Passive ko rule

[luigi]

1) If there are two to three moves ruled out by this, then there will be several times as many places where the best play is different, because illegal moves were present as threats.

2) Two to three moves that can't be played is already a lot!

1) Yes. Of course, this rule changes the game much more than superko rules, for instance.

2) It doesn't seem that much to me considering that those two or three moves are likely to be ko threats. (And three could be an overestimation anyway. Not every game features a complicated ko fight or lasts more than 300 moves.)

[luigi]

OK, all the better then. Passive ko is much less restrictive than Tibetan ko

??? how do you know this? it seems like apples and oranges to me, but even so I can't help but feeling that yours is more restrictive.

You're right that they're different things and comparing them is difficult. My reasoning was as follows:

Both rules have in common that they forbid making a capture by playing where your opponent just took one of your stones, and differ in the rest of forbidden plays after an enemy capture. Under Tibetan ko, those forbidden plays imply fundamental changes in life and death but leave the rest of the game mostly intact, while passive ko implies subtle changes in several areas of the game (most of them related to ko fights) but leaves the majority of basic life and death shapes intact.

My impression is that the latter feels closer to Go because I think the balance between life and death is what most strongly defines the character of the game, and it would seem that Tibetan ko tips that balance a bit too much towards life, whilst the diffuse changes introduced by passive ko don't alter that balance one way or the other in any easily identifiable manner.

Most notably, reducing the number of available ko threats can't do it because it also hinders the other player's efforts to kill, and since that's by far the most changed aspect of the game, the opportunities to tip that balance are greatly reduced.

Maybe it all boils down to a question of strategy versus tactics. Making it easier to live requires different long term planning and changes your global picture of the game, whereas making ko fights behave differently and increasing the incentive to capture affect your tactical priorities in the current local battles.

Of course, this is all arguable and you have much more experience with the game anyway, so your impressions may be more accurate.

[skydyr]

While life and death may be relatively unchanged under passive ko, it could greatly affect exchanges and many joseki, as suddenly you can't trade, say, a ponnuki in one area for one in another without making a ko threat, and the capture of a dead group or stone could completely change the outcome of a semeai, as when a capture creates an eye but removes the 2nd to last liberty.

[jts]

While life and death may be relatively unchanged under passive ko, it could greatly affect exchanges and many joseki, as suddenly you can't trade, say, a ponnuki in one area for one in another without making a ko threat, and the capture of a dead group or stone could completely change the outcome of a semeai, as when a capture creates an eye but removes the 2nd to last liberty.

More damning, any ko threat involving capture would become false if the ko could be resolved through capture.

[Javaness2]

Play it out on L19x19 and see what happens. One game with Passive ko, one with Tibetan ko

[luigi]

While life and death may be relatively unchanged under passive ko, it could greatly affect exchanges and many joseki, as suddenly you can't trade, say, a ponnuki in one area for one in another without making a ko threat

On the bright side, since ko fights are said by many people to be one of the most interesting aspects of the game, turning simple (but actually infrequent) exchanges into pseudo-ko fights could even be a good thing in some sense. They wouldn't be more or less obvious local replies anymore, but their assessment would depend on the whole board situation.

[Mef]

Perhaps even bigger than snapbacks is that a great many trades become impossible.

That's indeed an important difference, but I think such trades are actually much less frequent than one might expect.

In fact, I've reviewed a fair bunch of pro games from my small database of 5000 games (included with the SmartGo demo version) in search of moves that would have been illegal with the passive ko rule (from now on divergent moves)

I don't think your analysis is going to be correct here, because you ignore the possibility that players would strategically set up these positions. A simple endgame example:

$$B Black gets a and b
$$ ---------------------
$$ | . . . X b X X X O
$$ | X X X X O O O O O
$$ | O X O O O . . . .
$$ | a X O . . . . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .

Click Here To Show Diagram Code

[go]$$B Black gets a and b
$$ ---------------------
$$ | . . . X b X X X O
$$ | X X X X O O O O O
$$ | O X O O O . . . .
$$ | a X O . . . . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .[/go]

Assuming the rest of the board is settled B can take 1 and 3.

Or we can make it an L&D problem:

$$W W to kill
$$ -----------
$$ | . . O X O
$$ | X X X X O
$$ | . X O O O
$$ | . X O . .
$$ | X O O . .
$$ | O . . . .
$$ | . . . . .

Click Here To Show Diagram Code

[go]$$W W to kill
$$ -----------
$$ | . . O X O
$$ | X X X X O
$$ | . X O O O
$$ | . X O . .
$$ | X O O . .
$$ | O . . . .
$$ | . . . . .[/go]

[luigi]

Perhaps even bigger than snapbacks is that a great many trades become impossible.

That's indeed an important difference, but I think such trades are actually much less frequent than one might expect.

In fact, I've reviewed a fair bunch of pro games from my small database of 5000 games (included with the SmartGo demo version) in search of moves that would have been illegal with the passive ko rule (from now on divergent moves)

I don't think your analysis is going to be correct here, because you ignore the possibility that players would strategically set up these positions. A simple endgame example:

$$B Black gets a and b
$$ ---------------------
$$ | . . . X b X X X O
$$ | X X X X O O O O O
$$ | O X O O O . . . .
$$ | a X O . . . . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .

Click Here To Show Diagram Code

[go]$$B Black gets a and b
$$ ---------------------
$$ | . . . X b X X X O
$$ | X X X X O O O O O
$$ | O X O O O . . . .
$$ | a X O . . . . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .[/go]

Assuming the rest of the board is settled B can take 1 and 3.

Or we can make it an L&D problem:

$$W W to kill
$$ -----------
$$ | . . O X O
$$ | X X X X O
$$ | . X O O O
$$ | . X O . .
$$ | X O O . .
$$ | O . . . .
$$ | . . . . .

Click Here To Show Diagram Code

[go]$$W W to kill
$$ -----------
$$ | . . O X O
$$ | X X X X O
$$ | . X O O O
$$ | . X O . .
$$ | X O O . .
$$ | O . . . .
$$ | . . . . .[/go]

That's true. Making capturing trades impossible makes sente more valuable, which is perhaps the main strategic difference.

BTW, another interesting feature of this rule is that the mirror Go strategy is much harder to sustain.

[luigi]

As a curiosity, either of the following similar rules would make cycles mathematically impossible:

If your opponent has captured n stones on his turn, you can't make a capture on your next n turns. (EDIT: Further stones captured by the opponent are added to n.)

If your opponent has captured n stones on his turn, neither player can make a capture until the total number of non-capturing moves played afterwards equals n.

These rules would change the game completely, but they could be interesting experiments anyway. I suspect the first one would give far too much of an incentive to make captures, but the second one should be more balanced.

[skydyr]

If your opponent has captured n stones on his turn, neither player can make a capture until the total number of non-capturing moves played afterwards equals n.

Those last rules would have an interesting feature in that it is capture-sente (for lack of a better term) to capture odd numbers of stones, and capture-gote to capture even numbers.

[Mef]

If your opponent has captured n stones on his turn, you can't make a capture on your next n turns.

This rule needs slight modification to deal with one player capturing on consecutive turns. A contrived example:

$$B Four x send 3 return 1
$$ --------------------
$$ | X X O O O . O . O|
$$ | . X X X X O O O O|
$$ | X X . . X O . O X|
$$ | . X X X X O . O X|
$$ | X O O O X O O O X|
$$ | X O . O X X X X .|
$$ | X O . O X . . X X|
$$ | O O O O X X X X .|
$$ | O . O . O O O X X|
$$ --------------------

Click Here To Show Diagram Code

[go]$$B Four x send 3 return 1
$$ --------------------
$$ | X X O O O . O . O|
$$ | . X X X X O O O O|
$$ | X X . . X O . O X|
$$ | . X X X X O . O X|
$$ | X O O O X O O O X|
$$ | X O . O X X X X .|
$$ | X O . O X . . X X|
$$ | O O O O X X X X .|
$$ | O . O . O O O X X|
$$ --------------------[/go]

$$B Cooperative cycle B5, 7, 9 are pass
$$ --------------------
$$ | X X 8 6 . 3 O . O|
$$ | . X X X X O O O O|
$$ | X X . . X O . O X|
$$ | 0 X X X X O . O X|
$$ | X O O O X O O O X|
$$ | X O . O X X X X .|
$$ | X O . O X . . X X|
$$ | O O O O X X X X .|
$$ | O . O 1 . 2 4 X X|
$$ --------------------

Click Here To Show Diagram Code

[go]$$B Cooperative cycle B5, 7, 9 are pass
$$ --------------------
$$ | X X 8 6 . 3 O . O|
$$ | . X X X X O O O O|
$$ | X X . . X O . O X|
$$ | 0 X X X X O . O X|
$$ | X O O O X O O O X|
$$ | X O . O X X X X .|
$$ | X O . O X . . X X|
$$ | O O O O X X X X .|
$$ | O . O 1 . 2 4 X X|
$$ --------------------[/go]

B captures at 1 (W cannot capture for 3 moves), W2 (2 moves), B captures at 3 (W cannot capture for 3 moves), W4, 6, 8 remove the capture ban. 10 means B cannot capture for 3 moves.

$$Bm11 Cycle completes W18, 20 are pass, B21 marked
$$ --------------------
$$ | X X O O 4 X O . O|
$$ | . X X X X O O O O|
$$ | X X . . X O . O 7|
$$ | O X X X X O . O 5|
$$ | B O O O X O O O 9|
$$ | 1 O . O X X X X 2|
$$ | 3 O . O X . . X X|
$$ | O O O O X X X X .|
$$ | O . O X 6 O O X X|
$$ --------------------

Click Here To Show Diagram Code

[go]$$Bm11 Cycle completes W18, 20 are pass, B21 marked
$$ --------------------
$$ | X X O O 4 X O . O|
$$ | . X X X X O O O O|
$$ | X X . . X O . O 7|
$$ | O X X X X O . O 5|
$$ | B O O O X O O O 9|
$$ | 1 O . O X X X X 2|
$$ | 3 O . O X . . X X|
$$ | O O O O X X X X .|
$$ | O . O X 6 O O X X|
$$ --------------------[/go]

After 12, B can't play for 3 moves, W captures at 14 and 16, but those don't increase the length of the capture ban. B21 gets us back to the original position.


If you reword the rule to track of the total number of moves a player cannot play (add them together in the case of consecutive capture) it would prevent cycles for certain.

[luigi]

If you reword the rule to track of the total number of moves a player cannot play (add them together in the case of consecutive capture) it would prevent cycles for certain.

Yes, you're right. Actually that's what I was trying to say, but I didn't realize that my oversimplified wording didn't convey that meaning.

When I tested this idea, I had captured pieces put in a cumulative "reserve" next to the board. At the start of a player's turn, if there were any stones of his color in the reserve, he had to pass or use a stone from the reserve to make a non-capturing move on the board, after which his turn ended.

That would be the proper wording.

[luigi]

As a curiosity, either of the following similar rules would make cycles mathematically impossible:

If your opponent has captured n stones on his turn, you can't make a capture on your next n turns. (EDIT: Further stones captured by the opponent are added to n.)

If your opponent has captured n stones on his turn, neither player can make a capture until the total number of non-capturing moves played afterwards equals n.

These rules would change the game completely, but they could be interesting experiments anyway. I suspect the first one would give far too much of an incentive to make captures, but the second one should be more balanced.

There's of course a third alternative:

If you capture n stones on your turn, you can't make a capture again on your next n turns.

This has some curious implications regarding ko.

Also, in all three versions, instead of banning capturing moves, you can allow them but not let players remove the surrounded stones until n is 0.

[skydyr]

Also, in all three versions, instead of banning capturing moves, you can allow them but not let players remove the surrounded stones until n is 0.

Does this mean that the capture of, say, a 20-stone group could be prevented by capturing and removing a 4-stone group surrounding it before the 20 turn wait expires? Because my head just broke

[badukJr]

So basically anyone losing a capturing race can delay it and huge swaths of the board will become empty as both groups are simultaneously removed from the board when the ban is up.

Sounds Crazy.