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8.7 Eye Local Game Values

The game values in eyes.db are given in a simplified scheme which is flexible enough to represent most possibilities in a useful way.

The colon line below the pattern gives the eye value of the matched eye shape. This consists of four digits, each of which is the number of eyes obtained during the following conditions:

  1. The attacker moves first and is allowed yet another move because the defender plays tenuki.
  2. The attacker moves first and the defender responds locally.
  3. The defender moves first and the attacker responds locally.
  4. The defender moves first and is allowed yet another move because the attacker plays tenuki.

The first case does not necessarily mean that the attacker is allowed two consecutive moves. This is explained with an example later.

Also, since two eyes suffice to live, all higher numbers also count as two.

The following 15 cases are of interest:

The 3/4, 5/4, and 1* eye values are the same as in Howard Landman's paper Eyespace Values in Go. Attack and defense points are only marked in the patterns when they have definite effects on the eye value, i.e. pure threats are not marked.

Examples of all different cases can be found among the patterns in this file. Some of them might be slightly counterintuitive, so we explain one important case here. Consider

     Pattern 6141
     
      X
     XX.@x
     
     :1122

which e.g. matches in this position:

     .OOOXXX
     OOXOXOO
     OXXba.O
     OOOOOOO

Now it may look like `X' could take away both eyes by playing `a' followed by `b', giving 0122 as eye value. This is where the subtlety of the definition of the first digit in the eye value comes into play. It does not say that the attacker is allowed two consecutive moves but only that he is allowed to play "another move". The crucial property of this shape is that when `X' plays at a to destroy (at least) one eye, `O' can answer at `b', giving:

     
     .OOOXXX
     OO.OXOO
     O.cOX.O
     OOOOOOO

Now `X' has to continue at `c' in order to keep `O' at one eye. After this `O' plays tenuki and `X' cannot destroy the remaining eye by another move. Thus the eye value is indeed 1122.

As a final note, some of the eye values indicating a threat depend on suicide to be allowed, e.g.

     
     Pattern 301
     
     X.X
     
     :1222

We always assume suicide to be allowed in this database. It is easy enough to sort out such moves at a higher level when suicide is disallowed.