import { lookup } from "node:dns/promises";
import http, { type IncomingMessage } from "node:http";
import https from "node:https";
import { DatabaseSync } from "node:sqlite";
import ipaddr from "ipaddr.js";
import { type DefaultTreeAdapterTypes as Tree, parse } from "parse5";
import { HttpError, readBytes } from "./http.ts";
import { starUrl } from "./records.ts";

export type TitlePreview = { title: string; source: "og" | "html" | "url" };

const text = (node: Tree.Node) => ("value" in node ? node.value : "");
const clean = (raw: string | undefined) => {
  const title = raw?.replace(/\s+/gu, " ").trim() ?? "";
  return Array.from(title).slice(0, 300).join("");
};

export function parseTitle(
  html: string,
): { title: string; source: "og" | "html" } | undefined {
  let og: string | undefined;
  let heading: string | undefined;
  const visit = (node: Tree.Node) => {
    if ("tagName" in node && node.tagName === "meta" && !og) {
      const attrs = new Map(node.attrs.map(({ name, value }) => [name, value]));
      if (attrs.get("property")?.toLowerCase() === "og:title") {
        og = attrs.get("content");
      }
    }
    if ("tagName" in node && node.tagName === "title" && !heading) {
      heading = node.childNodes.map(text).join("");
    }
    if ("childNodes" in node) node.childNodes.forEach(visit);
  };
  visit(parse(html));
  const title = clean(og);
  if (title) return { title, source: "og" };
  const fallback = clean(heading);
  if (fallback) return { title: fallback, source: "html" };
}

export function publicAddress(address: string): boolean {
  try {
    return ipaddr.parse(address).range() === "unicast";
  } catch {
    return false;
  }
}

type Address = { address: string; family: number };

// Resolves a URL to one public address, which the request then pins so that
// DNS can't rebind it to something private mid-flight.
async function publicDestination(url: URL): Promise<Address | undefined> {
  if (
    !["http:", "https:"].includes(url.protocol) || url.port ||
    url.username || url.password
  ) return;
  const hostname = url.hostname.replace(/^\[|\]$/g, "");
  const addresses = await lookup(hostname, { all: true });
  if (!addresses.length || !addresses.every((a) => publicAddress(a.address))) {
    return;
  }
  return addresses.find(({ family }) => family === 4) ?? addresses[0];
}

function request(url: URL, { address, family }: Address) {
  return new Promise<IncomingMessage>((resolve, reject) => {
    const transport = url.protocol === "https:" ? https : http;
    transport.get(url, {
      lookup: (_hostname, options, callback) =>
        options.all
          ? callback(null, [{ address, family }])
          : callback(null, address, family),
      signal: AbortSignal.timeout(8000),
      maxHeaderSize: 8192,
      headers: {
        accept: "text/html, application/xhtml+xml",
        "accept-encoding": "identity",
        "user-agent": "Starboard title preview/1.0",
      },
    }, resolve).on("error", reject);
  });
}

const REDIRECTS = new Set([301, 302, 303, 307, 308]);
const HTML = /^\s*(text\/html|application\/xhtml\+xml)(?:\s*;|\s*$)/i;

// The page's HTML, following up to three redirects and re-checking each hop.
async function page(url: URL): Promise<string | undefined> {
  for (let hops = 0;; hops++) {
    const destination = await publicDestination(url);
    if (!destination) return;
    const response = await request(url, destination);
    const { statusCode, headers } = response;
    const location = headers.location;
    if (REDIRECTS.has(statusCode ?? 0) && location && hops < 3) {
      response.destroy();
      url = new URL(location, url);
      continue;
    }
    const contentType = headers["content-type"] ?? "";
    const encoding = headers["content-encoding"] ?? "identity";
    if (
      statusCode !== 200 || !HTML.test(contentType) || encoding !== "identity"
    ) {
      response.destroy();
      return;
    }
    const bytes = await readBytes(response, 8 * 1024 * 1024);
    const charset = /charset\s*=\s*([^;\s]+)/i.exec(contentType)?.[1];
    return new TextDecoder(charset ?? "utf-8").decode(bytes);
  }
}

export async function lookupTitle(link: string): Promise<TitlePreview> {
  try {
    const html = await page(new URL(link));
    const found = html && parseTitle(html);
    if (found) return found;
  } catch {
    /* Network and markup failures leave the URL as the editable title. */
  }
  return { title: link, source: "url" };
}

export class TitleRateLimit extends HttpError {
  constructor() {
    super(429, "Title lookup limit reached", { "retry-after": "600" });
  }
}
export class TitleBusy extends HttpError {
  constructor() {
    super(503, "Too many title lookups");
  }
}

export class TitleLookup {
  private active = 0;
  private lastPruned = -1;

  constructor(
    readonly db: DatabaseSync,
    readonly resolve: (url: string) => Promise<TitlePreview> = lookupTitle,
  ) {
    db.exec(`CREATE TABLE IF NOT EXISTS title_lookup (
      did TEXT PRIMARY KEY, bucket INTEGER NOT NULL, count INTEGER NOT NULL
    )`);
    db.exec(
      "CREATE INDEX IF NOT EXISTS title_lookup_bucket ON title_lookup(bucket)",
    );
  }

  async lookup(did: string, raw: string): Promise<TitlePreview> {
    const url = starUrl(raw);
    if (!url) throw new Error("Invalid link");
    if (this.active >= 8) throw new TitleBusy();
    const bucket = Math.floor(Date.now() / 600000);
    if (bucket !== this.lastPruned) {
      this.db.prepare("DELETE FROM title_lookup WHERE bucket < ?").run(
        bucket - 144,
      );
      this.lastPruned = bucket;
    }
    const row = this.db.prepare(`
      INSERT INTO title_lookup (did, bucket, count) VALUES (?, ?, 1)
      ON CONFLICT(did) DO UPDATE SET
        count = CASE WHEN title_lookup.bucket = excluded.bucket
          THEN title_lookup.count + 1 ELSE 1 END,
        bucket = excluded.bucket
      WHERE title_lookup.bucket != excluded.bucket OR title_lookup.count < 10
      RETURNING count
    `).get(did, bucket);
    if (!row) throw new TitleRateLimit();
    this.active++;
    try {
      return await this.resolve(url);
    } finally {
      this.active--;
    }
  }
}