if( X == Y ) {
    return depth;
} 
elsif ( ( X,Y ) is in a 4x4 block on the major diagonal ) {
    return $depth - 1;
}
else {
    ## Move the 4x4 block to the origin and treat as for depth -1
    X %= 4;
    Y %= 4;
    depth--;
    recurse;
}