In the tactical reading code in @file{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
@file{engine/optics.c} (@pxref{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 @file{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 @strong{semeai} is
a capturing race between two adjacent DEAD or CRITICAL
dragons of opposite colors. The principal function,
@code{owl_analyze_semeai()} is contained in @file{owl.c}.
Due to the complex nature of semeais, the results of
this function are more frequently wrong than the usual
owl code.

@menu
* The Owl Code::                Life and death reading
* Combinations::                Combinations
@end menu

@node The Owl Code
@section The Owl Code

The life and death code in @file{optics.c}, described elsewhere
(@pxref{Eyes}), works reasonably well as long as the position is in a
@dfn{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
@dfn{Optics With Limit-negotiation} (OWL). The module which
implements it is in @file{engine/owl.c}.

There are two reasonably small databases
@file{patterns/owl_defendpats.db} and @file{patterns/owl_attackpats.db}
of expanding and limiting moves. The code in @file{owl.c} generates a
small move tree, allowing the attacker only moves from
@file{owl_attackpats.db}, and the defender only moves from
@file{owl_defendpats.db}. In addition to the moves suggested by
patterns, vital moves from the eye space analysis are also tested.

A third database, @file{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 @file{owl_vital_apats.db} is the only available measure
to correct mistakes by the optics. Currently the patterns in
@file{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.

@findex owl_attack
@findex owl_defend
@findex compute_eyes_pessimistic
A node of the move tree is considered @code{terminal} if no further moves 
are found from @file{owl_attackpats.db} or @file{owl_defendpats.db}, or if
the function @code{compute_eyes_pessimistic()} reports that the group is
definitely alive. At this point, the status of the group is evaluated.
The functions @code{owl_attack()} and @code{owl_defend()}, with
usage similar to @code{attack()} and @code{find_defense()}, make
use of the owl pattern databases to generate the move tree and decide
the status of the group.

The function @code{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 @code{MAX_MOVES} defined at the beginning of
@file{engine/owl.c}. The most most valuable moves are
tried first, with the following restrictions:

@itemize @bullet
@item
If @code{stackp > owl_branch_depth} then only one move is tried per
variation. 
@item
If @code{stackp > owl_reading_depth} then the reading terminates,
and the situation is declared a win for the defender (since
deep reading may be a sign of escape).
@item
If the node count exceeds @code{owl_node_limit}, the reading also
terminates with a win for the defender.
@item
Any pattern with value 99 is considered a forced move: no
other move is tried, and if two such moves are found, the function
returns false. This is only relevant for the attacker.
@item
Any pattern in @file{patterns/owl_attackpats.db} and 
@file{patterns/owl_defendpats.db} with value 100 is considered a win: if
such a pattern is found by @code{owl_attack} or @code{owl_defend}, the
function returns true. This feature must be used most carefully.
@end itemize

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

When @code{owl_attack()} or @code{owl_defend()} is called,
the dragon under attack is marked in the array @code{goal}.
The stones of the dragon originally on the board are marked
with goal=1; those added by @code{owl_defend()} are marked
with goal=2. If all the original strings of the original dragon 
are captured, @code{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 @code{make_dragons()}.

The owl code can be conveniently tested using the 
@option{--decide-owl @var{location}} option. This should be used with 
@option{-t} to produce a useful trace, @option{-o} to produce
an SGF file of variations produced when the life and death of
the dragon at @var{location} is checked, or both. 
@option{--decide-position} performs the same analysis for all
dragons with small escape route. 

@node Combinations
@section Combination reading

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
@file{combination.c} is to find such moves.

For example, consider the following situation:

@example

+---------
|....OOOOX
|....OOXXX
|..O.OXX..
|.OXO.OX..
|.OX..OO..
|.XXOOOXO.
|..*XXOX..
|....XOX..
|.XX..X...
|X........

@end example

Every @samp{X} stone in this position is alive. However the move
at @samp{*} produces a position in which at least one of four
strings will get captured. This is a @emph{combination}.

The driving function is called @code{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 @samp{*} is @emph{not} an
atari, though after @samp{O} defends the four stones above, a
sequence of ataris ensues resulting in the capture of some
string.

Like the owl functions @code{atari_atari} does pattern-based
reading. The database generating the attacking moves is
@file{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.

@itemize @bullet
@item @code{void combinations(int color)}
@findex combinations
@quotation
Generate move reasons for combination attacks and defenses against
them. This is one of the move generators called from genmove().
@end quotation
@item @code{int atari_atari(int color, int *attack_move, char defense_moves[BOARDMAX], int save_verbose)}
@findex atari_atari
@quotation
Look for a combination for @code{color}. For the purpose of
the move generation, returns the size of the smallest of the
worms under attack.
@end quotation
@item @code{int atari_atari_confirm_safety(int color, int move, int *defense, int minsize, const char saved_dragons[BOARDMAX], const char saved_worms[BOARDMAX])}
@findex atari_atari_confirm_safety
@quotation
Tries to determine whether a move is a blunder.  Wrapper
around atari_atari_blunder_size. Check whether a combination
attack of size at least @code{minsize} appears after move at
@code{move} has been made.  The arrays @code{saved_dragons[]} and
@code{saved_worms[]} should be one for stones belonging to dragons
or worms respectively, which are supposedly saved by @code{move}.
@end quotation
@item @code{int atari_atari_blunder_size(int color, int move, int *defense, const char safe_stones[BOARDMAX])}
@findex atari_atari_blunder_size
@quotation
This function checks whether any new combination attack appears after
move at (move) has been made, and returns its size (in points).
@code{safe_stones} marks which of our stones are supposedly safe
after this move.
@end quotation
@end itemize