A check given by castling: is it a discovered check or a special case?

Question

As far as I know, there could be as many as four possible types of check if we want to be as specific as possible:

1) a simple "active" check. I am not sure if there is a separate term for this, so let me call it simple active. So, a simple active check arises when the piece that moves gives check by itself but does not open any lines of attack from any other friend piece.

2) a discovered check. It arises when recently moved piece does NOT give check by itself but opens exactly one line of attack. A rider-type piece (being either a queen, a rook or a bishop) was hidden behind the piece that just moved.

3) a common double check where we combine #1 and #2: we have a piece that moved. It gave a check by itself. Also, it opened exactly one line of attack of a distinct frienly piece.

4) a very rare type of double check when the just-moved piece did NOT give check by itself. However, it opened TWO straight lines of attack, each one by a different piece but of the same type! I am pretty sure this is only possible during en passant. Also, both checking pieces must be queens of same color. One line of attack on enemy king must be a file and another, a diagonal.

However, there is one quite important complication. I have recently realized that a check in chess could arise from castling.

I analyzed all list items again, and it seems that this last case does not fall under any of #1, #2, #3 or #4.

So I am confused. The better you know chess rules, the more you get confused.

If a check arose from castling, would it be a discovered check (see # 2), or is it a totally distinct type of check?

Essentially, I want to add that the rook in this case is a very specific chess piece type — a hopper. https://en.wikipedia.org/wiki/Fairy_chess_piece#Hoppers

Answer 1

OK, so this question is for chess problemists, especially fairy problemists.

First, why does the exact classification of castling horizontal check matter? Clearly it matters whether a move is check or not, but that's easy to determine. But the flavour of the check is not so easy, and usually not so important.

It's a bit like a soccer goal. It's very important we know whether a goal has been scored. But beyond that we might ask about the manner of the goal: was it a penalty kick, a header, an own goal or what? The manner is obvious 99% of the time, but in the 1% there are scrums of elbows and boots where it's hard to say exactly what happened.

Unless there is a fairy condition or tourney theme which is sensitive to discovery, I would say that it's up to you to classify. In the case of a tourney, of course you can consult the tourney director.

In the special case of rookhoppers replacing the rooks, I'm curious as to how castling is defined. Clearly one can have a rookhopper move Ra1-f1 or Rh1-d1, but that's not castling as the king remains stuck in the middle of the back rank. However it's defined, I still think it will be up to you to classify.

And of course separate from castling, triple checks are easily possible when you have hoppers on the board!