In order to remove the problem with unexpected occurrences of a signal from detecting obstacles, I propose to make the following addition to the story.

Unfortunately, obstacle detection is unreliable and signals are possible even when the door is not moving. In this case, these signals should be ignored.

Listen guys...
I am disappointed. Why are all solutions a simple enumeration of elements. I spent a lot of my time from the New Year holidays, remembered a little math to find an algorithm with logarithmic complexity. I expected to see something like this and admire the elegance of the solution.
Of course, no Paint-Squad could draw millions of numbers, but the task of counting digits for intervals of millions of elements could be interesting.
My respects to dinglemouse for having fun.