// A small HTML reader, pure: no DOM, no dependency. Shared by the X Article
// reader (xArticle.ts) and the XenForo thread parser (xenforoParse.ts).
//
// For HTML a browser serialised (outerHTML): attributes are double-quoted, void
// elements are unclosed, text escapes only & < > and nbsp. It tolerates more
// (single quotes, bare values, stray end tags), but it is not a general parser.

export type HtmlElement = {
  tag: string;
  attrs: Record<string, string>;
  children: HtmlNode[];
};
export type HtmlNode = HtmlElement | string;

const VOID_TAGS = new Set([
  "area", "base", "br", "col", "embed", "hr", "img", "input", "link", "meta",
  "param", "source", "track", "wbr",
]);
const RAW_TEXT_TAGS = new Set(["script", "style", "textarea", "title"]);

const NAMED_ENTITIES: Record<string, string> = {
  amp: "&", lt: "<", gt: ">", quot: '"', apos: "'", nbsp: "\u00a0",
  hellip: "…", mdash: "—", ndash: "–", lsquo: "‘", rsquo: "’", ldquo: "“",
  rdquo: "”", copy: "©", reg: "®", trade: "™",
};

export function decodeEntities(s: string): string {
  return s.replace(/&(#x[0-9a-f]+|#\d+|[a-z]+);/gi, (whole, name: string) => {
    if (name[0] === "#") {
      const code =
        name[1] === "x" || name[1] === "X"
          ? parseInt(name.slice(2), 16)
          : parseInt(name.slice(1), 10);
      return Number.isFinite(code) && code > 0 && code <= 0x10ffff
        ? String.fromCodePoint(code)
        : whole;
    }
    return NAMED_ENTITIES[name.toLowerCase()] ?? whole;
  });
}

const TAG_NAME_RE = /[A-Za-z][A-Za-z0-9:-]*/y;
const ATTR_RE = /\s*([^\s"'<>\/=]+)(?:\s*=\s*(?:"([^"]*)"|'([^']*)'|([^\s"'=<>`]+)))?/y;

// The document's top-level nodes, under a synthetic root element.
export function parseHtml(html: string): HtmlElement {
  const root: HtmlElement = { tag: "#root", attrs: {}, children: [] };
  // Lowered once: a raw-text element's end is searched for in it (a page holds
  // dozens of scripts, and lowering the whole page per script is quadratic).
  let lower: string | undefined;
  const stack: HtmlElement[] = [root];
  const top = () => stack[stack.length - 1];
  let i = 0;
  while (i < html.length) {
    const lt = html.indexOf("<", i);
    if (lt < 0) {
      top().children.push(decodeEntities(html.slice(i)));
      break;
    }
    if (lt > i) top().children.push(decodeEntities(html.slice(i, lt)));
    i = lt;
    if (html.startsWith("<!--", i)) {
      const end = html.indexOf("-->", i + 4);
      i = end < 0 ? html.length : end + 3;
      continue;
    }
    if (html[i + 1] === "!" || html[i + 1] === "?") {
      const end = html.indexOf(">", i);
      i = end < 0 ? html.length : end + 1;
      continue;
    }
    if (html[i + 1] === "/") {
      const end = html.indexOf(">", i);
      const name = html.slice(i + 2, end < 0 ? html.length : end).trim().toLowerCase();
      i = end < 0 ? html.length : end + 1;
      // Close up to the matching open element; a stray end tag is ignored.
      for (let k = stack.length - 1; k > 0; k--) {
        if (stack[k].tag === name) {
          stack.length = k;
          break;
        }
      }
      continue;
    }
    TAG_NAME_RE.lastIndex = i + 1;
    const nameMatch = TAG_NAME_RE.exec(html);
    if (!nameMatch) {
      // A "<" that opens no tag is text.
      top().children.push("<");
      i++;
      continue;
    }
    const tag = nameMatch[0].toLowerCase();
    let j = TAG_NAME_RE.lastIndex;
    const attrs: Record<string, string> = {};
    for (;;) {
      ATTR_RE.lastIndex = j;
      const a = ATTR_RE.exec(html);
      if (!a || a[0].length === 0) break;
      attrs[a[1].toLowerCase()] = decodeEntities(a[2] ?? a[3] ?? a[4] ?? "");
      j = ATTR_RE.lastIndex;
    }
    const close = html.indexOf(">", j);
    const selfClosing = close > 0 && html[close - 1] === "/";
    i = close < 0 ? html.length : close + 1;
    const el: HtmlElement = { tag, attrs, children: [] };
    top().children.push(el);
    if (RAW_TEXT_TAGS.has(tag)) {
      lower ??= html.toLowerCase();
      const end = lower.indexOf(`</${tag}`, i);
      const stop = end < 0 ? html.length : end;
      if (tag === "title" || tag === "textarea") el.children.push(decodeEntities(html.slice(i, stop)));
      const gt = end < 0 ? -1 : html.indexOf(">", end);
      i = gt < 0 ? html.length : gt + 1;
      continue;
    }
    if (!selfClosing && !VOID_TAGS.has(tag)) stack.push(el);
  }
  return root;
}