sub follow_paths_unrolled {
    my ($x_s,$y_s,$s_s,$paths,$regs) = @_;
                    
    my $cur_x = $x_s;
    my $cur_y = $y_s; 
    my $cur_s = $s_s;
    my $index = 0;
    my $current = [];
    my @STACK;   
    my $RETURN;
    my $BRANCH = 0;
         
    NEWCALL:                
    while(1) {
        my $branches = $paths->[$cur_x][$cur_y][$cur_s];
     
        if (not defined $branches) {
            # pad endpoint (base case)}
            $RETURN = [[[$x_s,$y_s,$s_s,'pad']]];
        }
        
        elsif (exists $regs->{$x_s}{$y_s}{$s_s}) {
            # register endpoint (base case)
            $RETURN = [[[$x_s,$y_s,$s_s,'reg']]];
        }               
    
        else {
            for (my $i=$index; $i < @$branches; $i++) {
                if ($BRANCH == 0) {
                    my $branch = $branches->[$i];
    
                    # push @STACK
                    $BRANCH = 1;
                    push @STACK, [$cur_x,$cur_y,$cur_s,$i,$current,$BRANCH];

                    # set new values
                    ($cur_x,$cur_y,$cur_s) = @$branch;
                    $index = 0;
                    $current = [];
                    $BRANCH = 0;
                    next NEWCALL;
                }
                else {
                    my $branch_paths = $RETURN;
                    push @$current, map { unshift @$_, [$cur_x,$cur_y,$cur_s]; $_ }
                                    @$branch_paths;
                    $BRANCH = 0;
                }
            }
            $RETURN = $current;
        }
        
        return $RETURN unless @STACK;
        ($cur_x,$cur_y,$cur_s,$index,$current,$BRANCH) = @{pop @STACK};
    }    
}