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Asia rule wrote:1) Checking is considers a worse offence than chasing another piece. So if one side is perpetually checking, and the other is perpetually chasing, the checking side loses. All chases are equal, though: if one side perpetually chases a Rook, and the other a Pawn, it is draw.

2) To make a forbidden perpetual chase, you must really chase the same piece on every move of the repeat cycle. (And it does not matter which piece does the chasing: two (or multiple) pieces taking turns to chase one is still forbidden. ) Having a single non-forcing move in the cycle ("one check, one idle", "one chase, one idle") completely acquits you of chase charges. Also alternately chasing two different pieces (including "one check, one chase") is allowed.

3) A chase must be a _new_ attack on a piece. If you could already capture piece A with piece B before move M, then being able to capture A with B after M does not threaten anything new, and is not a chase. E.g. moving a Rook along the attack ray is not creating a new attack. (Checks, obviously, always are new.) Note that one move can result in mutiple chases, both on multiple targets, (e.g. forks) and by multiple perpetrators (e.g. discovered attacks). They all must be judged separately.

4) A newly created attack is in general only considered forcing (i.e. a chase) if the target is not protected. If it can be recaptured on the same square, the attack is not a chase. (Note that the possibility to recapture might depend on how the piece is captured, in case of multiple attackers, so this has to be judged per (attacker, victim)-pair, and is not an intrinsic property of the victim.) But there are some exceptions that are declared either always a chase or never a chase, irrespective of the protected status of the victim:
- Attacks of Horse or Cannon on Rook are always a chase
- Attacks on a Pawn that has not crossed the River are never a chase
- Attacks by a Pawn or King are never a chase
- Attacks on a King are of course always checks

5) An attack on a piece is also not considered a chase if the piece can capture its attacker (which must be of equal type) back. The possibility for such pre-emptive self-defence is considered as good as having a protector that can retaliate. Such attacks are called "sacrifices" or "offers to exchange", not chases. Note that it does not matter if you can pre-emptively capture the attacker with another piece than the chased one, or if the chase victim can capture a piece other than the one chasing it.

6) In all cases only legal moves are taken under consideration. Pseudo-legal captures that are not legal are to be completely ignored. This applies to the attacks itself, to recaptures by a protector, or to pre-emptive counter captures. On the other hand, if the move is legal, does not have to be a good one. Even if it gets you mated in one after making it, or loses you heavy material, the move should be taken into account. (E.g. when a piece is twice attacked, and only once protected, recapturing might be suicidal, but the piece still counts as protected.)

hgm wrote:This is a good example of a controversial call, that follows from strictly applying the rules given above:

3a1ke2/4a4/4e4/RH6p/p3c1p2/9/2C2h3/4H3E/4A4/1r1AK4 w 2 44



44. Kf0 Cf5+ 45. Ke0 Ce5 46. Kf0 Cf5+ 47. Ke0 Ce5
{Xboard adjudication: perpetual chasing} 1-0


WinBoard comes to this verdict because it considers the Advisor on e1 as being chased. For one, it is not protected: the Advisor on d0 is a "false protector", because it is pinned by the Rook on b0, and the King (when on e0) is a false protector because Ae1 is also under attack by the black Horse, so the King could not recapture after Ce5xe1.

So the move Ce5 is chasing Ae1, by creating a new attack on it with the moved Cannon. And Cf5+ undoes the pinning of the Horse on f3 by the King, (Kings are not allowed to directly face each other in Xianngqi), so that it creates an attack by that Horse on the same Advisor, which was not possible before (because of the pin). So this move also chases.

Both moves create a new attack on the same unprotected Advisor, hence perpetual chase. It does this next to a "one check, one idle", which in itself would be allowed.


There are two aspects that make this controversial:

1) Creating a new attack by resolving a pin. There is no explicit example amongst the 104 diagrams that come with the Asia rules where this method to create a new legal capture is used. There are plenty of other examples where creating an attack by one piece through moving another (Discovered attacks, Cannon activation) are defined to be chases. Even when the piece that moves would in itself be allowed to chase in direct attack (i.e. is a King or Pawn, diagram 71-73). There are also examples where a Rook creating new pseudo-legal attacks on a Cannon (in an attempt to block its passage) that are not legal attacks (because the Rook is pinned) is not considered a chase (diagram 63).

2) Normally aiming a second attacker on a singly protected piece is not considered a chase (diagram 86-89). But if the single protector is a King, a second attacker makes the recapture illegal. Normally a recapture that leaves your King in check would not count ("false protector", diagrams 83-85). These are all examples where you put yourself in check by moving a pinned piece away, though, not where you step into check with the King itself.

3) Can a move that delivers check be a chase at the same time? Judging if a move attacks something is normally done by having the opponent null move, and allowing the side that just moved two moves in a row. But when in check, the null move is illegal. So this can be very tricky.

hgm wrote:To illustrate point (3) mentioned above, I composed the following position:

Code: Select all

9 . . . . k . e . c
8 C . . . a . . C r
7 . . . . c . . R .
6 . . . . . . . . p
5 . . . . . . . . .
4 . . . . . . . . .
3 . . . . . . . . .
2 . . . . E . . . .
1 . . . . A . . . .
0 . . . . K A E . .
  a b c d e f g h i

(I switched to using ASCII diagrams, as the rendering of XQ diagrams does not seem to work in this forum.)

1.Ch9+ Af9 2.Ch8 Ae8 3. Ch9+ Af9 4. Ch8 Ae8

The move Ch9+ activates Ca8xi8 as well as delivering check. Af9 resolves both threats, but Ch8 reactivates it again. So next to a "one check, one idle" against the King, white could be considered chasing Rh8 with Ca8.

But now look at the issues this raises in a slightly modified position:

Code: Select all

9 . . . . k . e . c
8 C . . . . r . C h
7 . . . . c . . R .
6 . . . . . . . . p
5 . . . . . . . . .
4 . . . . . . . . .
3 . . . . . . . . .
2 . . . . E . . . .
1 . . . . A . . . .
0 . . . . K A E . .
  a b c d e f g h i

I replaced Ri8 by a Horse, so that the attack of Ca8 on it is not automatically a chase, but it depends on the Hh8 being protected. In addition I replaced Ae8 by Rf8, to provide a protector. Now

1. Ch9+ Rf9 2. Ch8 Re8 3. Ch9+ Rf9 4. Ch8 Re8

Again Ch8 activates Ca8xi8, and the Horse is not protected on this move. So that is a chase. But after Ch9+, unblocking Ca8xi8 again, the Horse is protected by Rf8. Or is it, really???

After Cf8+ <null> Ca8xi8 black cannot capture back very well with Rf8xi8. Because that leaves him in check, just as the null move did, and it was just his luck that we decided to capture the Horse, rather than his King.

So how to handle this?
1) We can decide that the Horse is indeed not protected, because after Ca8xi8 Rf8xi8 leaves black in check, and Rf8 thus is a false protector. In this case we would have a forbidden perpetual chase here (next to the check).
2) We can replace the null move by some other check evasion, for the purpose of judging if the newly attacked piece wasprotected or not. But what if there are none? Or if the only one interferes with the new attack we want to judge (like here: Ke8 blocks Ca8xi8.)
3) We could temporarily 'lift the King from the board' in stead of the null move, before judging. This does not seem to be the proper thing to do, though: it would also liberate genuinely pinned pieces, for example. It is also theoretically possible that removing the King interferes with the newly created capture, or with the recapture, because the King acted as platform for a Cannon in either of those.
4) We could introduce a new class of legality ('semi-legal moves'), made from positions where the mover is in check, in order to distinguish moves that leave your King in check into moves that merely leave your King in the existing check, and moves that create a new way to capture King (possibly, but not necessarily, resolving the pre-existing way). The protected status of a piece could then be redifined as the existence of a semi-legal move to recapture.
5) Of course we could decide that a check is never a chase to the same time, so the entire problem does never occur.

It seems to me that (2) and (3) are not viable. Solution (5) is by far the easiest, and actually what China mainland rules prescribe. But Asia rules leave out this clause, and this is usually taken as an indication that it does not apply. (4) is doable, but very cumbersome, and looks a bit artificial / arbitrary. (1) is what WinBoard currently would do.

phhnguyen wrote:The White Cannon is doing one check and one chase. The Black side has only one move to resolve the check and one move to resolve the chase. Unfortunately, both those moves can be considered as a chase to that Cannon.

What should we score that game? A draw or White's win?

Diagram 37: Red (in Capital) moves first
R8+1 c1+1
R8-1 c1-1
R8+1 c1+1
R8-1 c1-1
R8+1 ....

Explanation:
Black Cannon's "one check and one chase" is allowed but not Red Rook's perpetual chasing of the Black Cannon. Red has to change.