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In the tactical reading code in ‘reading.c’, the code generating the moves which are tried are all hand coded in C, for efficiency. There is much to be said for another type of reading, in which the moves to be tried are generated from a pattern database.
GNU Go does three main types of pattern based reading. First, there is the OWL code (Optics with Limit Negotiation) which attempts to read out to a point where the code in ‘engine/optics.c’ (see section Eyes and Half Eyes) can be used to evaluate it. Like the tactical reading code, a persistent cache is employed to maintain some of the owl data from move to move. This is an essential speedup without which GNU Go would play too slowly.
Secondly, there is the ‘engine/combination.c’ which attempts to find combinations—situations where a series of threats eventually culminates in one that cannot be parried.
Finally there is the semeai module. A semeai is
a capturing race between two adjacent DEAD or CRITICAL
dragons of opposite colors. The principal function,
owl_analyze_semeai() is contained in ‘owl.c’.
Due to the complex nature of semeais, the results of
this function are more frequently wrong than the usual
|12.1 The Owl Code||Life and death reading|
|12.2 Combination reading||Combinations|
The life and death code in ‘optics.c’, described elsewhere (see section Eyes and Half Eyes), works reasonably well as long as the position is in a terminal position, which we define to be one where there are no moves left which can expand the eye space, or limit it. In situations where the dragon is surrounded, yet has room to thrash around a bit making eyes, a simple application of the graph-based analysis will not work. Instead, a bit of reading is needed to reach a terminal position.
The defender tries to expand his eyespace, the attacker to limit it, and when neither finds an effective move, the position is evaluated. We call this type of life and death reading Optics With Limit-negotiation (OWL). The module which implements it is in ‘engine/owl.c’.
There are two reasonably small databases ‘patterns/owl_defendpats.db’ and ‘patterns/owl_attackpats.db’ of expanding and limiting moves. The code in ‘owl.c’ generates a small move tree, allowing the attacker only moves from ‘owl_attackpats.db’, and the defender only moves from ‘owl_defendpats.db’. In addition to the moves suggested by patterns, vital moves from the eye space analysis are also tested.
A third database, ‘owl_vital_apats.db’ includes patterns which override the eyespace analysis done by the optics code. Since the eyeshape graphs ignore the complications of shortage of liberties and cutting points in the surrounding chains, the static analysis of eyespace is sometimes wrong. The problem is when the optics code says that a dragon definitely has 2 eyes, but it isn't true due to shortage of liberties, so the ordinary owl patterns never get into play. In such situations ‘owl_vital_apats.db’ is the only available measure to correct mistakes by the optics. Currently the patterns in ‘owl_vital_apats.db’ are only matched when the level is 9 or greater.
The owl code is tuned by editing these three pattern databases, principally the first two.
A node of the move tree is considered
terminal if no further moves
are found from ‘owl_attackpats.db’ or ‘owl_defendpats.db’, or if
compute_eyes_pessimistic() reports that the group is
definitely alive. At this point, the status of the group is evaluated.
usage similar to
use of the owl pattern databases to generate the move tree and decide
the status of the group.
compute_eyes_pessimistic() used by the owl
code is very conservative and only feels certain about eyes if the
eyespace is completely closed (i.e. no marginal vertices).
The maximum number of moves tried at each node is limited by
MAX_MOVES defined at the beginning of
‘engine/owl.c’. The most most valuable moves are
tried first, with the following restrictions:
stackp > owl_branch_depththen only one move is tried per variation.
stackp > owl_reading_depththen the reading terminates, and the situation is declared a win for the defender (since deep reading may be a sign of escape).
owl_node_limit, the reading also terminates with a win for the defender.
owl_defend, the function returns true. This feature must be used most carefully.
owl_defend() may, like
find_defense(), return an attacking or
defending move through their pointer arguments. If the position is
owl_attack() may or may not return an attacking
move. If it finds no move of interest, it will return
0. The same goes for
owl_defend() is called,
the dragon under attack is marked in the array
The stones of the dragon originally on the board are marked
with goal=1; those added by
owl_defend() are marked
with goal=2. If all the original strings of the original dragon
owl_attack() considers the dragon to be defeated,
even if some stones added later can make a live group.
Only dragons with small escape route are studied when the
functions are called from
The owl code can be conveniently tested using the ‘--decide-owl location’ option. This should be used with ‘-t’ to produce a useful trace, ‘-o’ to produce an SGF file of variations produced when the life and death of the dragon at location is checked, or both. ‘--decide-position’ performs the same analysis for all dragons with small escape route.
It may happen that no single one of a set of worms can be killed, yet there is a move that guarantees that at least one can be captured. The simplest example is a double atari. The purpose of the code in ‘combination.c’ is to find such moves.
For example, consider the following situation:
+--------- |....OOOOX |....OOXXX |..O.OXX.. |.OXO.OX.. |.OX..OO.. |.XXOOOXO. |..*XXOX.. |....XOX.. |.XX..X... |X........
Every ‘X’ stone in this position is alive. However the move at ‘*’ produces a position in which at least one of four strings will get captured. This is a combination.
The driving function is called
atari_atari because typically
a combination involves a sequence of ataris culminating in a capture,
though sometimes the moves involved are not ataris. For example in
the above example, the first move at ‘*’ is not an
atari, though after ‘O’ defends the four stones above, a
sequence of ataris ensues resulting in the capture of some
Like the owl functions
atari_atari does pattern-based
reading. The database generating the attacking moves is
‘aa_attackpats.db’. One danger with this function is
that the first atari tried might be irrelevant to the actual
combination. To detect this possibility, once we've found a
combination, we mark that first move as forbidden, then try
again. If no combination of the same size or larger turns
up, then the first move was indeed essential.
void combinations(int color)
Generate move reasons for combination attacks and defenses against them. This is one of the move generators called from genmove().
int atari_atari(int color, int *attack_move, char defense_moves[BOARDMAX], int save_verbose)
Look for a combination for
color. For the purpose of the move generation, returns the size of the smallest of the worms under attack.
int atari_atari_confirm_safety(int color, int move, int *defense, int minsize, const char saved_dragons[BOARDMAX], const char saved_worms[BOARDMAX])
Tries to determine whether a move is a blunder. Wrapper around atari_atari_blunder_size. Check whether a combination attack of size at least
minsizeappears after move at
movehas been made. The arrays
saved_wormsshould be one for stones belonging to dragons or worms respectively, which are supposedly saved by
int atari_atari_blunder_size(int color, int move, int *defense, const char safe_stones[BOARDMAX])
This function checks whether any new combination attack appears after move at (move) has been made, and returns its size (in points).
safe_stonesmarks which of our stones are supposedly safe after this move.
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