QR Code Generator — JavaScript source

/**
 * qr-code-generator - QR format information (BCH).
 *
 * Display snippet: assembling the module matrix is too long to show here;
 * this ports formatInfoBCH(data5) from the TypeScript lib (src/lib/qr.ts).
 * The full encoder (modes, Reed-Solomon EC, masking, matrix) lives in the
 * TypeScript/Go sources.
 *
 * The 15-bit format string protects 5 data bits — 2 error-correction level
 * bits (L=01 M=00 Q=11 H=10) followed by the 3-bit mask pattern — with a
 * BCH(15,5) code using generator 0x537, then XORs the result with the
 * fixed 0x5412 pattern so an all-zero string is never valid.
 */

const EC_FORMAT_BITS = { L: 0b01, M: 0b00, Q: 0b11, H: 0b10 };

/** 15-bit format info string (MSB first) for (ecLevel, mask). */
function formatInfo(ecLevel, mask) {
  return formatInfoBCH((EC_FORMAT_BITS[ecLevel] << 3) | mask);
}

/** 15-bit format info string (MSB first) for a 5-bit data value. */
function formatInfoBCH(data5) {
  let v = data5 << 10;
  const G = 0x537; // BCH(15,5) generator polynomial
  for (let i = 14; i >= 10; i--) {
    if (v & (1 << i)) v ^= G << (i - 10); // polynomial long division
  }
  const full = ((data5 << 10) | (v & 0x3ff)) ^ 0x5412; // apply the mask pattern
  return full.toString(2).padStart(15, '0');
}

// Example:
//   formatInfo('L', 0) -> '111011111000100' (0x77C4, matching the published tables)