polycompress function

Uint8List polycompress(Poly a)

Compresses a polynomial into a byte array representation.

This function takes a polynomial a and reduces its coefficients before compressing them into a compact byte array format. The polynomial is processed in chunks of four coefficients, each being mapped to a 10-bit value. These values are packed into a sequence of bytes, with five bytes encoding four coefficients. The resulting byte array is suitable for storage or transmission in applications where space efficiency is critical.

• Parameter a: The polynomial to be compressed.
• Returns: A byte array containing the compressed representation of the polynomial.

Implementation

Uint8List polycompress(Poly a) {
  Uint8List r = Uint8List(KYBER_POLYCOMPRESSEDBYTES);
  Poly t = polyreduce(Poly()..coeffs = List<int>.from(a.coeffs));
  int pos = 0;
  for (int i = 0; i < KYBER_N; i += 4) {
    int d0 = ((t.coeffs[i] << 10) + (KYBER_Q >> 1)) ~/ KYBER_Q & 0x3FF;
    int d1 = ((t.coeffs[i + 1] << 10) + (KYBER_Q >> 1)) ~/ KYBER_Q & 0x3FF;
    int d2 = ((t.coeffs[i + 2] << 10) + (KYBER_Q >> 1)) ~/ KYBER_Q & 0x3FF;
    int d3 = ((t.coeffs[i + 3] << 10) + (KYBER_Q >> 1)) ~/ KYBER_Q & 0x3FF;

    int packed = d0 | (d1 << 10) | (d2 << 20) | ((d3 & 0x3F) << 30);
    r[pos] = packed & 0xFF;
    r[pos + 1] = (packed >> 8) & 0xFF;
    r[pos + 2] = (packed >> 16) & 0xFF;
    r[pos + 3] = (packed >> 24) & 0xFF;
    r[pos + 4] = (d3 >> 6) & 0xFF;
    pos += 5;
  }
  return r;
}