/**
 * A small Markdown renderer, for the player-facing documentation only.
 *
 * WHY NOT A LIBRARY. This project has no runtime dependencies at all, and the guide uses a small,
 * known subset of Markdown — headings, paragraphs, lists, tables, links, code spans, block quotes
 * and rules. A Markdown library would be the first dependency in the tree, pulled in to render five
 * files whose whole vocabulary fits below. `docs/` is written by hand, not by users, so this does
 * not have to survive hostile input; it has to render what those five documents actually contain
 * and fail loudly on anything else.
 *
 * WHY NOT HAND-WRITTEN HTML. The Markdown is the one copy (TODO #15a). An HTML twin drifts from it
 * on the first edit, which is the failure the whole documentation pass was about.
 *
 * ESCAPING IS UNCONDITIONAL. Every scrap of text goes through `esc` before any markup is added, and
 * the inline pass only ever inserts tags around already-escaped content. A `<` in the prose is a
 * less-than sign, not the start of an element — there is no raw-HTML passthrough, deliberately.
 */

/** HTML-escape. Ampersand first, or it double-escapes the entities added after it. */
export function esc(s: string): string {
  return s
    .replace(/&/g, '&amp;')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;');
}

/** A heading found while rendering, for the contents list the page builds from it. */
export type Heading = { level: number; text: string; id: string };

/**
 * `## 3. The shape of a Stage` → `the-shape-of-a-stage`.
 *
 * The leading number is dropped: it is a position in the document, and a link that carries it
 * breaks when a section is inserted above. Duplicate slugs get a numeric suffix rather than
 * silently pointing at the first one.
 */
function slug(text: string, taken: Set<string>): string {
  const base =
    text
      .toLowerCase()
      // A whole section number, dotted or not: "4.2 Local Operations" and "7. FAQ" both lose it.
      .replace(/^\d+(?:\.\d+)*[.)]?\s+/, '')
      .replace(/[^a-z0-9]+/g, '-')
      .replace(/^-+|-+$/g, '') || 'section';
  let id = base;
  for (let n = 2; taken.has(id); n++) id = `${base}-${n}`;
  taken.add(id);
  return id;
}

/**
 * Inline markup, applied to text that is ALREADY escaped.
 *
 * Order matters: code spans are taken out first and put back last, so `**` inside backticks stays
 * literal. That is not a corner case here — the rules reference quotes field names like
 * `**Version**` when describing the page header.
 */
function inline(escaped: string, linkHref: (href: string) => string): string {
  const code: string[] = [];
  let s = escaped.replace(/`([^`]+)`/g, (_m, body: string) => {
    code.push(`<code>${body}</code>`);
    return `\u0000${code.length - 1}\u0000`;
  });

  // Links: [text](target). The target is rewritten so a link between documents lands on the
  // rendered page rather than the Markdown source.
  s = s.replace(/\[([^\]]+)\]\(([^)\s]+)\)/g, (_m, text: string, href: string) => {
    const target = linkHref(href);
    const external = /^https?:/.test(target);
    return `<a href="${target}"${external ? ' target="_blank" rel="noopener"' : ''}>${text}</a>`;
  });

  // Bold before italic, or `**x**` is read as an empty italic wrapping a bold.
  s = s.replace(/\*\*([^*]+)\*\*/g, '<strong>$1</strong>');
  s = s.replace(/(^|[\s(])\*([^*\n]+)\*/g, '$1<em>$2</em>');

  return s.replace(/\u0000(\d+)\u0000/g, (_m, i: string) => code[Number(i)]!);
}

/** One table row's cells, from `| a | b |`. */
function cells(line: string): string[] {
  return line
    .replace(/^\s*\|/, '')
    .replace(/\|\s*$/, '')
    .split('|')
    .map((c) => c.trim());
}

/** `---`, `:--`, `--:` and `:-:` → the CSS alignment a column wants. */
function alignments(sep: string): (string | null)[] {
  return cells(sep).map((c) => {
    const left = c.startsWith(':');
    const right = c.endsWith(':');
    if (left && right) return 'center';
    if (right) return 'right';
    if (left) return 'left';
    return null;
  });
}

const isTableSep = (line: string): boolean => /^\s*\|?[\s:|-]+\|[\s:|-]*$/.test(line) && line.includes('-');

export type Rendered = { html: string; headings: Heading[] };

/**
 * Render a Markdown document to HTML.
 *
 * `linkHref` rewrites link targets — the build uses it to send `rules.md` to `rules.html` — and
 * defaults to leaving them alone so the function is testable on its own.
 */
export function renderMarkdown(src: string, linkHref: (href: string) => string = (h) => h): Rendered {
  const lines = src.replace(/\r\n/g, '\n').split('\n');
  const out: string[] = [];
  const headings: Heading[] = [];
  const taken = new Set<string>();
  const ln = (s: string): void => void out.push(s);
  const text = (s: string): string => inline(esc(s), linkHref);

  let i = 0;
  while (i < lines.length) {
    const line = lines[i]!;

    // The generated-card markers, and any other HTML comment: structural, never shown.
    if (/^\s*<!--/.test(line)) {
      while (i < lines.length && !lines[i]!.includes('-->')) i++;
      i++;
      continue;
    }

    if (line.trim() === '') {
      i++;
      continue;
    }

    if (/^\s*(---|\*\*\*|___)\s*$/.test(line)) {
      ln('<hr>');
      i++;
      continue;
    }

    const heading = /^(#{1,6})\s+(.*)$/.exec(line);
    if (heading) {
      const level = heading[1]!.length;
      const raw = heading[2]!.trim();
      const id = slug(raw, taken);
      headings.push({ level, text: raw.replace(/[*`]/g, ''), id });
      // The anchor is a link to itself, so a section can be pointed at without a separate widget.
      ln(
        `<h${level} id="${id}">${text(raw)}` +
          `<a class="anchor" href="#${id}" aria-label="Link to this section">#</a></h${level}>`,
      );
      i++;
      continue;
    }

    // Fenced code.
    if (/^\s*```/.test(line)) {
      i++;
      const body: string[] = [];
      while (i < lines.length && !/^\s*```/.test(lines[i]!)) body.push(lines[i++]!);
      i++;
      ln(`<pre><code>${esc(body.join('\n'))}</code></pre>`);
      continue;
    }

    // Tables: a header row, an alignment row, then body rows.
    if (line.includes('|') && i + 1 < lines.length && isTableSep(lines[i + 1]!)) {
      const head = cells(line);
      const align = alignments(lines[i + 1]!);
      i += 2;
      const body: string[][] = [];
      while (i < lines.length && lines[i]!.includes('|') && lines[i]!.trim() !== '') body.push(cells(lines[i++]!));
      const th = head
        .map((c, n) => `<th${align[n] ? ` class="ta-${align[n]}"` : ''}>${text(c)}</th>`)
        .join('');
      const rows = body
        .map(
          (r) =>
            '<tr>' +
            r.map((c, n) => `<td${align[n] ? ` class="ta-${align[n]}"` : ''}>${text(c)}</td>`).join('') +
            '</tr>',
        )
        .join('');
      // Wrapped so a wide table scrolls inside the page rather than widening it on a phone.
      ln(`<div class="tablewrap"><table><thead><tr>${th}</tr></thead><tbody>${rows}</tbody></table></div>`);
      continue;
    }

    // Block quote: consecutive `>` lines, rendered through this same function so a quote may hold
    // a list or a table — the rules reference puts both inside one.
    if (/^\s*>/.test(line)) {
      const body: string[] = [];
      while (i < lines.length && /^\s*>/.test(lines[i]!)) body.push(lines[i++]!.replace(/^\s*>\s?/, ''));
      ln(`<blockquote>${renderMarkdown(body.join('\n'), linkHref).html}</blockquote>`);
      continue;
    }

    // Lists. A bullet or a number opens one; continuation lines are indented under their item.
    const bullet = /^(\s*)([-*+]|\d+[.)])\s+(.*)$/.exec(line);
    if (bullet) {
      const ordered = /\d/.test(bullet[2]!);
      const baseIndent = bullet[1]!.length;
      const items: string[] = [];
      let current: string[] | null = null;
      while (i < lines.length) {
        const l = lines[i]!;
        if (l.trim() === '') {
          // A blank line ends the list unless the next line is still inside it.
          const next = lines[i + 1] ?? '';
          const continues = /^(\s*)([-*+]|\d+[.)])\s+/.test(next) || /^\s{2,}\S/.test(next);
          if (!continues) break;
          i++;
          continue;
        }
        const m = /^(\s*)([-*+]|\d+[.)])\s+(.*)$/.exec(l);
        if (m && m[1]!.length <= baseIndent) {
          if (current) items.push(current.join(' '));
          current = [m[3]!];
          i++;
          continue;
        }
        if (m || /^\s{2,}\S/.test(l)) {
          // A nested item or a wrapped continuation. Nesting is rendered by recursion on the block.
          if (!current) break;
          current.push(l.trim());
          i++;
          continue;
        }
        break;
      }
      if (current) items.push(current.join(' '));
      const tag = ordered ? 'ol' : 'ul';
      ln(`<${tag}>${items.map((it) => `<li>${text(it)}</li>`).join('')}</${tag}>`);
      continue;
    }

    // Anything else is a paragraph, running to the next blank line or block opener.
    const para: string[] = [];
    while (i < lines.length) {
      const l = lines[i]!;
      if (
        l.trim() === '' ||
        /^(#{1,6})\s/.test(l) ||
        /^\s*>/.test(l) ||
        /^\s*```/.test(l) ||
        /^\s*<!--/.test(l) ||
        /^\s*(---|\*\*\*|___)\s*$/.test(l) ||
        /^(\s*)([-*+]|\d+[.)])\s+/.test(l) ||
        (l.includes('|') && isTableSep(lines[i + 1] ?? ''))
      ) {
        break;
      }
      para.push(l.trim());
      i++;
    }
    if (para.length) ln(`<p>${text(para.join(' '))}</p>`);
  }

  return { html: out.join('\n'), headings };
}