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15.1 The Owl Code

The life and death code in `optics.c', described elsewhere (see section 11. 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 `apats.db' or `dpats.db', or if the function compute_eyes_pessimistic() reports that the group is definitely alive or dead. At this point, the status of the group is evaluated. The functions owl_attack() and owl_defend(), with usage similar to attack() and find_defense(), make use of the owl pattern databases to generate the move tree and decide the status of the group.

The function 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 the parameter MAX_MOVES defined at the beginning of `engine/owl.c'. The most most valuable moves are tried first, with the following restrictions:

The functions owl_attack() and owl_defend() may, like attack() and find_defense(), return an attacking or defending move through their pointer arguments. If the position is already won, owl_attack() may or may not return an attacking move. If it finds no move of interest, it will return PASS, that is, 0. The same goes for owl_defend().

When owl_attack() or owl_defend() is called, the dragon under attack is marked in the array goal. 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 are captured, 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 make_dragons().

The owl code can be conveniently tested using the `--decidedragon location' 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. `--decideposition' performs the same analysis for all dragons with small escape route.

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