/// halves-optimized method unrolled for 32-bit integer
unsigned rbH1 (unsigned arg)
{
  arg = ((arg << 1)  & 0xAAAAAAAA) | ((arg >> 1)  & 0x55555555);
  arg = ((arg << 2)  & 0xCCCCCCCC) | ((arg >> 2)  & 0x33333333);
  arg = ((arg << 4)  & 0xF0F0F0F0) | ((arg >> 4)  & 0x0F0F0F0F);
  arg = ((arg << 8)  & 0xFF00FF00) | ((arg >> 8)  & 0x00FF00FF);
  arg = ((arg << 16) & 0xFFFF0000) | ((arg >> 16) & 0x0000FFFF);
  return arg;
}