Formulating the ko rule without off-board information

[luigi]

One quibble I have with the ko rule is that you can accidentally make an illegal move if you forget what your opponent's move was, as in game four of the 5th Meijin tournament in 1980. It occurs to me this could be easily avoided by using the following conventions:

- If your move is a ko capture, the surrounded enemy stone is not removed immediately.
- If, at the start of your turn, there is a surrounded stone of your color on the board, you must remove it before making your move.

[John Fairbairn]

It's a neat idea but what do you do about other such infringements that have occurred in professional play? E.g. a player plays a suicide move, or a player removes enemy stones wrongly thinking he had captured them.

It seems simpler just to stick with the present rules which implicitly say that a professional must behave like a professional or suffer the consequences.

Given that there are only about 50 cases of forfeiture by pros in over 100,000 games in the GoGoD database, and these cover quite a wide range of different infractions, it doesn't seem to be much of an issue in practice.

The Cho Chikun case you mentioned was special in that he asked the scorekeeper for guidance, but there were plenty of fellow professionals who thought that was wrong and unprofessional, and that he should have forfeited the game.

[luigi]

It's a neat idea but what do you do about other such infringements that have occurred in professional play? E.g. a player plays a suicide move, or a player removes enemy stones wrongly thinking he had captured them.

There are all sorts of illegal things a player can do, of course. My quibble with ko is simply that it forces a player to remember the previous move. It's the same with en passant capture in chess, for instance, and even worse with castling in that game.

I just think that, in a perfect information game, all relevant information should be visible to the players at all times. If this can be achieved by fully encoding the game state in the board position (as opposed to by using off-board markers), all the better.

[Pio2001]

I'm the kind of player who recaptures in a ko after having thought for 5 minutes.

With your rule, when it's my turn, I can remove my stone, however, 5 minutes later, I may yet recapture where I just removed the stone

The idea to encode the whole information on the board is interesting, but an information is still missing with your system : whose turn is it ? Both stones in the ko have no liberty.

[luigi]

I'm the kind of player who recaptures in a ko after having thought for 5 minutes.

With your rule, when it's my turn, I can remove my stone, however, 5 minutes later, I may yet recapture where I just removed the stone

The idea is to remove your stone right before placing another. Or, even better, you can move the stone in the ko to where you want to play (and give your opponent one prisoner, if applicable).

The idea to encode the whole information on the board is interesting, but an information is still missing with your system : whose turn is it ? Both stones in the ko have no liberty.

If the sum of the number of stones on the board and the number of prisoners is even, it's Black to move; if it's odd, it's White to move (assuming no one has passed).

[jlt]

To know whose turn it is, count the number of stones that have already been played (stones on the board + prisoners). If there are as many black stones than white stones, then it's black's turn, otherwise it's white's turn.

If you use a clock, then the information is also on the clock.

Of course, in amateur play this is generally not a problem, you can usually ask your opponent if you are not sure.

[mhlepore]

If is too much to ask Cho Chikun to remember who played the last ko threat, isn't it also too much to ask that we count the total stones played to determine who has the next move?

That is, if the point of the original post is to make the game easier to evaluate in practical circumstances, it seems we've gone off course in discussing total stones played to determine who has the next move. If the point of the post was more academic, then the Cho Chikun example seems irrelevant.

[gowan]

Properly programmed bots have no difficulty with any of these issues. IMO, knowing whose turn it is, the status of the ko, etc., is all a part of skill at go. Of course, human players are human and make mistakes, even pros do it, too. In some way you are asking a system to avoid making mistakes. These mistakes are most likely to happen during time trouble. Handling the clock is still another skill.

[luigi]

If is too much to ask Cho Chikun to remember who played the last ko threat, isn't it also too much to ask that we count the total stones played to determine who has the next move?

That is, if the point of the original post is to make the game easier to evaluate in practical circumstances, it seems we've gone off course in discussing total stones played to determine who has the next move. If the point of the post was more academic, then the Cho Chikun example seems irrelevant.

They're different issues. If you don't know whose turn it is, you can find it out by yourself by counting the stones. If you don't remember your opponent's last move, you cannot find it out by yourself.

[John Fairbairn]

If you don't know whose turn it is, you can find it out by yourself by counting the stones.

No, you can't. Depending on the rule set, prisoners may be tossed back into the bowls, or if kept they may not be in view (hidden behind a traditional board) and you're not supposed to interfere with your opponent. And what about Kibi no Makibi who swallowed a prisoner?

But why should we obsess about perfect information anyway?

[luigi]

If you don't know whose turn it is, you can find it out by yourself by counting the stones.

No, you can't. Depending on the rule set, prisoners may be tossed back into the bowls, or if kept they may not be in view (hidden behind a traditional board) and you're not supposed to interfere with your opponent. And what about Kibi no Makibi who swallowed a prisoner?

That was not the point of my original post anyway. I think players should just know when it's their turn to play. Counting hundreds of stones to find that out is obviously very impractical.

But why should we obsess about perfect information anyway?

It's a matter of principle. As a game designer, I care about this stuff. I'm not saying we should obsess over it. I just like the fact that there is an easy way to remove the memory factor from ko.

[moha]

One quibble I have with the ko rule is that you can accidentally make an illegal move if you forget what your opponent's move was

Actually you have much more to remember, if there is some kind of superko rule in use.

With the normal ko rule the burden doesn't seem too high (en passant is good analogy), the issue doesn't even exist in online/electronic play.

If, at the start of your turn, there is a surrounded stone of your color on the board, you must remove it before making your move.

What if the player(s) forget to remove either stone with no liberty for a few turns?