Table to Spreadsheet

function run(selected_text) {
  // Rendered tables may already arrive as spreadsheet clipboard text.
  if (selected_text.includes("\t") && !selected_text.trimStart().startsWith("|")) {
    const rows = [], row = [];
    let cell = "", quoted = false, closed = false, start = true;
    const finish = () => { row.push(cell); cell = ""; closed = false; start = true; };
    for (let index = 0; index < selected_text.length; index += 1) {
      const char = selected_text[index];
      if (quoted) {
        if (char === '"') {
          if (selected_text[index + 1] === '"') { cell += '"'; index += 1; }
          else { quoted = false; closed = true; }
        } else cell += char;
      } else if (char === "\t" || char === "\r" || char === "\n") {
        finish();
        if (char !== "\t") { rows.push(row.splice(0)); }
        if (char === "\r" && selected_text[index + 1] === "\n") index += 1;
      } else if (closed) {
        throw new Error("Invalid quoted spreadsheet cell. Select the table again and retry.");
      } else if (start && char === '"') { quoted = true; start = false; }
      else { cell += char; start = false; }
    }
    if (quoted) throw new Error("Incomplete quoted spreadsheet cell. Select the table again and retry.");
    if (row.length || cell.length || !/[\r\n]$/.test(selected_text)) { finish(); rows.push(row); }
    const width = rows[0].length;
    if (rows.some((values) => values.length !== width)) throw new Error("The selected rows have different numbers of columns.");
    const quote = (value) => /[\t\r\n"]/.test(value) ? '"' + value.replace(/"/g, '""') + '"' : value;
    return rows.map((values) => values.map(quote).join("\t")).join("\n");
  }
  const unescape = (text) => text.replace(/\\([^\w\s])/g, "$1");
  const entities = (text) => text.replace(/&(#x[\da-f]+|#\d+|amp|lt|gt|quot|apos|nbsp);/gi, (match, code) => {
    const names = {amp: "&", lt: "<", gt: ">", quot: '"', apos: "'", nbsp: " "};
    if (code[0] !== "#") return names[code.toLowerCase()];
    const value = code[1].toLowerCase() === "x" ? parseInt(code.slice(2), 16) : parseInt(code.slice(1), 10);
    return value > 0 && value <= 0x10ffff && !(value >= 0xd800 && value <= 0xdfff)
      ? String.fromCodePoint(value) : match;
  });

  // Read inline Markdown links without truncating URLs containing parentheses.
  function linkAt(text, start) {
    if (text[start] !== "[") return null;
    let depth = 1, index = start + 1;
    for (; index < text.length && depth; index += 1) {
      if (text[index] === "\\") { index += 1; continue; }
      if (text[index] === "[") depth += 1;
      if (text[index] === "]") depth -= 1;
    }
    if (depth || text[index] !== "(") return null;
    index += 1;
    while (/\s/.test(text[index] || "") && index < text.length) index += 1;
    let url = "";
    if (text[index] === "<") {
      const end = text.indexOf(">", index + 1);
      if (end < 0) return null;
      url = text.slice(index + 1, end);
      index = end + 1;
    } else {
      depth = 0;
      for (; index < text.length; index += 1) {
        const char = text[index];
        if (char === "\\" && index + 1 < text.length) { url += char + text[++index]; continue; }
        if ((char === ")" || /\s/.test(char)) && depth === 0) break;
        if (char === "(") depth += 1;
        if (char === ")") depth -= 1;
        url += char;
      }
      if (depth) return null;
    }
    while (/\s/.test(text[index] || "") && index < text.length) index += 1;
    if (text[index] === '"' || text[index] === "'") {
      const quote = text[index++];
      while (index < text.length && text[index] !== quote) {
        if (text[index] === "\\") index += 1;
        index += 1;
      }
      if (text[index++] !== quote) return null;
      while (/\s/.test(text[index] || "") && index < text.length) index += 1;
    }
    return text[index] === ")" ? {end: index + 1, url: entities(unescape(url))} : null;
  }

  function codeAt(text, start) {
    if (text[start] !== "`") return null;
    const fence = text.slice(start).match(/^`+/)[0];
    let end = text.indexOf(fence, start + fence.length);
    while (end >= 0 && (text[end - 1] === "`" || text[end + fence.length] === "`")) {
      end = text.indexOf(fence, end + fence.length);
    }
    return end < 0 ? null : {end: end + fence.length, value: text.slice(start + fence.length, end)};
  }

  function splitRow(line) {
    const cells = [];
    let cell = "", separators = 0;
    for (let index = 0; index < line.length;) {
      const char = line[index];
      if (char === "\\" && index + 1 < line.length) { cell += line.slice(index, index + 2); index += 2; continue; }
      const token = codeAt(line, index) || linkAt(line, index);
      if (token) { cell += line.slice(index, token.end); index = token.end; continue; }
      if (char === "|") { cells.push(cell.trim()); cell = ""; separators += 1; }
      else cell += char;
      index += 1;
    }
    cells.push(cell.trim());
    if (!separators) throw new Error("Select a Markdown table with columns separated by | characters.");
    if (line.startsWith("|")) cells.shift();
    if (cells[cells.length - 1] === "" && line.endsWith("|") && cells.length > 1) cells.pop();
    return cells;
  }

  function plainCell(text) {
    let output = "";
    for (let index = 0; index < text.length;) {
      if (text[index] === "\\" && index + 1 < text.length) {
        output += unescape(text.slice(index, index + 2)); index += 2; continue;
      }
      const code = codeAt(text, index);
      if (code) { output += code.value; index = code.end; continue; }
      const isImage = text[index] === "!" && text[index + 1] === "[";
      const link = linkAt(text, index + (isImage ? 1 : 0));
      if (link) { output += link.url; index = link.end; continue; }
      const autolink = text.slice(index).match(/^<(https?:\/\/[^>]+|mailto:[^>]+)>/i);
      if (autolink) { output += entities(autolink[1]); index += autolink[0].length; continue; }
      const html = text.slice(index).match(/^<\/?(?:br|strong|b|em|i|s|del|code)(?:\s*\/?)>/i);
      if (html) { if (/^<br/i.test(html[0])) output += "\n"; index += html[0].length; continue; }
      const marker = ["**", "__", "~~", "*", "_"].find((value) => text.startsWith(value, index));
      if (marker && (marker !== "_" || index === 0 || !/\w/.test(text[index - 1]))) {
        const end = text.indexOf(marker, index + marker.length);
        const inner = end < 0 ? "" : text.slice(index + marker.length, end);
        if (inner && inner === inner.trim()) { output += plainCell(inner); index = end + marker.length; continue; }
      }
      output += text[index++];
    }
    return entities(output);
  }

  let lines = selected_text.replace(/\r\n|[\r\u2028\u2029]/g, "\n").trim().split("\n");
  // Accept a table copied with its enclosing Markdown code fence.
  const fence = (lines[0] || "").trim().match(/^(`{3,}|~{3,})(?:markdown|md|text)?\s*$/i);
  if (fence && lines.length > 1 && lines[lines.length - 1].trim() === fence[1]) lines = lines.slice(1, -1);
  const rows = lines.filter((line) => line.trim()).map((line) => splitRow(line.trim()));
  if (!rows.length) throw new Error("Select a Markdown table first.");
  if (rows.length > 1 && rows[1].every((value) => /^:?-{3,}:?$/.test(value))) rows.splice(1, 1);
  const width = rows[0].length;
  if (!width || rows.some((row) => row.length !== width)) {
    throw new Error("The table rows have different numbers of columns. Escape literal pipes as \\| and select one complete table.");
  }
  const quote = (text) => /[\t\r\n"]/.test(text) ? '"' + text.replace(/"/g, '""') + '"' : text;
  return rows.map((row) => row.map((cell) => quote(plainCell(cell))).join("\t")).join("\n");
}