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GNU Go Documentation

14.3 Ko Handling

The principles of ko handling are the same for tactical reading and owl reading.

We have already mentioned (see section 14.1 Reading Basics) that GNU Go uses a return code of KO_A or KO_B if the result depends on ko. The return code of KO_B means that the position can be won provided the player whose move calls the function can come up with a sufficiently large ko threat. In order to verify this, the function must simulate making a ko threat and having it answered by taking the ko even if it is illegal. We call such an experimental taking of the ko a conditional ko capture.

Conditional ko captures are accomplished by the function tryko(). This function is like trymove() except that it does not require legality of the move in question.

The static reading functions, and the global functions do_attack and do_find_defense have arguments komaster, kom_pos. These mediate ko captures to prevent the occurrence of infinite loops.

Normally komaster is EMPTY but it can also be BLACK, WHITE or GRAY. The komaster is set to color when color makes a conditional ko capture. In this case kom_pos is set to the location of the captured ko stone.

If the opponent is komaster, the reading functions will not try to take the ko at kom_pos. Also, the komaster is normally not allowed to take another ko. The exception is a nested ko, characterized by the condition that the captured ko stone is at distance 1 both vertically and horizontally from kom_pos, which is the location of the last stone taken by the komaster. Thus in this situation:

.OX OX*X OmOX OO

Here if `m' is the location of kom_pos, then the move at `*' is allowed.

The rationale behind this rule is that in the case where there are two kos on the board, the komaster cannot win both, and by becoming komaster he has already chosen which ko he wants to win. But in the case of a nested ko, taking one ko is a precondition to taking the other one, so we allow this.

If the komaster's opponent takes a ko, then both players have taken one ko. In this case komaster is set to GRAY and after this further ko captures are not allowed.

If the ko at kom_pos is filled, then the komaster reverts to EMPTY.

The komaster scheme used in GNU Go 3.0 is known as komaster scheme 1. It may be summarized as follows. It is assumed that `O' is about to move.

• 1. Komaster is EMPTY.
  • 1a) Unconditional ko capture is allowed. Komaster remains EMPTY.
  • 1b) Conditional ko capture is allowed. Komaster is set to `O' and kom_pos to the location of the ko, where a stone was just removed.
• 2. Komaster is `O':
  • 2a) Only nested ko captures are allowed.
  • 2b) If komaster fill the ko at kom_pos then komaster reverts to EMPTY.
• 3. Komaster is `X':
  • Play at kom_pos is not allowed. Any other ko capture is allowed. If `O' takes another ko, komaster becomes GRAY.
• 4. Komaster is GRAY:
  • Ko captures are not allowed. If the ko at kom_pos is filled then the komaster reverts to EMPTY.

In GNU Go 3.2 a new komaster scheme 5 is used. It may be described as follows.

• 1. Komaster is EMPTY.
  • 1a. Unconditional ko capture is allowed.

    Komaster remains EMPTY if previous move was not a ko capture. Komaster is set to WEAK_KO if previous move was a ko capture and kom_pos is set to the old value of board_ko_pos.

  • 1b) Conditional ko capture is allowed.

    Komaster is set to O and kom_pos to the location of the ko, where a stone was just removed.

• 2. Komaster is O:
  • 2a) Only nested ko captures are allowed. Kom_pos is moved to the new removed stone.
  • 2b) If komaster fills the ko at kom_pos then komaster reverts to EMPTY.
• 3. Komaster is X:

  Play at kom_pos is not allowed. Any other ko capture is allowed. If O takes another ko, komaster becomes GRAY_X.

• 4. Komaster is GRAY_O or GRAY_X:

  Ko captures are not allowed. If the ko at kom_pos is filled then the komaster reverts to EMPTY.

• 5. Komaster is WEAK_KO:
  • 5a) After a non-ko move komaster reverts to EMPTY.
  • 5b) Unconditional ko capture is only allowed if it is nested ko capture.

    Komaster is changed to WEAK_X and kom_pos to the old value of board_ko_pos.

  • 5c) Conditional ko capture is allowed according to the rules of 1b.