export type CsvDelimiter = "," | ";" | "\t";

function detectDelimiter(text: string): CsvDelimiter {
  const counts: Record<CsvDelimiter, number> = { ",": 0, ";": 0, "\t": 0 };
  let inQuotes = false;
  for (const ch of text) {
    if (ch === '"') inQuotes = !inQuotes;
    else if (!inQuotes) {
      if (ch === "\n" || ch === "\r") break;
      if (ch === "," || ch === ";" || ch === "\t") counts[ch]++;
    }
  }
  let best: CsvDelimiter = ",";
  for (const d of [";", "\t"] as const) if (counts[d] > counts[best]) best = d;
  return best;
}

/** RFC 4180 parser. Strips a BOM, auto-detects the delimiter from the header line, skips blank lines. */
export function parseCsv(text: string, opts: { delimiter?: CsvDelimiter } = {}): string[][] {
  if (text.charCodeAt(0) === 0xfeff) text = text.slice(1);
  const delim = opts.delimiter ?? detectDelimiter(text);
  const rows: string[][] = [];
  let row: string[] = [];
  let field = "";
  let quoted = false; // current field was quoted
  let inQuotes = false;
  let pending = false; // something has been read for the current row

  const endField = () => {
    row.push(field);
    field = "";
  };
  const endRow = () => {
    const blank = row.length === 0 && field === "" && !quoted;
    if (!blank) {
      endField();
      rows.push(row);
    }
    row = [];
    field = "";
    quoted = false;
    pending = false;
  };

  for (let i = 0; i < text.length; i++) {
    const ch = text[i]!;
    if (inQuotes) {
      if (ch === '"') {
        if (text[i + 1] === '"') {
          field += '"';
          i++;
        } else inQuotes = false;
      } else field += ch;
    } else if (ch === '"' && field === "" && !quoted) {
      inQuotes = true;
      quoted = true;
      pending = true;
    } else if (ch === delim) {
      endField();
      quoted = false;
      pending = true;
    } else if (ch === "\r" || ch === "\n") {
      if (ch === "\r" && text[i + 1] === "\n") i++;
      endRow();
    } else {
      field += ch;
      pending = true;
    }
  }
  if (pending || field !== "" || row.length > 0) endRow();
  return rows;
}

function quoteCell(s: string, single: boolean): string {
  if (s === "" ? single : /[",;\t\r\n]/.test(s) || s !== s.trim()) return `"${s.replaceAll('"', '""')}"`;
  return s;
}

/** Serialises with `,` and `\r\n`, quoting only where needed. */
export function toCsv(rows: string[][]): string {
  return rows.map((r) => r.map((c) => quoteCell(c, r.length === 1)).join(",") + "\r\n").join("");
}