubnetMask: '255.255.255.255',
            port: 53,
            protocol: 'udp',
            action: 'accept',
          }),
        ],
      }),
    );
    const udp = (identity: number, sender: string, receiver: string, sourcePort: number, destinationPort: number): IPv4Packet => ({
      identity,
      senderIPAddress: sender,
      receiverIPAddress: receiver,
      timeToLive: 64,
      protocol: 'udp',
      payload: { kind: 'udp', datagram: { sourcePort, destinationPort, payload: new Uint8Array() } },
    });

    expect(engine.evaluateFirewall(udp(183, '10.0.0.20', '8.8.8.8', 50_001, 53), routerId).accepted).toBe(true);
    expect(engine.evaluateFirewall(udp(184, '8.8.8.8', '10.0.0.20', 53, 50_001), routerId).accepted).toBe(true);
  });
});

describe('packet loss and trace retention', () => {
  it('drops every physical delivery while global packet loss is on', () => {
    const firstNodeId = uuid(0x101);
    const firstPortId = uuid(0x102);
    const secondNodeId = uuid(0x103);
    const secondPortId = uuid(0x104);
    const engine = startedEngine({
      nodes: [pc(firstNodeId, firstPortId), pc(secondNodeId, secondPortId)],
      links: [link(firstNodeId, firstPortId, secondNodeId, secondPortId)],
      deviceConfigurations: new Map([
        [firstNodeId, device('10.0.0.1')],
        [secondNodeId, device('10.0.0.2')],
      ]),
    });
    const frame: EthernetFrame = {
      identity: engine.allocateFrameIdentity(),
      sourceMacAddress: stableMacAddress(firstPortId),
      destinationMacAddress: ETHERNET_BROADCAST_MAC_ADDRESS,
      payload: {
        kind: 'arp',
        packet: {
          operation: 'request',
          senderMacAddress: stableMacAddress(firstPortId),
          senderIPAddress: '10.0.0.1',
          targetMacAddress: UNSPECIFIED_MAC_ADDRESS,
          targetIPAddress: '10.0.0.2',
        },
      },
    };

    engine.setGlobalPacketLossEnabled(true);
    engine.sendEthernetFrame(firstNodeId, firstPortId, frame);

    expect(engine.state.globalPacketLossEnabled).toBe(true);
    expect(engine.state.arpCachesByNodeId.get(secondNodeId)?.size ?? 0).toBe(0);
    expect(
      engine.state.packetTraces.some(
        (trace) =>
          trace.frameIdentity === frame.identity &&
          trace.nodeId === secondNodeId &&
          trace.operation === 'dropped' &&
          trace.detail === 'global packet-loss simulation',
      ),
    ).toBe(true);

    engine.setGlobalPacketLossEnabled(false);
    engine.sendEthernetFrame(firstNodeId, firstPortId, frame);

    expect(engine.state.globalPacketLossEnabled).toBe(false);
    expect(engine.state.arpCachesByNodeId.get(secondNodeId)?.get('10.0.0.1')?.macAddress).toBe(
      stableMacAddress(firstPortId),
    );
  });

  it('resets packet loss across stop and restart', () => {
    const engine = new NetworkRuntimeEngine(42);
    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 42, initialTimeMilliseconds: 0 });
    engine.setGlobalPacketLossEnabled(true);
    expect(engine.state.globalPacketLossEnabled).toBe(true);

    engine.handle({ type: 'stop' });
    expect(engine.state.globalPacketLossEnabled).toBe(false);

    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 42, initialTimeMilliseconds: 0 });
    expect(engine.state.globalPacketLossEnabled).toBe(false);
  });

  it('evicts the oldest trace events in configured batches', () => {
    const nodeId = uuid(0x2001);
    const interfaceId = uuid(0x2002);
    const engine = new NetworkRuntimeEngine(20, traceRetentionPolicy(3, 2));

    for (let identity = 1; identity <= 6; identity += 1) {
      engine.recordTrace({
        frameIdentity: identity,
        nodeId,
        interfaceId,
        direction: 'outbound',
        layer: 'dataLink',
        operation: 'sent',
        detail: `trace-${identity}`,
      });
    }

    expect(engine.state.packetTraces.map((trace) => trace.id)).toEqual([5, 6]);
    expect(engine.state.packetTraces.map((trace) => trace.detail)).toEqual(['trace-5', 'trace-6']);
    expect(engine.state.discardedPacketTraceCount).toBe(4);
    expect(engine.state.nextTraceIdentity).toBe(7);
  });
});

describe('TCP sockets', () => {
  /** Two PCs on one cable, started. */
  const pair = (first: number, second: number, prefix: string) => {
    const [clientId, clientPortId, serverId, serverPortId] = [first, first + 0x10, second, second + 0x10].map(uuid) as [
      string,
      string,
      string,
      string,
    ];
    const engine = startedEngine({
      nodes: [pc(clientId, clientPortId), pc(serverId, serverPortId)],
      links: [link(clientId, clientPortId, serverId, serverPortId)],
      deviceConfigurations: new Map([
        [clientId, device(`${prefix}.1`)],
        [serverId, device(`${prefix}.2`)],
      ]),
    });
    return { engine, clientId, serverId };
  };

  it('makes an established client and an accepted server socket with the three-way handshake', () => {
    const { engine, clientId, serverId } = pair(0x1701, 0x1702, '10.17.0');
    const listenerId = engine.openTcpServerSocket(serverId, 8080)!;
    const clientSocketId = engine.openTcpClientSocket(clientId, '10.17.0.2', 8080)!;

    expect(engine.connectTcp(clientSocketId)).toBe(true);
    const acceptedSocketId = engine.state.tcpAcceptedSocketIdsByListenerId.get(listenerId)![0]!;
    expect(engine.state.socketsById.get(clientSocketId)?.tcpState).toBe('ESTABLISHED');
    expect(engine.state.socketsById.get(acceptedSocketId)?.tcpState).toBe('ESTABLISHED');
    expect(engine.state.packetTraces.some((trace) => trace.detail === 'TCP SYN_SENT -> ESTABLISHED')).toBe(true);
    expect(engine.state.packetTraces.some((trace) => trace.detail === 'TCP SYN_RCVD -> ESTABLISHED')).toBe(true);
  });

  it("sends segments of Java's MSS and reassembles one message up to the push", () => {
    const { engine, clientId, serverId } = pair(0x1721, 0x1722, '10.18.0');
    const listenerId = engine.openTcpServerSocket(serverId, 8081)!;
    const clientSocketId = engine.openTcpClientSocket(clientId, '10.18.0.2', 8081)!;
    expect(engine.connectTcp(clientSocketId)).toBe(true);
    const acceptedSocketId = engine.state.tcpAcceptedSocketIdsByListenerId.get(listenerId)![0]!;
    const payload = Uint8Array.from({ length: 3_001 }, (_, index) => index % 251);

    expect(engine.sendTcp(clientSocketId, payload)).toBe(true);
    expect(engine.receiveTcp(acceptedSocketId)).toEqual(payload);
    const payloadLengths = engine.state.packetTraces
      .filter((trace) => trace.detail?.startsWith('TCP data') === true)
      .map((trace) => Number(trace.afterHeaders.find((header) => header.name === 'payloadLength')?.value));
    expect(payloadLengths).toEqual([1_460, 1_460, 81]);
  });

  it('retransmits a connection attempt at most three times and releases the port', () => {
    const clientId = uuid(0x1741);
    const engine = startedEngine({ nodes: [pc(clientId, uuid(0x1751))], links: [], deviceConfigurations: new Map([[clientId, device('10.19.0.1')]]) });
    const socketId = engine.openTcpClientSocket(clientId, '10.19.0.99', 80)!;
    const localPort = engine.state.socketsById.get(socketId)!.localPort;

    expect(engine.connectTcp(socketId)).toBe(false);
    expect(engine.state.tcpSessionsById.get(socketId)?.sendAttempts).toBe(3);
    expect(engine.state.socketsById.get(socketId)?.tcpState).toBe('CLOSED');
    expect(engine.isTcpPortReserved(clientId, localPort)).toBe(false);
    expect(engine.state.packetTraces.filter((trace) => trace.detail?.includes('TCP SYN retransmission') === true)).toHaveLength(2);
  });

  it('drops a SYN+ACK at a listening port with no connection of its own', () => {
    const { engine, clientId, serverId } = pair(0x1781, 0x1782, '10.21.0');
    const listenerId = engine.openTcpServerSocket(serverId, 7000)!;
    const segment = { sourcePort: 80, destinationPort: 7000, sequenceNumber: 5, acknowledgementNumber: 9, flags: TCP_FLAG.synchronize | TCP_FLAG.acknowledgement, payload: new Uint8Array() };
    engine.sendIPv4Packet(clientId, {
      identity: engine.allocatePacketIdentity(),
      senderIPAddress: '10.21.0.1',
      receiverIPAddress: '10.21.0.2',
      timeToLive: 64,
      protocol: 'tcp',
      payload: { kind: 'tcp', segment },
    });
    engine.handle({ type: 'advance', toMilliseconds: engine.state.currentTimeMilliseconds + 1_000 });

    expect(engine.state.tcpAcceptedSocketIdsByListenerId.get(listenerId)).toEqual([]);
    expect(engine.state.packetTraces.some((trace) => trace.detail === 'TCP destination port closed')).toBe(true);
  });

  it('names the flags of a segment in Swift\'s order', () => {
    expect(tcpFlagDescription(TCP_FLAG.acknowledgement | TCP_FLAG.push | TCP_FLAG.reset | TCP_FLAG.synchronize | TCP_FLAG.finish)).toBe(
      'FIN+SYN+RST+PSH+ACK',
    );
    expect(tcpFlagDescription(TCP_FLAG.synchronize | TCP_FLAG.acknowledgement)).toBe('SYN+ACK');
    expect(tcpFlagDescription(0)).toBe('');
  });

  it("closes with Java's FIN states: the other end closes by itself, TIME_WAIT lasts three round trips", () => {
    const { engine, clientId, serverId } = pair(0x1761, 0x1762, '10.20.0');
    const listenerId = engine.openTcpServerSocket(serverId, 9090)!;
    const clientSocketId = engine.openTcpClientSocket(clientId, '10.20.0.2', 9090)!;
    expect(engine.connectTcp(clientSocketId)).toBe(true);
    const serverSocketId = engine.state.tcpAcceptedSocketIdsByListenerId.get(listenerId)![0]!;

    engine.closeTcpSocket(clientSocketId);
    // Java's socket thread leaves listen on the FIN and closes without the application.
    const serverStates = engine.state.packetTraces
      .filter((trace) => trace.nodeId === serverId && trace.detail?.startsWith('TCP ') === true && trace.detail.includes(' -> '))
      .map((trace) => trace.detail);
    expect(serverStates.slice(-3)).toEqual(['TCP ESTABLISHED -> CLOSE_WAIT', 'TCP CLOSE_WAIT -> LAST_ACK', 'TCP LAST_ACK -> CLOSED']);
    expect(engine.state.socketsById.get(clientSocketId)?.tcpState).toBe('TIME_WAIT');
    engine.closeTcpSocket(serverSocketId);
    expect(engine.state.socketsById.get(serverSocketId)?.tcpState).toBe('CLOSED');

    // Java's defaultTimeout: three round trips of 2750 ms at 90 %.
    const timeWait = engine.state.packetTraces.find((trace) => trace.nodeId === clientId && trace.detail?.endsWith('-> TIME_WAIT') === true)!
      .timeMilliseconds;
    engine.handle({ type: 'advance', toMilliseconds: timeWait + 3 * 2_750 - 1 });
    expect(engine.state.socketsById.get(clientSocketId)?.tcpState).toBe('TIME_WAIT');
    engine.handle({ type: 'advance', toMilliseconds: timeWait + 3 * 2_750 });
    expect(engine.state.socketsById.get(clientSocketId)?.tcpState).toBe('CLOSED');
    expect(engine.state.socketsById.get(listenerId)?.tcpState).toBe('LISTEN');
  });

  it('ends TIME_WAIT with the next segment that comes', () => {
    const { engine, clientId, serverId } = pair(0x1771, 0x1772, '10.21.0');
    engine.openTcpServerSocket(serverId, 9090);
    const clientSocketId = engine.openTcpClientSocket(clientId, '10.21.0.2', 9090)!;
    expect(engine.connectTcp(clientSocketId)).toBe(true);
    engine.closeTcpSocket(clientSocketId);
    expect(engine.state.socketsById.get(clientSocketId)?.tcpState).toBe('TIME_WAIT');

    const localPort = engine.state.socketsById.get(clientSocketId)!.localPort;
    engine.sendIPv4Packet(serverId, {
      identity: 77,
      senderIPAddress: '10.21.0.2',
      receiverIPAddress: '10.21.0.1',
      timeToLive: 64,
      protocol: 'tcp',
      payload: {
        kind: 'tcp',
        segment: { sourcePort: 9090, destinationPort: localPort, sequenceNumber: 1, acknowledgementNumber: 0, flags: TCP_FLAG.acknowledgement, payload: new Uint8Array() },
      },
    });
    engine.handle({ type: 'advance', toMilliseconds: engine.state.currentTimeMilliseconds + 100 });

    expect(engine.state.socketsById.get(clientSocketId)?.tcpState).toBe('CLOSED');
  });

  it('accepts on the socket that listened, and listens on with the next sequence number', () => {
    const [firstId, firstPortId, secondId, secondPortId, serverId, serverPortId, switchId] = [
      0x1781, 0x1782, 0x1783, 0x1784, 0x1785, 0x1786, 0x1787,
    ].map(uuid) as [string, string, string, string, string, string, string];
    const switchPorts = [0x1791, 0x1792, 0x1793].map(uuid) as [string, string, string];
    const engine = startedEngine({
      nodes: [
        pc(firstId, firstPortId),
        pc(secondId, secondPortId),
        pc(serverId, serverPortId),
        { id: switchId, kind: 'networkSwitch', ports: switchPorts.map((id, index) => portSnapshot(id, `sw${index + 1}`)) },
      ],
      links: [
        link(firstId, firstPortId, switchId, switchPorts[0]),
        link(secondId, secondPortId, switchId, switchPorts[1]),
        link(serverId, serverPortId, switchId, switchPorts[2]),
      ],
      deviceConfigurations: new Map([
        [firstId, device('10.22.0.1')],
        [secondId, device('10.22.0.2')],
        [serverId, device('10.22.0.3')],
      ]),
    });
    // Java counts initial sequence numbers over all TCP sockets: the listener 1, the first client 2,
    // the listener's successor 3 (made when the first is accepted), the second client 4.
    const listenerId = engine.openTcpServerSocket(serverId, 80)!;
    expect(engine.connectTcp(engine.openTcpClientSocket(firstId, '10.22.0.3', 80)!)).toBe(true);
    expect(engine.connectTcp(engine.openTcpClientSocket(secondId, '10.22.0.3', 80)!)).toBe(true);

    const accepted = engine.state.tcpAcceptedSocketIdsByListenerId.get(listenerId)!;
    expect(accepted.map((id) => engine.state.tcpSessionsById.get(id)?.nextSendSequenceNumber)).toEqual([1_000_001, 3_000_001]);
  });
});

describe('DHCP', () => {
  // Java's round trip time at the default speed of 90 %: 50 cables of 55 ms.
  const RTT = 2_750;

  const serverAndClient = (first: number, serverIp: string, client: DeviceConfiguration, server: Partial<DhcpServerConfiguration>) => {
    const [serverId, serverPortId, clientId, clientPortId] = [first, first + 1, first + 2, first + 3].map(uuid) as [string, string, string, string];
    const engine = startedEngine(
      {
        nodes: [pc(serverId, serverPortId), pc(clientId, clientPortId)],
        links: [link(serverId, serverPortId, clientId, clientPortId)],
        deviceConfigurations: new Map([
          [serverId, device(serverIp)],
          [clientId, client],
        ]),
        dhcpClientConfigurationsByNodeId: new Map([[clientId, { isEnabled: true }]]),
        dhcpServerConfigurationsByNodeId: new Map([[serverId, dhcpServerConfiguration({ isActive: true, ...server })]]),
      },
      16,
    );
    return { engine, serverId, clientId, clientPortId };
  };

  /** Hosts on one switch, each with its device configuration; the first is the DHCP client. */
  const onSwitch = (first: number, devices: DeviceConfiguration[], servers: Map<string, Partial<DhcpServerConfiguration>> = new Map()) => {
    const switchId = uuid(first);
    const hosts = devices.map((_, index) => ({ id: uuid(first + 1 + 2 * index), portId: uuid(first + 2 + 2 * index), switchPortId: uuid(first + 0x80 + index) }));
    const switchNode: NodeSnapshot = {
      id: switchId,
      kind: 'networkSwitch',
      ports: hosts.map((host, index) => portSnapshot(host.switchPortId, `sw${index + 1}`)),
    };
    const hostIds = hosts.map((host) => host.id);
    const engine = startedEngine(
      {
        nodes: [switchNode, ...hosts.map((host) => pc(host.id, host.portId))],
        links: hosts.map((host) => link(host.id, host.portId, switchId, host.switchPortId)),
        deviceConfigurations: new Map(devices.map((each, index) => [hostIds[index]!, each])),
        dhcpClientConfigurationsByNodeId: new Map([[hostIds[0]!, { isEnabled: true }]]),
        dhcpServerConfigurationsByNodeId: new Map(
          [...servers].map(([ipAddress, server]) => [
            hostIds[devices.findIndex((each) => each.ipAddress === ipAddress)]!,
            dhcpServerConfiguration({ isActive: true, ...server }),
          ]),
        ),
      },
      16,
    );
    return { engine, hostIds };
  };

  /** When the node sent a DHCP message of the type. */
  const sentAt = (engine: NetworkRuntimeEngine, nodeId: string, type: string) =>
    engine.state.packetTraces
      .filter((trace) => trace.nodeId === nodeId && trace.layer === 'application' && trace.operation === 'sent' && trace.detail === type)
      .map((trace) => trace.timeMilliseconds);

  /** When the node broadcast an ARP packet with the detail. */
  const arpAt = (engine: NetworkRuntimeEngine, nodeId: string, detail: string) =>
    engine.state.packetTraces
      .filter((trace) => trace.nodeId === nodeId && trace.layer === 'dataLink' && trace.operation === 'created' && trace.detail === detail)
      .map((trace) => trace.timeMilliseconds);

  /** The link-local addresses the client draws with seed 16, as Java's selectArbitraryLinkLocalAddress skips known ones. */
  const linkLocalDraws = (count: number) => {
    const random = new JavaCompatibleRandom(16n ^ 0x4c4c_4144n);
    const draws: string[] = [];
    while (draws.length < count) {
      const address = `169.254.1.${Math.round(random.nextDouble() * 253) + 1}`;
      if (!draws.includes(address)) draws.push(address);
    }
    return draws;
  };

  it('completes DISCOVER, OFFER, REQUEST and ACK and applies the configuration', () => {
    const { engine, clientId } = serverAndClient(0x1601, '192.168.50.1', device('169.254.1.2', { mask: '0.0.0.0' }), {
      lowerBoundIPAddress: '192.168.50.20',
      upperBoundIPAddress: '192.168.50.29',
      gatewayIPAddress: '192.168.50.1',
      dnsServerIPAddress: '192.168.50.53',
      useOwnSettings: true,
    });

    engine.handle({ type: 'advance', toMilliseconds: 1_000 });
    // A client that runs goes on waiting; it does not start over.
    expect(engine.runDhcpClient(clientId)?.state).toBe('VALIDATE');
    engine.handle({ type: 'advance', toMilliseconds: 5_000 });

    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)).toEqual(
      deviceConfiguration('192.168.50.20', '255.255.255.0', '192.168.50.1', '192.168.50.53'),
    );
    const status = engine.state.dhcpClientStatusesByNodeId.get(clientId);
    expect([status?.state, status?.errorCount, status?.succeeded, status?.linkLocal]).toEqual(['FINISH', 0, true, false]);
    const leaseExpiry = [...engine.state.dhcpLeasesByIPAddress.values()][0]?.expiresAtMilliseconds;
    expect(leaseExpiry).toBeGreaterThan(engine.state.currentTimeMilliseconds);
    expect(leaseExpiry! - engine.state.currentTimeMilliseconds).toBeLessThanOrEqual(DHCP_DYNAMIC_LEASE_LIFETIME_MILLISECONDS);
    // One message per attempt.
    expect(sentAt(engine, clientId, 'DHCPDISCOVER')).toEqual([0]);
    // The OFFER comes after two cables' delay; the ARP check waits one round trip before the REQUEST.
    expect(arpAt(engine, clientId, 'ARP request for 192.168.50.20')).toEqual([110]);
    expect(sentAt(engine, clientId, 'DHCPREQUEST')).toEqual([110 + RTT]);
  });

  it('matches a static assignment regardless of case, with a lease that does not expire', () => {
    const clientPortId = uuid(0x1611 + 3);
    const { engine, clientId } = serverAndClient(0x1611, '10.0.0.1', device('0.0.0.0'), {
      lowerBoundIPAddress: '10.0.0.20',
      upperBoundIPAddress: '10.0.0.20',
      staticAssignments: [{ id: 'static', macAddress: stableMacAddress(clientPortId).toLowerCase(), ipAddress: '10.0.0.99' }],
    });

    engine.handle({ type: 'advance', toMilliseconds: 5_000 });

    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)?.ipAddress).toBe('10.0.0.99');
    expect([...engine.state.dhcpLeasesByIPAddress.values()][0]?.expiresAtMilliseconds).toBeUndefined();
  });

  it('takes a link-local address after the fifth timeout and does not give back the old one', () => {
    const clientId = uuid(0x1621);
    const engine = startedEngine(
      {
        nodes: [pc(clientId, uuid(0x1622))],
        links: [],
        deviceConfigurations: new Map([[clientId, deviceConfiguration('169.254.5.7', '255.255.0.0', '169.254.0.1', '169.254.0.53')]]),
        dhcpClientConfigurationsByNodeId: new Map([[clientId, { isEnabled: true }]]),
      },
      16,
    );

    engine.handle({ type: 'advance', toMilliseconds: 0 });
    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)?.ipAddress).toBe('0.0.0.0');
    expect(engine.state.dhcpClientStatusesByNodeId.get(clientId)?.state).toBe('DISCOVER');

    engine.handle({ type: 'advance', toMilliseconds: 5 * RTT - 1 });
    expect(engine.state.dhcpClientStatusesByNodeId.get(clientId)?.errorCount).toBe(4);
    engine.handle({ type: 'advance', toMilliseconds: 20_000 });

    const [address] = linkLocalDraws(1);
    expect(sentAt(engine, clientId, 'DHCPDISCOVER')).toEqual([0, RTT, 2 * RTT, 3 * RTT, 4 * RTT]);
    expect(arpAt(engine, clientId, `ARP request for ${address}`)).toEqual([5 * RTT]);
    expect(arpAt(engine, clientId, `ARP reply for ${address}`)).toEqual([6 * RTT]);
    // Java: the address, its /16 mask and no gateway; the DNS server stays.
    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)).toEqual(
      deviceConfiguration(address!, '255.255.0.0', '', '169.254.0.53'),
    );
    const status = engine.state.dhcpClientStatusesByNodeId.get(clientId);
    expect([status?.state, status?.errorCount, status?.succeeded, status?.linkLocal]).toEqual(['FINISH', 5, true, true]);
    expect(engine.state.dhcpClientContextsByNodeId.has(clientId)).toBe(false);
    expect(engine.state.socketsById.size).toBe(0);
  });

  it('sends the REQUEST again to the same server when it does not answer', () => {
    const { engine, serverId, clientId } = serverAndClient(0x1631, '10.0.0.1', device('0.0.0.0'), {
      lowerBoundIPAddress: '10.0.0.20',
      upperBoundIPAddress: '10.0.0.29',
    });
    engine.handle({ type: 'advance', toMilliseconds: 150 });
    expect(engine.state.dhcpClientStatusesByNodeId.get(clientId)?.state).toBe('VALIDATE');
    engine.closeSocket(engine.state.dhcpServerSocketIdsByNodeId.get(serverId)!);

    engine.handle({ type: 'advance', toMilliseconds: 110 + 5 * RTT });

    const requests = [1, 2, 3, 4, 5].map((attempt) => 110 + attempt * RTT);
    expect(sentAt(engine, clientId, 'DHCPREQUEST')).toEqual(requests);
    expect(sentAt(engine, clientId, 'DHCPDISCOVER')).toEqual([0]);
    const status = engine.state.dhcpClientStatusesByNodeId.get(clientId);
    expect([status?.state, status?.errorCount, status?.offeredIPAddress]).toEqual(['REQUEST', 4, '10.0.0.20']);
  });

  it('declines at once when another card answers for the offered address', () => {
    const { engine, hostIds } = onSwitch(
      0x1641,
      [device('0.0.0.0'), device('192.168.0.10'), device('192.168.0.100')],
      new Map([['192.168.0.10', { lowerBoundIPAddress: '192.168.0.100', upperBoundIPAddress: '192.168.0.100' }]]),
    );
    const clientId = hostIds[0]!;

    engine.handle({ type: 'advance', toMilliseconds: 1_000 });

    // The OFFER takes two cables to the client, the ARP request and the
    // holder's reply two each; the DECLINE follows the reply, not the round trip.
    const [check] = arpAt(engine, clientId, 'ARP request for 192.168.0.100');
    expect(check).toBe(220);
    expect(sentAt(engine, clientId, 'DHCPDECLINE')).toEqual([check! + 220]);
    expect(sentAt(engine, clientId, 'DHCPDISCOVER')).toEqual([0, check! + 220]);
    expect(sentAt(engine, clientId, 'DHCPREQUEST')).toEqual([]);
  });

  it('starts over with DISCOVER after a NAK, which is no failure', () => {
    const { engine, serverId, clientId } = serverAndClient(0x1671, '10.0.0.1', device('0.0.0.0'), {
      lowerBoundIPAddress: '10.0.0.20',
      upperBoundIPAddress: '10.0.0.29',
    });
    engine.handle({ type: 'advance', toMilliseconds: 150 });
    // While the client checks 10.0.0.20, the server bars it: the REQUEST gets a NAK.
    engine.state.dhcpBlacklistByServerNodeId.set(serverId, new Map([['10.0.0.20', 1_000_000]]));

    engine.handle({ type: 'advance', toMilliseconds: 10_000 });

    const nak = 110 + RTT + 110;
    expect(sentAt(engine, clientId, 'DHCPREQUEST')).toEqual([110 + RTT, nak + 110 + RTT]);
    expect(sentAt(engine, clientId, 'DHCPDISCOVER')).toEqual([0, nak]);
    const status = engine.state.dhcpClientStatusesByNodeId.get(clientId);
    expect([status?.state, status?.errorCount, status?.succeeded]).toEqual(['FINISH', 0, true]);
    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)?.ipAddress).toBe('10.0.0.21');
  });

  it('ignores an answer from another server than the one it asked', () => {
    const { engine, hostIds } = onSwitch(
      0x1681,
      [device('0.0.0.0'), device('192.168.0.10'), device('192.168.0.66')],
      new Map([['192.168.0.10', { lowerBoundIPAddress: '192.168.0.100', upperBoundIPAddress: '192.168.0.100' }]]),
    );
    const [clientId, , rogueId] = hostIds as [string, string, string];
    const clientMac = stableMacAddress(uuid(0x1681 + 2));
    const request = 220 + RTT;
    engine.handle({ type: 'advance', toMilliseconds: request + 10 });
    expect(engine.state.dhcpClientStatusesByNodeId.get(clientId)?.state).toBe('REQUEST');

    // A NAK for the client, but from 192.168.0.66; it arrives before the server's ACK.
    const rogue = engine.bindUdpSocket(rogueId, { localPort: 67 })!;
    const nak = encodeDhcpMessage({ type: 'DHCPNAK', yourIPAddress: '192.168.0.100', clientMacAddress: clientMac, serverIdentifier: '192.168.0.66' });
    engine.sendUdp(rogue, nak, { ipAddress: '255.255.255.255', port: 68 });
    engine.handle({ type: 'advance', toMilliseconds: 10_000 });

    expect(sentAt(engine, clientId, 'DHCPDISCOVER')).toEqual([0]);
    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)?.ipAddress).toBe('192.168.0.100');
  });

  it("takes the other server's queued OFFER at once after a DECLINE", () => {
    // Server A offers the address a third card has, server B another one; B's
    // OFFER comes while the client checks A's and waits in its queue.
    const { engine, hostIds } = onSwitch(
      0x1691,
      [device('0.0.0.0'), device('192.168.0.10'), device('192.168.0.20'), device('192.168.0.100')],
      new Map([
        ['192.168.0.10', { lowerBoundIPAddress: '192.168.0.100', upperBoundIPAddress: '192.168.0.100' }],
        ['192.168.0.20', { lowerBoundIPAddress: '192.168.0.200', upperBoundIPAddress: '192.168.0.200' }],
      ]),
    );
    const clientId = hostIds[0]!;

    engine.handle({ type: 'advance', toMilliseconds: 10_000 });

    const decline = 220 + 220;
    expect(sentAt(engine, clientId, 'DHCPDECLINE')).toEqual([decline]);
    expect(sentAt(engine, clientId, 'DHCPDISCOVER')).toEqual([0, decline]);
    expect(arpAt(engine, clientId, 'ARP request for 192.168.0.200')).toEqual([decline]);
    expect(sentAt(engine, clientId, 'DHCPREQUEST')).toEqual([decline + RTT]);
    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)?.ipAddress).toBe('192.168.0.200');
  });

  it('skips a link-local address the ARP cache knows', () => {
    const [known, next] = linkLocalDraws(2) as [string, string];
    const { engine, hostIds } = onSwitch(0x16a1, [
      device('0.0.0.0'),
      device(known, { mask: '255.255.0.0' }),
      device('169.254.9.9', { mask: '255.255.0.0' }),
    ]);
    const clientId = hostIds[0]!;
    engine.handle({ type: 'advance', toMilliseconds: 1_000 });
    engine.resolveMacAddress(hostIds[1]!, '169.254.9.9', { maxRetries: 1 });
    expect(engine.state.arpCachesByNodeId.get(clientId)?.has(known)).toBe(true);

    engine.handle({ type: 'advance', toMilliseconds: 20_000 });

    expect(arpAt(engine, clientId, `ARP request for ${known}`)).toEqual([]);
    expect(arpAt(engine, clientId, `ARP request for ${next}`)).toEqual([5 * RTT]);
    expect(engine.state.dhcpClientStatusesByNodeId.get(clientId)?.errorCount).toBe(5);
    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)?.ipAddress).toBe(next);
  });

  // Java's selectArbitraryLinkLocalAddress loop counts occupied addresses
  // but checks its limit only after a timeout, which cannot come there: it
  // searches without end. The engine gives up after the fifth, as
  // LINK_LOCAL_MAX_ERROR_COUNT intends, and keeps 0.0.0.0.
  it('gives up after five occupied link-local addresses (Java searches on without end)', () => {
    const occupied = linkLocalDraws(5);
    const { engine, hostIds } = onSwitch(0x1651, [device('0.0.0.0'), ...occupied.map((address) => device(address, { mask: '255.255.0.0' }))]);
    const clientId = hostIds[0]!;

    engine.handle({ type: 'advance', toMilliseconds: 30_000 });

    for (const address of occupied) expect(arpAt(engine, clientId, `ARP request for ${address}`)).toHaveLength(1);
    const status = engine.state.dhcpClientStatusesByNodeId.get(clientId);
    expect([status?.state, status?.errorCount, status?.succeeded]).toEqual(['FINISH', 10, false]);
    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)?.ipAddress).toBe('0.0.0.0');
    expect(engine.state.dhcpClientContextsByNodeId.has(clientId)).toBe(false);
  });

  // Java's ARP lookup waits on the whole ARP table: any entry that changes
  // ends the wait, and the address counts as free. The engine waits for the
  // address it checks, or the round trip.
  it('waits out the address check when an unrelated ARP packet comes (Java would stop waiting)', () => {
    const { engine, hostIds } = onSwitch(
      0x1661,
      [device('0.0.0.0'), device('192.168.0.10'), device('192.168.0.30')],
      new Map([['192.168.0.10', { lowerBoundIPAddress: '192.168.0.100', upperBoundIPAddress: '192.168.0.100' }]]),
    );
    const clientId = hostIds[0]!;
    engine.handle({ type: 'advance', toMilliseconds: 300 });
    expect(engine.state.dhcpClientStatusesByNodeId.get(clientId)?.state).toBe('VALIDATE');

    engine.resolveMacAddress(hostIds[2]!, '192.168.0.77', { maxRetries: 1 });
    expect(engine.state.arpCachesByNodeId.get(clientId)?.has('192.168.0.30')).toBe(true);
    engine.handle({ type: 'advance', toMilliseconds: 5_000 });

    const [check] = arpAt(engine, clientId, 'ARP request for 192.168.0.100');
    expect(sentAt(engine, clientId, 'DHCPREQUEST')).toEqual([check! + RTT]);
    expect(engine.state.topologySnapshot.deviceConfigurations.get(clientId)?.ipAddress).toBe('192.168.0.100');
  });

  it("writes and reads a message in Swift's text format", () => {
    const message = {
      type: 'DHCPOFFER' as const,
      yourIPAddress: '10.0.0.5',
      clientMacAddress: '02:00:00:00:00:01',
      subnetMask: '255.255.255.0',
      routerIPAddress: '10.0.0.1',
      dnsServerIPAddress: '10.0.0.53',
      requestedIPAddress: '',
      serverIdentifier: '10.0.0.1',
    };
    const text = new TextDecoder().decode(encodeDhcpMessage(message));
    expect(text).toBe(
      'DHCPOFFER\nyiaddr=10.0.0.5\nchaddr=02:00:00:00:00:01\nrouter=10.0.0.1\nsubnetmask=255.255.255.0\ndnsserver=10.0.0.53\nserverident=10.0.0.1',
    );
    const { requestedIPAddress: _empty, ...withoutEmpty } = message;
    expect(decodeDhcpMessage(new TextEncoder().encode(text))).toEqual(withoutEmpty);
    expect(decodeDhcpMessage(new TextEncoder().encode('dhcpack\nyiaddr=1.2.3.4\nnoise'))).toEqual({ type: 'DHCPACK', yourIPAddress: '1.2.3.4' });
    expect(decodeDhcpMessage(new TextEncoder().encode('HELLO'))).toBeUndefined();
  });
});