gnubg uses a technique called "move filters" in order to prune the complete list of legal moves when analysing chequer play decisions. Move filters can be considering a generalising of the search space used in earlier versions of gnubg.
A move filter for a given ply, say, 2-ply, consists of four parameters for each subply:
For example, for 2-ply chequer play decisions there are two movefilters: one for pruning at 0-ply, and another for pruning at 1-ply. The predefined setting "Normal" has: accept 0 moves and add up to 8 moves within 0.16 at 0-ply, and no pruning at 1-ply.
Consider the opening position where 6-2 has been rolled:
GNU Backgammon Position ID: 4HPwATDgc/ABMA
Match ID : cAkZAAAAAAAA
+13-14-15-16-17-18------19-20-21-22-23-24-+ O: gnubg
| X O | | O X | 0 points
| X O | | O X |
| X O | | O |
| X | | O |
| X | | O |
v| |BAR| | (Cube: 1)
| O | | X |
| O | | X |
| O X | | X |
| O X | | X O | Rolled 24
| O X | | X O | 0 points
+12-11-10--9--8--7-------6--5--4--3--2--1-+ X: jth
Pip counts: O 167, X 167
gnubg starts by finding all possible moves and evaluate those at 0-ply:
1. Cubeful 0-ply 8/4 6/4 Eq.: +0.207
0.548 0.172 0.009 - 0.452 0.121 0.005
0-ply cubeful [expert]
2. Cubeful 0-ply 13/11 13/9 Eq.: +0.050 ( -0.156)
0.509 0.155 0.009 - 0.491 0.137 0.007
0-ply cubeful [expert]
3. Cubeful 0-ply 24/20 13/11 Eq.: +0.049 ( -0.158)
0.513 0.137 0.007 - 0.487 0.132 0.004
0-ply cubeful [expert]
4. Cubeful 0-ply 24/22 13/9 Eq.: +0.037 ( -0.170)
0.508 0.142 0.007 - 0.492 0.134 0.004
0-ply cubeful [expert]
5. Cubeful 0-ply 24/22 24/20 Eq.: -0.008 ( -0.215)
0.501 0.121 0.006 - 0.499 0.133 0.003
0-ply cubeful [expert]
6. Cubeful 0-ply 24/18 Eq.: -0.015 ( -0.222)
0.502 0.121 0.006 - 0.498 0.140 0.004
0-ply cubeful [expert]
7. Cubeful 0-ply 24/20 6/4 Eq.: -0.023 ( -0.229)
0.497 0.132 0.007 - 0.503 0.144 0.005
0-ply cubeful [expert]
8. Cubeful 0-ply 13/9 6/4 Eq.: -0.026 ( -0.233)
0.494 0.146 0.008 - 0.506 0.151 0.009
0-ply cubeful [expert]
[10 moves deleted]
Accoring to the move filter the first 0 moves are accepted. The equity of the best move is +0.207, and according to the movefilter we add up to 8 extra moves if they're within 0.160, that is, if they have equity higher than +0.047. Move 4-18 all have equity lower that +0.047, so the move list after pruning at 0-ply consists of move 1-3. According to the move filter we do not perform any pruning at 1-ply, so move 1-3 are submitted for evaluation at 2-ply;
1. Cubeful 2-ply 8/4 6/4 Eq.: +0.197
0.546 0.172 0.008 - 0.454 0.123 0.005
2-ply cubeful 100% speed [world class]
2. Cubeful 2-ply 24/20 13/11 Eq.: +0.058 ( -0.138)
0.515 0.141 0.007 - 0.485 0.130 0.005
2-ply cubeful 100% speed [world class]
3. Cubeful 2-ply 13/11 13/9 Eq.: +0.050 ( -0.147)
0.508 0.156 0.007 - 0.492 0.136 0.006
2-ply cubeful 100% speed [world class]
4. Cubeful 0-ply 24/22 13/9 Eq.: +0.037 ( -0.159)
0.508 0.142 0.007 - 0.492 0.134 0.004
0-ply cubeful [expert]
5. Cubeful 0-ply 24/22 24/20 Eq.: -0.008 ( -0.205)
0.501 0.121 0.006 - 0.499 0.133 0.003
0-ply cubeful [expert]
6. Cubeful 0-ply 24/18 Eq.: -0.015 ( -0.212)
0.502 0.121 0.006 - 0.498 0.140 0.004
0-ply cubeful [expert]
7. Cubeful 0-ply 24/20 6/4 Eq.: -0.023 ( -0.219)
0.497 0.132 0.007 - 0.503 0.144 0.005
0-ply cubeful [expert]
8. Cubeful 0-ply 13/9 6/4 Eq.: -0.026 ( -0.222)
0.494 0.146 0.008 - 0.506 0.151 0.009
0-ply cubeful [expert]
If we instead request a 4-ply chequer play decision, gnubg will use the move filters defined for 4-ply:
| Ply | Accept moves | Extra moves | Threshold for extra moves
|
| 0 | 0 | 8 | 0.160
|
| 1 | no pruning |
| |
| 2 | 0 | 2 | 0.040
|
| 3 | no pruning |
|
The 4-ply move filter is identical to the 2-ply for pruning at 0-ply, so after 0-ply we have the same three moves as above. Since there is no pruning at 1-ply these three moves are evaluated at 2-ply as above. There is no pruning at 3-ply.
At 4-ply we do not accept any moves, but add up to two moves if there within 0.040 from the best move. Since the second best move is -0.138 worse than the best move, we do not accept any moves to be evaluated at 4-ply. Hence gnubg will actually not evaluate any moves on 4-ply.
The predefined movefilters all have "accept 0 moves", in order to facilitate fast decisions and analysis, i.e., no need to waste much time over obvious moves.
For post-mortem analysis it may be worthwhile to ensure that gnubg analyses at least two moves at the specified ply. To do this, specify "accept 2 moves" in the move filters you use for analysis. However, do note that gnubg will force evaluation at the specified ply if the actual move made is doubtful. This ensures that all errors and blunders are evaluted at the same level.