/** Parses enough of RFC 4180 to show a CSV as a table: quoted fields, doubled quotes, CRLF. */
export function parseCsvPreview(
  text: string,
  maxRows = 500,
): { rows: string[][]; truncated: boolean } {
  const rows: string[][] = [];
  let row: string[] = [];
  let field = "";
  let quoted = false;
  let index = 0;
  const endRow = () => {
    row.push(field);
    field = "";
    if (row.length > 1 || row[0] !== "") rows.push(row);
    row = [];
  };
  while (index < text.length) {
    const char = text[index];
    if (quoted) {
      if (char === '"' && text[index + 1] === '"') {
        field += '"';
        index += 2;
        continue;
      }
      if (char === '"') quoted = false;
      else field += char;
      index += 1;
      continue;
    }
    if (char === '"' && field === "") quoted = true;
    else if (char === ",") {
      row.push(field);
      field = "";
    } else if (char === "\n" || char === "\r") {
      endRow();
      if (char === "\r" && text[index + 1] === "\n") index += 1;
      if (rows.length > maxRows) break;
    } else field += char;
    index += 1;
  }
  if (rows.length <= maxRows && (field !== "" || row.length > 0)) endRow();
  return { rows: rows.slice(0, maxRows), truncated: rows.length > maxRows };
}