The 50 highest scoring board points are then examined by the evaluation function. The board point which receives the highest score from the evaluation function is played as thenext move.
In some circumstances, Go4++ effectively evaluates more than 50 candidate moves per turn.Pattern matching is not applied to regions of the board where the stone safety (section 2.4) and territory (section 2.6) do not differ from those of the previous move. Thus after evaluating the 50 highest scoring board points after pattern matching, the next move is selected from the combined set of evaluation scores of the 50 board points just consideredand the evaluation scores of any board points in the undisturbed regions which were considered during the selection of the previous move.
2 Evaluation Function
The evaluation function is a six step process: 1. a connectivity probability map is generated, 2. groups are determined, 3. eye space is determined from which individual eyesare identified, 4. the safety of each stone is determined, 5. a radiation function combines the stone safety data and connectivitv probability map to radiate safety to empty points, and 6. territory is determined. Reiss describes the evaluation function as pessimistic since whenever it is applicable, the evaluation function considers that the opponent is the next to move.
2.1 Connectivity Probability Map
Each time a hypothetical move is played on the board, a connectivity probability map forall friendly and opponent stones (including the hypothetical stone) is generated and stored. The connectivity probability map for a stone contains the probability of connecting to nearby friendly stones already on the board or to an empty point if it was occupied by a friendly stone. Stones and empty points which can be reached directly (nobi),by a diagonal move (kosumi link), a one point jump (ikken-tobi link), a two point jump (nikken-tobi link), a small knight's move (kogeima link), or a large knight's move (ogeima link) are examined (total of 32 possible points).
The probability of connecting two stones is calculated empirically by playing out the sequences of moves needed to determine whether the stones can be connected or whether the opponent can cut them. This process requires the use of lookahead, particularly when ladders are generated by cutting moves (e.g., cutting a kogeima link generates two ladders,one for each cutting point). If all possible cutting stones are captured, then the probability of connection is l00%. However, if the cutting stones can not be captured, a hand-tuned algorithm estimates the connection probability.
A massive amount of data is generated in calculating the probability map for each stone on the board for every hypothetical stone being evaluated. The process is very computationally expensive and requires a powerful computer (Go4++ currently runs on a Pentium-Pro 200).
