*/
static unsigned int checksum(const char *zIn, size_t N){
  const unsigned char *z = (const unsigned char *)zIn;
  unsigned sum0 = 0;
  unsigned sum1 = 0;
  unsigned sum2 = 0;
  unsigned sum3 = 0;
  while(N >= 16){
  unsigned sum;
  while( N>=4 ){
    sum0 += ((unsigned)z[0] + z[4] + z[8] + z[12]);
    sum1 += ((unsigned)z[1] + z[5] + z[9] + z[13]);
    sum2 += ((unsigned)z[2] + z[6] + z[10]+ z[14]);
    sum3 += ((unsigned)z[3] + z[7] + z[11]+ z[15]);
    z += 16;
    N -= 16;
  }
  while(N >= 4){
    sum0 += z[0];
    sum1 += z[1];
    sum2 += z[2];
    sum3 += z[3];
    N -= 4;
    z += 4;
    z += 4;
    N -= 4;
  }
  sum = (sum0 << 24) + (sum1 << 16) + (sum2 << 8) + sum3;
  sum3 += (sum2 << 8) + (sum1 << 16) + (sum0 << 24);
  switch(N){
    case 3:   sum += (z[2] << 8);
    case 2:   sum += (z[1] << 16);
    case 1:   sum += (z[0] << 24);
    case 3:   sum3 += (z[2] << 8);
    case 2:   sum3 += (z[1] << 16);
    case 1:   sum3 += (z[0] << 24);
    default:  ;
  }
  return sum;
  return sum3;
}

/*
** Create a new delta.
**
** The delta is written into a preallocated buffer, zDelta, which 
** should be at least 60 bytes longer than the target file, zOut.