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9.6 Valuation of suggested moves

At the end of the move generation process, the function value_move_reasons() tries to assign values to the moves for the purpose of selecting the best move. The single purpose of the move valuation is to try to rank the moves so that the best move gets the highest score. In principle these values could be arbitrary, but in order to make it easier to evaluate how well the valuation performs, not to mention simplify the tuning, we try to assign values which are consistent with the usual methods of counting used by human Go players, as explained for example in The Endgame by Ogawa and Davies.

Moves are valued with respect to four different criteria. These are

All of these are floats and should be measured in terms of actual points.

The territorial value is the total change of expected territory caused by this move. This includes changes in the status of groups if the move is an attack or a defense move.

Beginning with GNU Go 3.0, the influence function plays an important role in estimating territory (see section 16.3 Influence and Territory). It is used to make a guess at each intersection how likely it is that it will become black or white territory. The territorial value sums up the changes in these valuations.

Strategical value is a measure of the effect the move has on the safety of all groups on the board. Typically cutting and connecting moves have their main value here. Also edge extensions, enclosing moves and moves towards the center have high strategical value. The strategical value should be the sum of a fraction of the territorial value of the involved dragons. The fraction is determined by the change in safety of the dragon.

Shape value is a purely local shape analysis. An important role of this measure is to offset mistakes made by the estimation of territorial values. In open positions it's often worth sacrificing a few points of (apparent) immediate profit to make good shape. Shape value is implemented by pattern matching, the Shape patterns.

Secondary value is given for move reasons which by themselves are not sufficient to play the move. One example is to reduce the number of eyes for a dragon that has several or to attack a defenseless worm.

When all these values have been computed, they are summed, possibly weighted (secondary value should definitely have a small weight), into a final move value. This value is used to decide the move.

9.6.1 Territorial Value  How much territory does a move gain
9.6.2 Strategical Value  Strategical gains from a move
9.6.3 Shape Factor  Local shape
9.6.4 Minimum Value  Minimum value
9.6.5 Secondary Value  Other, more indirect, gains from a move
9.6.6 Threats and Followup Value  Valuation of attack and defense threats


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