EAN-13 barcode renderer in SVG

// Left-hand "odd parity" patterns; R is the inverse, G is R reversed.
const L = ['0001101', '0011001', '0010011', '0111101', '0100011', '0110001', '0101111', '0111011', '0110111', '0001011'];
const R = L.map((p) => [...p].map((b) => (b === '1' ? '0' : '1')).join(''));
const G = R.map((p) => [...p].reverse().join(''));
// Which set encodes each of the six left digits, chosen by the first digit.
const PARITY = ['LLLLLL', 'LLGLGG', 'LLGGLG', 'LLGGGL', 'LGLLGG', 'LGGLLG', 'LGGGLL', 'LGLGLG', 'LGLGGL', 'LGGLGL'];

export function checkDigit(first12: string): number {
  const sum = [...first12].reduce((s, d, i) => s + Number(d) * (i % 2 ? 3 : 1), 0);
  return (10 - (sum % 10)) % 10;
}

export function encodeEan13(input: string): { bits: string; digits: string } {
  if (!/^\d{12,13}$/.test(input)) throw new Error('EAN-13 needs 12 or 13 digits');
  const body = input.slice(0, 12);
  const digits = body + checkDigit(body);
  if (input.length === 13 && input !== digits) throw new Error('Check digit mismatch');

  const parity = PARITY[Number(digits[0])]!;
  let bits = '101';
  for (let i = 1; i <= 6; i++) bits += (parity[i - 1] === 'L' ? L : G)[Number(digits[i])];
  bits += '01010';
  for (let i = 7; i <= 12; i++) bits += R[Number(digits[i])];
  return { bits: bits + '101', digits };
}

export function Ean13({ code, moduleWidth = 2, height = 90 }: { code: string; moduleWidth?: number; height?: number }) {
  const { bits, digits } = encodeEan13(code);
  const quiet = 9;
  const bars: { x: number; w: number }[] = [];
  for (let i = 0; i < bits.length; i++) {
    if (bits[i] !== '1') continue;
    const last = bars[bars.length - 1];
    if (last && last.x + last.w === i) last.w += 1; // merge runs of adjacent bars
    else bars.push({ x: i, w: 1 });
  }
  const width = (bits.length + quiet * 2) * moduleWidth;
  return (
    <svg xmlns="http://www.w3.org/2000/svg" width={width} height={height + 18} role="img" aria-label={`Barcode ${digits}`}>
      <rect width={width} height={height + 18} fill="#fff" />
      {bars.map((b) => (
        <rect key={b.x} x={(b.x + quiet) * moduleWidth} y={0} width={b.w * moduleWidth} height={height} fill="#000" />
      ))}
      <text x={width / 2} y={height + 14} textAnchor="middle" fontFamily="monospace" fontSize={13} letterSpacing={3}>
        {digits}
      </text>
    </svg>
  );
}