sub BuildKey {
    my( $radius, $x, $y, $z )= @_;
    my $sizeRange=
        $radius <= 2 ? 0
      : $radius <= 10 ? 1
      : 2;
    my $width= $bandWidth[$sizeRange];
    if(  0 == $width  ) {
        return pack "c", $sizeRange;
    }
    for my $c (  $x, $y, $z  ) {
        $c= int($c/$width);
        $c ^= 0x8000_0000;
    }
    return return pack "cNNN", $sizeRange, $x, $y, $z;
}