const esc = (s: string) =>
  s.replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;");

function inline(s: string): string {
  return esc(s)
    .replace(/`([^`]+)`/g, (_, c) => `<code>${c}</code>`)
    .replace(/\*\*([^*]+)\*\*/g, "<strong>$1</strong>")
    .replace(/(^|[\s(])_([^_]+)_/g, "$1<em>$2</em>")
    .replace(
      /\[([^\]]+)\]\((https?:\/\/[^\s)]+)\)/g,
      '<a href="$2" target="_blank" rel="noopener">$1</a>',
    );
}

export function renderMarkdown(src: string): string {
  const lines = src.replace(/\r\n/g, "\n").split("\n");
  const out: string[] = [];
  let i = 0;

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

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

    if (line.trim().startsWith("```")) {
      const buf: string[] = [];
      i++;
      while (i < lines.length && !lines[i].trim().startsWith("```")) {
        buf.push(esc(lines[i]));
        i++;
      }
      i++;
      out.push(`<pre><code>${buf.join("\n")}</code></pre>`);
      continue;
    }

    const h = line.match(/^(#{2,4})\s+(.*)$/);
    if (h) {
      const level = h[1].length;
      out.push(`<h${level}>${inline(h[2])}</h${level}>`);
      i++;
      continue;
    }

    if (/^\s*[-*]\s+/.test(line)) {
      const items: string[] = [];
      while (i < lines.length && /^\s*[-*]\s+/.test(lines[i])) {
        items.push(`<li>${inline(lines[i].replace(/^\s*[-*]\s+/, ""))}</li>`);
        i++;
      }
      out.push(`<ul>${items.join("")}</ul>`);
      continue;
    }

    const para: string[] = [];
    while (
      i < lines.length &&
      lines[i].trim() !== "" &&
      !lines[i].trim().startsWith("```") &&
      !/^(#{2,4})\s+/.test(lines[i]) &&
      !/^\s*[-*]\s+/.test(lines[i])
    ) {
      para.push(lines[i]);
      i++;
    }
    out.push(`<p>${inline(para.join(" "))}</p>`);
  }

  return out.join("\n");
}