CRC32 Combine implementation from zlib in Dart

crc32_combine.dart

/// zlib crc32 combine implementation
/// Usage:
///
///   CRC32().combine(checksum1, checksum2, length2); // length of part behind checksum2
///
class CRC32 {
  List<int> x2ntable = List<int>.filled(32, 0);

  CRC32() {
    int n, p;

    /* initialize the x^2^n mod p(x) table */
    p = 1 << 30; /* x^1 */
    x2ntable[0] = p;
    for (n = 1; n < 32; n++) {
      x2ntable[n] = p = multmodp(p, p);
    }
  }

  static const int poly = 0xedb88320;
  int combine(int crc1, int crc2, int length) {
    return multmodp(x2nmodp(length, 3), crc1) ^ (crc2 & 0xffffffff);
  }

  /*
    Return a(x) multiplied by b(x) modulo p(x), where p(x) is the CRC polynomial,
    reflected. For speed, this requires that a not be zero.
  */
  int multmodp(int a, int b) {
    int m, p;

    m = 1 << 31;
    p = 0;
    for (;;) {
      if ((a & m) != 0) {
        p ^= b;
        if ((a & (m - 1)) == 0) break;
      }
      m >>= 1;
      if ((b & 1) != 0) {
        b = (b >> 1) ^ poly;
      } else {
        b >>= 1;
      }
    }
    return p;
  }

/*
  Return x^(n * 2^k) modulo p(x). Requires that x2n_table[] has been
  initialized.
 */
  int x2nmodp(int n, int k) {
    int p;

    p = 1 << 31;
    while (n != 0) {
      if ((n & 1) != 0) p = multmodp(x2ntable[k & 31], p);
      n >>= 1;
      k++;
    }
    return p;
  }
}