core/crc.ts

/**
 * CRC computation for payload integrity verification
 * Supports CRC-4, CRC-8, and CRC-16
 */

// CRC-4 polynomial: x^4 + x + 1 (0x13, used bits: 0x03)
const CRC4_POLY = 0x03;
// CRC-8 polynomial: x^8 + x^2 + x + 1 (CRC-8-CCITT)
const CRC8_POLY = 0x07;
// CRC-16 polynomial: x^16 + x^15 + x^2 + 1 (CRC-16-IBM)
const CRC16_POLY = 0x8005;

/**
 * Compute CRC over a bit array
 * Uses non-zero initial value to prevent all-zeros from having CRC=0
 */
function computeCrc(bits: Uint8Array, poly: number, crcBits: number): number {
  const mask = (1 << crcBits) - 1;
  let crc = mask; // Non-zero init: all 1s (0xF for CRC-4, 0xFF for CRC-8, 0xFFFF for CRC-16)

  for (let i = 0; i < bits.length; i++) {
    crc ^= (bits[i] & 1) << (crcBits - 1);
    if (crc & (1 << (crcBits - 1))) {
      crc = ((crc << 1) ^ poly) & mask;
    } else {
      crc = (crc << 1) & mask;
    }
  }

  return crc;
}

/**
 * Compute CRC and append to bit array
 */
export function crcAppend(bits: Uint8Array, crcBits: 4 | 8 | 16): Uint8Array {
  const poly = crcBits === 4 ? CRC4_POLY : crcBits === 8 ? CRC8_POLY : CRC16_POLY;
  const crc = computeCrc(bits, poly, crcBits);

  const result = new Uint8Array(bits.length + crcBits);
  result.set(bits);

  // Append CRC bits (MSB first)
  for (let i = 0; i < crcBits; i++) {
    result[bits.length + i] = (crc >> (crcBits - 1 - i)) & 1;
  }

  return result;
}

/**
 * Verify CRC on a bit array (last crcBits bits are the CRC)
 * Returns the payload bits (without CRC) if valid, null otherwise
 */
export function crcVerify(bits: Uint8Array, crcBits: 4 | 8 | 16): Uint8Array | null {
  if (bits.length <= crcBits) return null;

  const payload = bits.subarray(0, bits.length - crcBits);
  const poly = crcBits === 4 ? CRC4_POLY : crcBits === 8 ? CRC8_POLY : CRC16_POLY;
  const computed = computeCrc(payload, poly, crcBits);

  // Extract received CRC
  let received = 0;
  for (let i = 0; i < crcBits; i++) {
    received |= (bits[bits.length - crcBits + i] & 1) << (crcBits - 1 - i);
  }

  if (computed === received) {
    return new Uint8Array(payload);
  }
  return null;
}