/**
 * Put the declared addresses on the machines.
 *
 * The lab provides the underlay, and an address is underlay — it is what a hosting
 * provider or a home router would have given the machine before any of our software ran.
 * So the lab assigns it, and the mesh is left to build everything above it.
 *
 * Addresses are applied by matching on MAC rather than interface name. A guest names its
 * interfaces by bus position — `enp5s0`, not `eth0` — so the name the hypervisor uses and
 * the name the guest uses are different, and matching by name silently configures the
 * wrong interface on a multi-homed machine.
 *
 * See novox/hq 02-DECISIONS/0031-the-lab-provides-the-underlay.md
 */

import type { Attachment, Scenario } from "../declaration/types.ts";
import { incus } from "../incus/client.ts";
import { macFor } from "./names.ts";
import { transitAddress } from "./router.ts";

export interface Wire {
  device: string;
  mac: string;
  addresses: string[];
  mtu: number | undefined;
}

/**
 * A systemd-networkd unit per interface. Static, because the declaration is the authority:
 * a scenario that let the hypervisor hand out addresses would be the lab supplying facts
 * the declaration is supposed to own.
 */
function networkUnit(wire: Wire): string {
  const lines = [
    "[Match]",
    `MACAddress=${wire.mac}`,
    "",
    "[Network]",
    ...wire.addresses.map((address) => `Address=${address}`),
    // No gateway and no DNS on purpose. Routing off the segment is a gateway's job, and a
    // scenario that pre-wired it would be arranging what it is supposed to observe.
    "IPv6AcceptRA=no",
  ];
  if (wire.mtu !== undefined) {
    lines.push("", "[Link]", `MTUBytes=${wire.mtu}`);
  }
  return lines.join("\n") + "\n";
}

/**
 * Give one link a static address through systemd-networkd, and wait until networkd says it is
 * configured.
 *
 * **Not `ip addr add`**, which is what this replaced and what cost a lab that could not finish.
 * An address set by hand leaves the link `configuring` for ever, because networkd is still
 * waiting to configure something it was never told about. `systemd-networkd-wait-online` then
 * never returns — its timeout is `infinity` — so `network-online.target` is never reached, and
 * **anything ordered after it never starts**. On these machines that is Docker, which meant
 * `docker load` blocked on a socket whose daemon was queued behind a target that would never
 * come. The lab stalled for thirty-five minutes with nothing to say.
 *
 * Every machine already did it this way. The registry did not, and it was the only one that
 * needed Docker before anything else ran.
 */
export async function addressLink(
  instanceName: string,
  wire: Wire,
  index = 0,
): Promise<void> {
  const unit = networkUnit(wire);
  await incus(
    ["exec", instanceName, "--", "sh", "-c",
     `mkdir -p /etc/systemd/network && cat > /etc/systemd/network/10-mlab-${index}.network <<'MLAB'\n${unit}MLAB`],
    30_000,
  );
  await incus(["exec", instanceName, "--", "systemctl", "enable", "--now", "systemd-networkd"], 60_000);
  await incus(["exec", instanceName, "--", "systemctl", "restart", "systemd-networkd"], 60_000);
}

export async function applyAddresses(
  scenario: Scenario,
  instanceId: string,
  machineNames: Map<string, string>,
  log: (message: string) => void = () => {},
): Promise<void> {
  for (const [machine, spec] of Object.entries(scenario.machines)) {
    if (spec.at === "detached") continue;
    const name = machineNames.get(machine);
    if (!name) continue;

    const wires: Wire[] = [];
    for (const [index, attachment] of spec.at.entries()) {
      wires.push({
        device: `eth${index}`,
        mac: macFor(instanceId, machine, index),
        addresses: attachment.address.map((address) => withPrefix(scenario, attachment.segment, address)),
        mtu: scenario.segments[attachment.segment]?.mtu,
      });
    }

    for (const [index, wire] of wires.entries()) {
      const unit = networkUnit(wire);
      await incus(
        ["exec", name, "--", "sh", "-c",
         `mkdir -p /etc/systemd/network && cat > /etc/systemd/network/10-mlab-${index}.network <<'MLAB'\n${unit}MLAB`],
        30_000,
      );
    }

    await incus(["exec", name, "--", "systemctl", "enable", "--now", "systemd-networkd"], 60_000);
    await incus(["exec", name, "--", "systemctl", "restart", "systemd-networkd"], 60_000);
    log(`  addressed ${machine} (${wires.map((w) => w.addresses.join(",")).join(" | ")})`);
  }
}

/**
 * systemd-networkd wants a prefix length on the address. The declaration gives a bare
 * address and the segment gives the range, so the two are combined here rather than making
 * every scenario repeat the prefix on every machine.
 */
function withPrefix(scenario: Scenario, segment: string, address: string): string {
  const ranges = scenario.segments[segment]?.cidr ?? [];
  const wantV6 = address.includes(":");
  for (const range of ranges) {
    const slash = range.lastIndexOf("/");
    if (slash === -1) continue;
    const isV6 = range.slice(0, slash).includes(":");
    if (isV6 === wantV6) return `${address}${range.slice(slash)}`;
  }
  return address;
}

/**
 * Point each machine at the router serving its segment.
 *
 * The route is the machine's, not the mesh's — a default route is what a home network hands
 * out, and a machine that could not reach beyond its own segment would be reproducing the
 * wrong topology. What the lab still does not supply is the overlay: no peers, no hub, no
 * names.
 *
 * The router's inside address is the first host address of the range, chosen rather than
 * declared because a scenario has nothing to say about it.
 */
export async function applyDefaultRoutes(
  scenario: Scenario,
  machineNames: Map<string, string>,
  log: (message: string) => void = () => {},
): Promise<void> {
  for (const [machine, spec] of Object.entries(scenario.machines)) {
    if (spec.at === "detached") continue;
    const name = machineNames.get(machine);
    if (!name) continue;

    // A machine behind a gateway routes through it. A machine sitting directly on a public
    // segment routes through transit instead — otherwise it can reach its own network and
    // nothing else, which is not what being on the internet means.
    const behind = spec.at.find((a) => scenario.segments[a.segment]?.gateway);
    if (!behind) {
      await routeViaTransit(scenario, spec, name);
      continue;
    }

    const index = spec.at.indexOf(behind);
    for (const range of scenario.segments[behind.segment]?.cidr ?? []) {
      const slash = range.lastIndexOf("/");
      if (slash === -1) continue;
      const base = range.slice(0, slash);
      const via = base.includes(":")
        ? `${base.replace(/::$/, "")}::1`
        : (() => { const o = base.split("."); o[3] = "1"; return o.join("."); })();
      const family = base.includes(":") ? "-6" : "-4";
      await incus(
        ["exec", name, "--", "sh", "-c",
         `ip ${family} route replace default via ${via} dev $(ip -o link | awk -F': ' 'NR==${index + 2}{print $2}') 2>/dev/null || true`],
        30_000,
      );
    }
    log(`  routed ${machine} via its gateway on ${behind.segment}`);
  }
}

/** A machine on a public segment reaches the other public networks through transit. */
async function routeViaTransit(
  scenario: Scenario,
  spec: { at: Attachment[] | "detached" },
  name: string,
): Promise<void> {
  if (spec.at === "detached") return;
  const onPublic = spec.at.find((a) => scenario.segments[a.segment]?.kind === "public");
  if (!onPublic) return;

  const index = spec.at.indexOf(onPublic);
  for (const cidr of scenario.segments[onPublic.segment]?.cidr ?? []) {
    const via = transitAddress(cidr);
    if (!via) continue;
    const gateway = via.slice(0, via.lastIndexOf("/"));
    const family = gateway.includes(":") ? "-6" : "-4";
    await incus(
      ["exec", name, "--", "sh", "-c",
       `ip ${family} route replace default via ${gateway} dev $(ip -o link | awk -F': ' 'NR==${index + 2}{print $2}') 2>/dev/null || true`],
      30_000,
    );
  }
}