// The route adapter: it provides `route` by writing route files for the predecessor's proxy.
//
// **Migration scaffolding, and it says so** (novox/hq ADR 0104). The mesh has one provider of
// `route`, its own proxy, and that proxy binds the two public ports on the machine's own network.
// On a node being adopted (ADR 0100) the predecessor's proxy holds those ports and serves every
// public name there, so the two cannot run at once — and until one of them does, no module that
// must be reachable by name can be taken at all.
//
// So this module answers the same provision by writing into the predecessor's own configuration
// instead of proxying itself. It receives exactly what the proxy receives — the contributions file
// at `receives.route` — and turns each contribution into one route file where the predecessor's
// file provider reads. The predecessor keeps every name it already serves and keeps renewing its
// certificates; a name whose module has migrated is served by the same proxy, pointing at the
// mesh's container instead of the predecessor's.
//
// Two rules hold the whole thing safe, and both are tested:
//
//   - **one file per contribution**, named so the mesh can recognise its own, and
//   - **it never touches a file it did not write** — not a file of the predecessor's, and not even
//     a `mesh-…` file that carries no marker of this module's.
//
// It is removed when the predecessor's proxy retires, at which point the mesh's own proxy takes
// the ports and already knows every route.

import { readdir, readFile, rename, rm, stat, writeFile } from "node:fs/promises";
import { join } from "node:path";

/**
 * One entry of the contributions file the mesh writes at `receives.route`.
 *
 * The same shape the mesh's own proxy reads (`mesh-control/examples/route-proxy`), because the
 * contract is the file and not the program — which is the whole reason one provider can stand in
 * for another without a module that publishes through it noticing.
 */
export interface Contribution {
  /** The module that asked for a route. */
  from?: string;
  /** The node it was assigned to. */
  node?: string;
  /**
   * Where the mesh says that machine is on the private network. Empty means this one — a workload
   * beside the predecessor's proxy is the ordinary case during a migration.
   */
  at?: string;
  values?: Record<string, unknown>;
}

/** What the predecessor needs to be told, and the facts about this one machine. */
export interface Settings {
  /** The directory the predecessor's file provider reads, on this machine. */
  dynamic: string;
  /** The entry point it serves public HTTPS on. */
  entrypoint: string;
  /**
   * The predecessor's certificate resolver, by its own name.
   *
   * **Its own, deliberately.** The predecessor holds the certificates for every public name and
   * renews them; naming its resolver means a migrated name is served from the certificate that
   * already exists. A resolver of the mesh's would ask a public authority for a certificate in the
   * same window the module is cut over, which is the risk ADR 0104 exists to remove.
   */
  resolver: string;
  /**
   * How the predecessor's proxy reaches this machine. It runs in a container, so the machine is
   * not `127.0.0.1` from where it stands.
   */
  machine: string;
}

/** One route this module is to keep in force. */
export interface Route {
  /** The public name, lowercased. */
  name: string;
  /** The module that contributed it, for the note inside the file. */
  from: string;
  /** Where the predecessor's proxy is to send it. */
  target: string;
}

/** What one pass changed. */
export interface Pass {
  written: string[];
  removed: string[];
  skipped: string[];
}

/** The line that marks a file as this module's. Recognition is by name *and* by this. */
export const marker = "# written by the novox mesh route-adapter (novox/hq ADR 0104)";

/** What this module's files are called, and so what it will consider removing. */
const ours = /^mesh-[A-Za-z0-9._*-]+\.yml$/;

/** What a public name may be made of. Anything else is refused rather than turned into a path. */
const aName = /^[a-z0-9*][a-z0-9.*-]*$/;

/** The defaults, which are also what the manifest's settings file says. */
export const defaults: Settings = {
  dynamic: "/services/traefik/dynamic",
  entrypoint: "websecure",
  resolver: "le",
  machine: "host.docker.internal",
};

/**
 * The settings, as the node set them.
 *
 * **A per-node fact, not a constant** (novox/hq ADR 0100, whose `ports` setting is the precedent).
 * Where the predecessor keeps the directory its file provider reads is true of one machine and of
 * nothing else, so it is a setting laid over the module's default rather than a number in the
 * catalogue. It reaches the module the way cloudflare-dns's does: a `merge: "json"` file the mesh
 * composes and the container reads.
 */
export function settingsFrom(raw: unknown): Settings {
  const given = (raw ?? {}) as Record<string, unknown>;
  const text = (key: string, fallback: string): string => {
    const value = given[key];
    return typeof value === "string" && value.trim() !== "" ? value.trim() : fallback;
  };
  return {
    dynamic: text("dynamic", defaults.dynamic),
    entrypoint: text("entrypoint", defaults.entrypoint),
    resolver: text("certificate-resolver", defaults.resolver),
    machine: text("machine", defaults.machine),
  };
}

/**
 * The routes a contributions document asks for.
 *
 * A contribution the adapter cannot act on is skipped and said aloud rather than guessed at: the
 * mesh's proxy does the same, and a route invented from half a contribution is a name that answers
 * for something nobody asked for.
 */
export function routesFrom(document: unknown, machine: string): { routes: Route[]; skipped: string[] } {
  const given = ((document ?? {}) as { given?: unknown }).given;
  const entries: Contribution[] = Array.isArray(given) ? (given as Contribution[]) : [];
  const routes: Route[] = [];
  const skipped: string[] = [];
  for (const entry of entries) {
    const from = typeof entry.from === "string" && entry.from !== "" ? entry.from : "an unnamed module";
    const asked = entry.values?.["name"];
    const name = typeof asked === "string" ? asked.trim().toLowerCase() : "";
    if (name === "") {
      skipped.push(`${from} asked for a route and named nothing`);
      continue;
    }
    if (!aName.test(name)) {
      // Refused rather than sanitised: this name becomes a file name in a directory belonging to
      // something else, and quietly rewriting it is how a module ends up serving a name it never
      // asked for — or writing outside the directory altogether.
      skipped.push(`${from} asked for ${JSON.stringify(name)}, which is not a name this can write`);
      continue;
    }
    const port = asPort(entry.values?.["port"]);
    if (port === undefined) {
      skipped.push(`${from} asked for route ${name} and gave no usable port`);
      continue;
    }
    // Where the mesh says that machine is. Empty means this one, and this one is reached from
    // inside the predecessor's container by the machine's own name, not by loopback.
    const at = typeof entry.at === "string" && entry.at.trim() !== "" ? entry.at.trim() : machine;
    routes.push({ name, from, target: `http://${at}:${port}` });
  }
  return { routes, skipped };
}

/** The file one route is written to. The prefix is how the mesh recognises its own. */
export function fileNameFor(name: string): string {
  return `mesh-${name}.yml`;
}

/** The router and service name inside that file. One name, so the two halves cannot drift. */
export function routerNameFor(name: string): string {
  return `mesh-${name.replace(/[^a-z0-9-]/g, "-")}`;
}

/**
 * One route, in the shape the predecessor already reads.
 *
 * Written out rather than produced by a YAML library: the document has one shape, every value in
 * it is the mesh's own, and a dependency whose output could drift is a dependency that could stop
 * the predecessor parsing the directory it serves everything else from.
 */
export function routeFile(route: Route, settings: Settings): string {
  const id = routerNameFor(route.name);
  return [
    marker,
    `# ${route.from} contributed this route. It is removed when that contribution goes.`,
    "http:",
    "  routers:",
    `    ${id}:`,
    `      entryPoints: [${settings.entrypoint}]`,
    `      rule: Host(\`${route.name}\`)`,
    `      service: ${id}`,
    "      tls:",
    `        certResolver: ${settings.resolver}`,
    "        domains:",
    `          - main: ${route.name}`,
    "  services:",
    `    ${id}:`,
    "      loadBalancer:",
    "        servers:",
    `          - url: ${route.target}`,
    "",
  ].join("\n");
}

/**
 * Whether a file in the predecessor's directory is this module's to remove.
 *
 * **Two tests, not one.** The name says which files this module may consider at all; the marker
 * inside says it actually wrote this one. A file somebody else happened to call `mesh-something`
 * is not this module's, and deleting it because the name matched would be exactly the fault the
 * rule exists to prevent — the mesh is a guest in this directory.
 */
export function isOurs(fileName: string, body: string): boolean {
  return ours.test(fileName) && body.startsWith(marker);
}

/**
 * Bring the predecessor's directory in line with what the mesh contributed: one file per route,
 * this module's files that no route wants any more removed, and everything else untouched.
 *
 * A file whose content is already right is not rewritten, so an unchanged pass does not wake the
 * predecessor's watcher for nothing.
 */
export async function reconcile(routes: Route[], settings: Settings): Promise<Pass> {
  const directory = settings.dynamic;
  const present = await stat(directory).catch(() => undefined);
  if (!present?.isDirectory()) {
    // Named, and named in the operator's terms. On an adopted node this directory is the
    // predecessor's and the mesh only mounts it; absent, there is nothing to write into and
    // writing anyway would put route files somewhere nothing reads.
    throw new Error(
      `the predecessor's dynamic directory ${directory} is not there. It is this node's ` +
        `\`dynamic\` setting, and the module's container mounts it at the same path — so either ` +
        `the setting names somewhere else, or the mount does not follow it`,
    );
  }

  const wanted = new Map<string, string>();
  for (const route of routes) {
    wanted.set(fileNameFor(route.name), routeFile(route, settings));
  }

  const pass: Pass = { written: [], removed: [], skipped: [] };
  for (const [fileName, body] of [...wanted].sort()) {
    const at = join(directory, fileName);
    const already = await readFile(at, "utf8").catch(() => undefined);
    if (already === body) {
      continue;
    }
    if (already !== undefined && !isOurs(fileName, already)) {
      // A file of this module's name that this module did not write. Left exactly as it is: the
      // directory is the predecessor's, and the one thing this module may never do is overwrite
      // something it cannot show it owns.
      pass.skipped.push(`${fileName} is not this module's to write, so the route it wants is not served`);
      continue;
    }
    // Renamed into place rather than written in place: the predecessor watches this directory and
    // would otherwise read a file half written.
    const staged = at + ".mesh-staged";
    await writeFile(staged, body, { mode: 0o644 });
    await rename(staged, at);
    pass.written.push(fileName);
  }

  for (const fileName of (await readdir(directory)).sort()) {
    if (wanted.has(fileName) || !ours.test(fileName)) {
      continue;
    }
    const body = await readFile(join(directory, fileName), "utf8").catch(() => undefined);
    if (body === undefined || !isOurs(fileName, body)) {
      continue;
    }
    await rm(join(directory, fileName));
    pass.removed.push(fileName);
  }
  return pass;
}

/** What JSON makes of a port, which is a float even where it was written 8080. */
function asPort(value: unknown): number | undefined {
  const port = typeof value === "number" ? value : typeof value === "string" ? Number(value) : NaN;
  if (!Number.isInteger(port) || port < 1 || port > 65535) {
    return undefined;
  }
  return port;
}