@menu
* Influential Concepts::        Conceptual Outline of Influence
* Territory and Moyo::		Territory, Moyo and Area
* Influence Usage::		Where influence gets used in the engine
* Influence and Territory::     Influence and Territory
* Territorial Details::		Details of the Territory Valuation
* The Influence Core::          The Core of the Influence Function
* The Influence Algorithm::     The algorithm of @code{accumlate_influence()}
* Permeability::                Permeability
* Escape::                      Escape
* Break Ins::                   Break Ins
* Surrounded Dragons::          Surrounded Dragons
* Influential Patterns::	Patterns used by the Influence module
* Influential Display::         Colored display and debugging of influence
* Influence Tuning::            Influence tuning with view.pike
@end menu

@node Influential Concepts
@section Conceptual Outline of Influence

We define call stones @dfn{lively} if they cannot be tactically
attacked, or if they have a tactical defense and belong to the player
whose turn it is. Similarly, stones that cannot be strategically attacked
(in the sense of the life-and-death analysis), or that have a strategical
defense and belong to the player to move, are called @dfn{alive}.
If we want to use the influence function before deciding the strategical
status, all lively stones count as alive.

Every alive stone on the board works as an influence source, with
influence of its color radiating outwards in all directions. The
strength of the influence declines exponentially with the distance
from the source.

Influence can only flow unhindered if the board is empty, however. All
lively stones (regardless of color) act as influence barriers, as do
connections between enemy stones that can't be broken through. For
example the one space jump counts as a barrier unless either of the
stones can be captured. Notice that it doesn't matter much if the
connection between the two stones can be broken, since in that case
there would come influence from both directions anyway.

From the influence of both colors we compute a territorial value between
-1.0 and +1.0 for each intersection, which can be seen as the likely hood
of it becoming territory for either color.

In order to avoid finding bogus territory, we add extra influence
sources at places where an invasion can be launched, e.g. at 3-3 under
a handicap stone, in the middle of wide edge extensions and in the
center of large open spaces anywhere. Similarly we add extra influence
sources where intrusions can be made into what otherwise looks as
solid territory, e.g. monkey jumps. These intrusions depend on whose
turn we assume it to be.

All these extra influence sources, as well as connections, are controlled
by a pattern database, which consists of the two files patterns/influence.db
and patterns/barriers.db. The details are explained in
@ref{Influential Patterns}.

@node Territory and Moyo
@section Territory, Moyo and Area
@cindex territory
@cindex moyo
@cindex area

Using the influence code, empty regions of the board are partitioned
in three ways. A vertex may be described as White or Black's 
@dfn{territory}, @dfn{moyo} or @dfn{area}. The functions
@code{whose_territory()}, @code{whose_moyo()} and @code{whose_area()}
will return a color, or EMPTY if it belongs to one player or the
other in one of these classifications.

@itemize @bullet
@item Territory
@quotation
Those parts of the board which are expected to materialize
as actual points for one player or the other at the end of
the game are considered @dfn{territory}.
@end quotation
@item Moyo
@quotation
Those parts of the board which are either already territory or more generally
places where a territory can easily materialize if the opponent neglects to
reduce are considered @dfn{moyo}.
@dfn{moyo}.
@end quotation
@item Area
@quotation
Those parts of the board where one player or the other has a
stronger influence than his opponent are considered @dfn{area}.
@end quotation
@end itemize

Generally territory is moyo and moyo is area. To get a feeling
for these concepts, load an sgf file in a middle game position
with the option @option{-m 0x0180} and examine the resulting
diagrams (@pxref{Influential Display}).

@node Influence Usage
@section Where influence gets used in the engine

The information obtained from the influence computation is used in a variety
of places in the engine, and the influence module is called several times
in the process of the move generation. The details of the influence
computation vary according to the needs of the calling function.

After GNU Go has decided about the tactical stability of strings, the
influence module gets called the first time. Here all lively stones act
as an influence source of default strength 100. The result is stored in
the variables @code{initial_influence} and @code{initial_opposite_influence},
and it is used as an important information for guessing the strength of
dragons. For example, a dragon that is part of a moyo of size 25 is
immediately considered alive.  For dragons with a smaller moyo size, a
life-and-death analysis will be done by the owl code (see @ref{Pattern Based
Reading}). A dragon with a moyo size of only 5 will be considered weak, even
if the owl code has decided that it cannot be killed.

As a tool for both the owl code and the strength estimate of dragons,
an "escape" influence gets computed for each dragon (@pxref{Escape}).

Once all dragons have been evaluated, the influence module is called again
and the variables @code{initial_influence} and
@code{initial_opposite_influence} get overwritten. Of course, the dragon
status', which are available now, are taken into account. Stones belonging
to a dead dragon will not serve as an influence source, and the strengths of
other stones get adjusted according to the strength of their respective
dragon.

The result of this run is the most important tool for move evaluation. All
helper functions of patterns as explained in @ref{Patterns} that
refer to influence results (e. g. @code{olib(*)} etc.) actually use these
results. Further, @code{initial_influence} serves as the reference for
computing the territorial value of a move. That is, from the influence
strengths stored in @code{initial_influence}, a territory value is
assigned to each intersection. This value is supposed to estimate the
likelyhood that this intersection will become white or black territory.

Then, for each move that gets considered in the function @code{value_moves},
the influence module is called again via the function
@code{compute_move_influence} to assess the likely territorial balance after
this move, and the result is compared with the state before that move.

An additional influence computation is done in order to compute the followup
value of a move. Some explainations are in @ref{Territorial Details}.

Some of the public functions from @file{influence.c} which are used
throughout the engine are listed in @ref{Influence Utilities}.

@node Influence and Territory
@section Influence and Territory

In this section we consider how the influence function is used to
estimate territory in the function @code{estimate_territorial_value()}.

A move like @samp{*} by @samp{O} below is worth one point:

@example
OXXX.
OX.XX
O*a.X
OX.XX
OXXX.
@end example

This is evaluated by the influence function in the following way:
We first assign territory under the assumption that X moves first in all
local positions in the original position;  then we reassing territory,
again under the assumption that @samp{X} moves first in all local positions,
but after we let @samp{O} make the move at @samp{*}. These two
territory assignments are compared and the difference gives the
territorial value of the move.

Technically, the assumption that @samp{X} plays first everywhere is
implemented via an asymmetric pattern database in @code{barriers.db}.
What exactly is a safe connection that stops hostile influence from
passing through is different for @samp{O} and @samp{X}; of course such a
connection has to be tighter for stones with color @samp{O}. Also,
additional intrusion influence sources are added for @samp{X} in places
where @samp{X} stones have natural followup moves.

In this specific example above, the asymmetry (before any move has been made)
would turn out as follows: If @samp{X} is in turn to move, the white influence
would get stopped by a barrier at @samp{*}, leaving 4 points of territory
for @samp{X}.  However, if @samp{O} was next to move, then a followup move
for the white stones at the left would be assumed in the form of an extra
("intrusion") influence source at @samp{*}. This would get stopped at
@samp{a}, leaving three points of territory.

Returning to the valuation of a move by @samp{O} at @samp{*}, we get a
value of 1 for the move at @samp{*}.
However, of course this move is sente once it is worth playing, and should
therefore (in miai counting) be awarded an effective value of 2. Hence we
need to recognize the followup value of a move. GNU Go 3.0 took care of
this by using patterns in @code{patterns.db} that enforced an explicit
followup value. Versions from 3.2 on instead compute a seperate followup
influence to each move considered. In the above example, an intrusion source
will be added at @samp{a} as a followup move to @samp{*}. This destroys all of
Black's territory and hence gives a followup value of 3.

The pattern based followup value are still needed at some places, however.

To give another example, consider this position where we want to
estimate the value of an @samp{O} move at @samp{*}:

@example
OOOXXX
..OX..
..OX..
...*..
------
@end example

Before the move we assume @samp{X} moves first in the local position (and
that @samp{O} has to connect), which gives territory like this (lower case
letter identify territory for each player):

@example
OOOXXX
ooOXxx
o.OXxx
o...xx
------
@end example

Then we let @samp{O} make the move at @samp{*} and assume
@samp{X} moves first again next.  The territory then becomes (@samp{X}
is also assumed to have to connect):

@example
OOOXXX
ooOXxx
ooOX.x
oo.O.x
------
@end example

We see that this makes a difference in territory of 4, which is what
influence_delta_territory() should report. Then again, we have followup
value, and here also a reverse followup value. The reverse followup value,
which in this case will be so high that the move is treated as reverse
sente, is added by an explicit pattern. Other sources for followup or
reverse followup values are threats to capture a rescue a string of stones.
See the code and comments in the function @code{value_move_reaons} for how
followup and reverse followup values are used to adjust the effective 
move value.

To give an example of territorial value where something is captured,
consider the @samp{O} move at @samp{*} here,

@example
XXXXXXXO
X.OOOOXO
X.O..O*O
--------
@end example

As before we first let the influence function determine territory
assuming X moves first, i.e. with a captured group:

@example
XXXXXXXO
XxyyyyXO
Xxyxxy.O
--------
@end example

Here @samp{y} indicates @samp{X} territory + captured stone,
i.e. these count for two points. After the @samp{O} move at @samp{*} we
instead get

@example
XXXXXXXO
X.OOOOXO
X.OooOOO
--------
@end example

and we see that @samp{X} has 16 territory fewer and @samp{O}
has two territory more, for a total difference of 18 points.

That the influence function counts the value of captured stones was
introduced in GNU Go 3.2. Previously this was instead done using the
effective_size heuristic. The effective size is the number of
stones plus the surrounding empty spaces which are closer to
this string or dragon than to any other stones. Here the @samp{O}
string would thus have effective size 6 (number of stones) + 2
(interior eye) + 2*0.5 (the two empty vertices to the left of
the string, split half each with the surrounding X string) +
1*0.33 (the connection point, split between three strings) =
9.33. As noted this value was doubled, giving 18.67 which is
reasonably close to the correct value of 18. The effective size
heuristic is still used in certain parts of the move valuation
where we can't easily get a more accurate value from the
influence function (e. g. attacks depending on a ko, attack threats).

Note that this section only describes the territorial valuation of a move.
Apart from that, GNU Go uses various heuristics in assigning a strategical
value (weakening and strengthening of other stones on the board) to a move.
Also, the influence function isn't quite as well tuned as the examples above
may seem to claim. But it should give a fairly good idea of how the design
is intended.

Another matter is that so far we have only considered the change in secure
territory. GNU Go 3.2 and later versions use a revised heuristic, which
is explained in the next section, to assign probable territory to each
player.

@node Territorial Details
@section Details of the Territory Valuation

This section explains how GNU Go assigns a territorial value to an
intersection once the white and black influence have been computed.
The intention is that an intersection that has a chance of xx% of
becoming white territory is counted as 0.xx points of territory for
white, and similar for black.

The algorithm in the function @code{new_value_territory} goes roughly
as follows:

If @code{wi} is the white influence at a point, and @code{bi} the black
influence, then @code{ value = ( (wi-bi)/ (wi+bi) )^3} (positive values
indicates likley white territory, negative stand for black territory)
turns out to be very simple first guess that is still far off, but
reasonable enough to be useful.

This value is then suspect a number of corrections. Assume that this first
guess resulted in a positive value.

If both @code{bi} and @code{wi} are small, it gets reduced. What exactly is
"small" depends on whether the intersection is close to a corner or an edge
of the board, since it is easier to claim territory in the corner than in
the center.

Then the value at each intersection is degraded to the minimum value of
its neighbors. This can be seen as a second implementation of the proverb
saying that there is no territory in the center of the board. This step
substantially reduces the size of spheres of territory that are open at
several sides.

Finally, there are a number of patterns that explicitly forbid GNU Go to
count territory at some intersections. This is used e. g. for false eyes that
will eventually have to be filled in. Also, points for prisoners are added.

To fine tune this scheme, some revisions have been made to the influence
computations that are relevant for territorial evaluation. This includes
a reduced default attenuation and some revised pattern handling.

@node The Influence Core
@section The Core of the Influence Function

The basic influence radiation process can efficiently be implemented
as a breadth first search for adjacent and more distant points, using
a queue structure.

Influence barriers can be found by pattern matching, assisted by
reading through constraints and/or helpers. Wall structures, invasion
points and intrusion points can be found by pattern matching as well.

When influence is computed, the basic idea is that there are a number
of influence sources on the board, whose contributions are summed to
produce the influence values. For the time being we can assume that
the living stones on the board are the influence sources, although
this is not the whole story.

The function @code{compute_influence()} contains a loop over the
board, and for each influence source on the board, the function
@code{accumulate_influence()} is called. This is the core of the
influence function. Before we get into the details, this is how
the influence field from a single isolated influence source of
strength 100 turns out (with an attenuation of 3.0):

@example
  0  0  0  0  0  0  0  0  0  0  0
  0  0  0  0  1  1  1  0  0  0  0
  0  0  0  1  2  3  2  1  0  0  0
  0  0  1  3  5 11  5  3  1  0  0
  0  1  2  5 16 33 16  5  2  1  0
  0  1  3 11 33  X 33 11  3  1  0
  0  1  2  5 16 33 16  5  2  1  0
  0  0  1  3  5 11  5  3  1  0  0
  0  0  0  1  2  3  2  1  0  0  0
  0  0  0  0  1  1  1  0  0  0  0
  0  0  0  0  0  0  0  0  0  0  0
@end example

These values are in reality floating point numbers but have been
rounded down to the nearest integer for presentation. This means that
the influence field does not stop when the numbers become zeroes.

Internally @code{accumulate_influence()} starts at the influence source and
spreads influence outwards by means of a breadth first propagation,
implemented in the form of a queue. The order of propagation and the
condition that influence only is spread outwards guarantee that no
intersection is visited more than once and that the process
terminates. In the example above, the intersections are visited in the
following order:

@example
  +  +  +  +  +  +  +  +  +  +  +
  + 78 68 66 64 63 65 67 69 79  +
  + 62 46 38 36 35 37 39 47 75  +
  + 60 34 22 16 15 17 23 43 73  +
  + 58 32 14  6  3  7 19 41 71  +
  + 56 30 12  2  0  4 18 40 70  +
  + 57 31 13  5  1  8 20 42 72  +
  + 59 33 21 10  9 11 24 44 74  +
  + 61 45 28 26 25 27 29 48 76  +
  + 77 54 52 50 49 51 53 55 80  +
  +  +  +  +  +  +  +  +  +  +  +
@end example

The visitation of intersections continues in the same way on the
intersections marked '@samp{+} and further outwards. In a real
position there will be stones and tight connections stopping the
influence from spreading to certain intersections. This will
disrupt the diagram above, but the main property of the
propagation still remains, i.e. no intersection is visited more
than once and after being visited no more influence will be
propagated to the intersection.

@node The Influence Algorithm
@section The Influence Algorithm

Let @code{(m, n)} be the coordinates of the influence source and
@code{(i, j)} the coordinates of a an intersection being visited
during propagation, using the same notation as in the
@code{accumulate_influence()} function.  Influence is now propagated to
its eight closest neighbors, including the diagonal ones,
according to the follow scheme:

For each of the eight directions @code{(di, dj)}, do:

@enumerate 
@item 
Compute the scalar product @code{di*(i-m) + dj*(j-n)}
between the vectors @code{(di,dj)} and @code{(i,j) - (m,n)}
@item If this is negative or zero, the direction is not outwards and
we continue with the next direction. The exception is when we
are visiting the influence source, i.e. the first intersection,
when we spread influence in all directions anyway.
@item If @code{(i+di, j+dj)} is outside the board or occupied we
also continue with the next direction.
@item Let S be the strength of the influence at @code{(i, j)}. The influence
propagated to @code{(i+di, j+dj)} from this intersection is given by
@code{P*(1/A)*D*S}, where the three different kinds of damping are:

@itemize @bullet
@item The permeability @samp{P}, which is a property of the board
intersections. Normally this is one, i.e. unrestricted
propagation, but to stop propagation through e.g. one step
jumps, the permeability is set to zero at such intersections
through pattern matching. This is further discussed below.
@item The attenuation @samp{A}, which is a property of the influence
source and different in different directions. By default this has the
value 3 except diagonally where the number is twice as much. By
modifying the attenuation value it is possible to obtain influence
sources with a larger or a smaller effective range.
@item The directional damping @samp{D}, which is the squared cosine of the
angle between @code{(di,dj)} and @code{(i,j) - (m,n)}. The idea is to
stop influence from "bending" around an interfering stone and
get a continuous behavior at the right angle cutoff. The
choice of the squared cosine for this purpose is rather
arbitrary, but has the advantage that it can be expressed as a
rational function of @samp{m}, @samp{n}, @samp{i}, @samp{j},
@samp{di}, and @samp{dj}, without involving any trigonometric or
square root computations. When we are visiting the influence
source we let by convention this factor be one.
@end itemize
@end enumerate

Influence is typically contributed from up to three neighbors
"between" this intersection and the influence source. These values are
simply added together. As pointed out before, all contributions will
automatically have been made before the intersection itself is
visited.

When the total influence for the whole board is computed by
@code{compute_influence()}, @code{accumulate_influence()} is
called once for each influence source. These invocations are
totally independent and the influence contributions from the
different sources are added together.

@node Permeability
@section Permeability

The permeability at the different points is initially one at all empty
intersections and zero at occupied intersections. To get a useful
influence function we need to modify this, however. Consider the
following position:

@example
|......
|OOOO..
|...O..
|...a.X   ('a' empty intersection)
|...O..
|...OOO
|.....O
+------
@end example

The corner is of course secure territory for @samp{O} and clearly
the @samp{X} stone has negligible effect inside this position. To
stop @samp{X} influence from leaking into the corner we use pattern
matching (pattern Barrier1/Barrier2 in @file{barriers.db}) to modify the
permeability for @samp{X} at this intersection to zero. @samp{O} can still
spread influence through this connection.

Another case that needs to be mentioned is how the permeability
damping is computed for diagonal influence radiation. For horizontal
and vertical radiation we just use the permeability (for the relevant
color) at the intersection we are radiating from. In the diagonal case
we additionally multiply with the maximum permeability at the two
intersections we are trying to squeeze between. The reason for this
can be found in the diagram below:

@example
|...X    |...X    
|OO..    |Oda.
|..O.    |.bc.
|..O.    |..O.
+----    +----
@end example

We don't want @samp{X} influence to be spread from @samp{a} to
@samp{b}, and since the permeability at both c and d is zero, the
rule above stops this.

@node Escape
@section Escape

One application of the influence code is in computing the
@code{dragon.escape_route} field. This is computed by the function
@code{compute_escape()} as follows.  First, every intersection is
assigned an escape value, ranging between 0 and 4, depending on
the influence value of the opposite color.

The @code{escape_route} field is modified by the code in @file{surround.c}
(@pxref{Surrounded Dragons}). It is divided by two for weakly surrounded
dragons, and set to zero for surrounded ones.

In addition to assiging an escape value to empty vertices,
we also assign an escape value to friendly dragons. This
value can range from 0 to 6 depending on the status of 
the dragon, with live dragons having value 6.

Then we sum the values of the resulting influence escape values
over the intersections (including friendly dragons) at distance 4,
that is, over those intersections which can be joined to the
dragon by a path of length 4 (and no shorter path) not passing
adjacent to any unfriendly dragon. In the following example, we
sum the influence escape value over the four vertices labelled
'4'.

@example
   
   . . . . . . . . .    . . . . . . . . .
   . . . . . X . . O    . . . . . X . . O
   . . X . . . . . O    . . X . 2 . 4 . O
   X . . . . . . . .    X . . 1 1 2 3 4 .
   X O . O . . . . O    X O 1 O 1 2 3 4 O
   X O . O . . . . .    X O 1 O 1 . 4 . .
   X O . . . X . O O    X O 1 . . X . . O
   . . . X . . . . .    . 1 . X . . . . .
   X . . . . X . . .    X . . . . X . . .
   . . . . . . . . .    . . . . . . . . .

@end example

Since the dragon is trying to reach safety, the reader might
wonder why @code{compute_influence()} is called with the opposite
color of the dragon contemplating escape.  To explain this point,
we first remind the reader why there is a color parameter to
@code{compute_influence()}. Consider the following example position:
@example

     ...XX...
     OOO..OOO
     O......O
     O......O
     --------

@end example

Whether the bottom will become O territory depends on who is in turn
to play. This is implemented with the help of patterns in barriers.db,
so that X influence is allowed to leak into the bottom if X is in turn
to move but not if O is. There are also ``invade'' patterns which add
influence sources in sufficiently open parts of the board which are
handled differently depending on who is in turn to move.

In order to decide the territorial value of an O move in the third
line gap above, influence is first computed in the original position
with the opponent (i.e. X) in turn to move. Then the O stone is played
to give:

@example

     ...XX...
     OOO.OOOO
     O......O
     O......O
     --------

@end example

Now influence is computed once more, also this time with X in turn to
move. The difference in territory (as computed from the influence
values) gives the territorial value of the move.

Exactly how influence is computed for use in the escape route
estimation is all ad hoc. But it makes sense to assume the opponent
color in turn to move so that the escape possibilities aren't
overestimated. After we have made a move in the escape direction
it is after all the opponent's turn.

The current escape route mechanism seems good enough to be useful
but is not completely reliable. Mostly it seems to err on the side of
being too optimistic.

@node Break Ins
@section Break Ins

The code in @file{breakin.c} break-ins into territories that require
deeper tactical reading and are thus impossible to detect for the
influence module. It gets run after the influence module and revises
its territory valuations.

The break-in code makes use of two public functions in @file{readconnect.c},

@itemize @bullet
@item int break_in(int str, const char goal[BOARDMAX], int *move)
@findex break_in
@quotation
Returns WIN if @code{str} can connect to the area @code{goal[]} (which may or
may not contain stones), if the string's owner gets the first move.
@end quotation
@item int block_off(int str, const char goal[BOARDMAX], int *move)
@findex block_off
@quotation
Returns WIN if @code{str} cannot connect to the area @code{goal[]} (which may
or may not contain stones), if the other color moves first.
@end quotation
@end itemize

These functions are public front ends to their counterparts
@code{recursive_break_in} and @code{recursive_block_off}, which
call each other recursively.

The procedure is as follows: We look at all big (>= 10) territory regions
as detected by the influence code. Using the computation of
connection distances from readconnect.c, we compute all nearby vertices
of this territory. We look for the closest safe stones belonging to
the opponent. 

For each such string @code{str} we call

@itemize @bullet
@item @code{break_in(str, territory)} if the opponent is assumed to be next to move,
@item @code{block_off(str, territory)} if the territory owner is next.
@end itemize

If the break in is successful resp. the blocking unsuccessful, we
shrink the territory, and see whether the opponent can still break in.
We repeat this until the territory is shrunk so much that the opponent
can no longer reach it.

To see the break in code in action run GNU Go on the file
@file{regression/games/break_in.sgf} with the option @code{-d0x102000}. Among
the traces you will find:

@example
  Trying to break in from D7 to:
E9 (1)  F9 (1)  G9 (1)  E8 (1)  F8 (1)  G8 (1)  
H8 (1)  G7 (1)  H7 (1)  J7 (1)  H6 (1)  J6 (1)
H5 (1)  J5 (1)  H4 (1)  J4 (1)  H3 (1)  J3 (1)
H2 (1)  J2 (1)    
block_off D7, result 0 PASS (355, 41952 nodes, 0.73 seconds)
E9 (1)  F9 (1)  G9 (1)  E8 (1)  F8 (1)  G8 (1)
H8 (1)  G7 (1)  H7 (1)  J7 (1)  H6 (1)  J6 (1)
H5 (1)  J5 (1)  H4 (1)  J4 (1)  H3 (1)  J3 (1)
H2 (1)  J2 (1)    
B:F4 
  Erasing territory at E8 -b.
  Erasing territory at G3 -b.
  Now trying to break to smaller goal:
F9 (1)  G9 (1)  F8 (1)  G8 (1)  H8 (1)  G7 (1)
H7 (1)  J7 (1)  H6 (1)  J6 (1)  H5 (1)  J5 (1)
H4 (1)  J4 (1)  H3 (1)  J3 (1)  H2 (1)  J2 (1)    
@end example

This means that the function @code{break_in} is called with the goal
marked 'a' in the following diagram. The code attempts to find out
whether it is possible to connect into this area from the string
at @code{D7}.

@example
   A B C D E F G H J
 9 . . . . a a a . . 9
 8 . . . . a a a a . 8
 7 . . . X O O a a a 7
 6 . . . X X X O a a 6
 5 . . . . + . . a a 5
 4 . . . X . . O a a 4
 3 . . . . X . . a a 3
 2 . . . . . . O a a 2
 1 . . . . . . . . . 1
   A B C D E F G H J
@end example

A breakin is found, so the goal is shrunk by removing
@code{E9} and @code{J2}, then break_in is called again.

In order to see what reading is actually done in order to
do this break in, you may load GNU Go in gtp mode, then
issue the commands:

@example
loadsgf break_in.sgf 
= black

start_sgftrace
= 

break_in D7 E9 F9 G9 E8 F8 G8 H8 G7 H7 J7 H6 J6 H5 J5 H4 J4 H3 J3 H2 J2
= 1 E8

finish_sgftrace vars.sgf
= 

start_sgftrace
= 

break_in D7 F9 G9 F8 G8 H8 G7 H7 J7 H6 J6 H5 J5 H4 J4 H3 J3 H2 J2
= 1 G7

finish_sgftrace vars1.sgf
@end example

This will produce two sgf files containing the variations caused
by these calls to the breakin code. The second file, @file{vars1.sgf}
will contain quite a few variations.

The break in code makes a list of break ins which are found.
When it is finished, the function @code{add_expand_territory_move}
is called for each break in, adding a move reason.

The break in code is slow, and only changes a few moves by the engine
per game. Nevertheless we believe that it contributes substantially
to the strength of the program. The break in code is enabled by default 
in GNU Go 3.6 at level 10, and disabled at level 9. In fact, this is the
@strong{only} difference between levels 9 and 10 in GNU Go 3.6.

@node Surrounded Dragons
@section Surrounded Dragons

When is a dragon surrounded?

As has been pointed out by Bruce Wilcox, the geometric lines connecting groups
of the opposite color are often important. It is very hard to prevent the
escape of this @samp{O} dragon:

@example
..........
.....O....
.X.......X
.X...O...X
..........
..........
----------
@end example

On the other hand, this dragon is in grave danger:

@example
..........
..........
.X.......X
.....O....
.X.......X
.X...O...X
..........
..........
----------
@end example

The difference between these two positions is that in the first, the @samp{O}
dragon crosses the line connecting the top two @samp{X} stones.

Code in @file{surround.c} implements a test for when a dragon is surrounded.
The idea is to compute the convex hull of the @emph{surround set}, that is,
the set stones belonging to unfriendly neighbor dragons. If the dragon is
contained within that hull. If it is, it is said to be @emph{surrounded}.

In practice this scheme is modified slightly. The implementation uses various
algorithms to compute distances and hostile stones are discarded from the 
surround set when a pair other hostile ones can be found which makes the
considered one useless. For example, in the following position
the bottom @samp{O} stone would get discarded.

@example
O.X.O  
.....
.O.O.
.....
..O..
@end example

Also, points are added to the surround set below stones on the
second and third lines. This should account for the edge being a
natural barrier.

In order to compute distances between corners of the convex hull
a sorting by angle algorithm has been implemented. If the distance
between a pair enclosing stones is large, the surround status gets
decreased to @code{WEAKLY_SURROUNDED}, or even 0 for very large ones.

The sorting by angle must be explained. A small diagram will probably help :

@example
.O.O.
O...O
..X..
O...O
.O.O.
@end example

The sorting algorithm will generate this:

@example
.4.5.
3...6
..X..
2...7
.1.8.
@end example

That is, the points are sorted by ascending order of the measure of the
angle S-G-O, where S is SOUTH, G the (approximated) gravity center of
the goal, and O the position of the considered hostile stones.

The necessity of such sorting appears when one tries to measure distances
between enclosing stones without sorting them, just by using directly the
existing left and right corners arrays. In some positions, the results will
be inconsistent. Imagine, for example a position where for instance the points
1,2,3,4,6 and 7 were in the left arrary, leaving only 5 and 8 in the right
array. Because of the large distance between 5 and 8, the dragon would have
declared weak surrounded or not surrounded at all. Such cases are rare but 
frequent enough to require the angle sorting.

The following position:

@example
O.X.O
.....
.O.O.
@end example      

This is "more" surrounded than the following position:

@example
O.XXXXXX.O
..........
.O......O.
@end example

In the second case, the surround status would be lowered to
@code{WEAKLY_SURROUNDED}.

The surround code is used to modify the escape_route field
in the dragon2 data array. When a dragon is WEAKLY_SURROUNDED,
the escape_route is divided by 2. If the dragon is SURROUNDED,
escape_route is simply set to 0.


@node Influential Patterns
@section Patterns used by the Influence module

This section explains the details of the pattern databases used for
the influence computation.

First, we have the patterns in @file{influence.db}, which get matched
symmetrically for both colors.

@itemize
@item @samp{E}
@quotation
These patterns add extra influence sources close to some shapes like walls.
This tries to reflect their extra strength. These patterns are not used
in the influence computations relevant for territory valuations, but they
are useful for getting a better estimate of strengths of groups.
@end quotation
@item @samp{I}
@quotation
These patterns add extra influence sources at typical invasion points. 
Usually they are of small strength. If they additionally have the class
@samp{s}, the extra influence source is added for both colors. Otherwise,
only the player assumed to be next to move gets the benefit.
@end quotation
@end itemize 

The patterns in @file{barriers.db} get matched only for @samp{O}
being the player next to move.

@itemize
@item @samp{A}
@quotation
Connections between @samp{X} stones that stop influence of @samp{O}. They
have to be tight enough that @samp{O} cannot break through, even though
he is allowed to move first.
@end quotation
@item @samp{D}
@quotation
Connections between @samp{O} stones that stop influence of @samp{X}. The
stones involved can be more loosely connected than those in @samp{A}
patterns.
@end quotation
@item @samp{B}
@quotation
These indicate positions of followup moves for the @samp{O} stone marked
with @samp{Q} in the pattern. They are used to reduce the territory e. g.
where a monkey jump is possible. Also, they are used in the computation
of the followup influence, if the @samp{Q} stone was the move played
(or a stone saved by the move played).
@end quotation
@item @samp{t}
@quotation
These patterns indicate intersections where one color will not be able
to get territory, for example in a false eye. The points are set with
a call to the helper non_oterritory or non_xterritory in the action of
the pattern.
@end quotation
@end itemize 

The intrusion patterns (@samp{B}) are more powerful than the description
above might suggest. They can be very helpful in identifying weak shapes
(by adding an intrusion source for the opponent where he can break through).
A negative inference for this is that a single bad @samp{B} pattern, e. g.
one that has a wrong constraint, typically causes 5 to 10 @code{FAIL}s in
the regression test suite.

Influence Patterns can have autohelper constraints as usual. As for
the constraint attributes, there are (additionally to the usual
ones @samp{O}, @samp{o}, @samp{X} and @samp{x}),
attributes @samp{Y} and @samp{FY}. A pattern marked with @samp{Y} will
only be used in the influence computations relevant for the territory
valuation, while @samp{FY} patterns only get used in the other influence
computations.

The action of an influence pattern is at the moment only used for
non-territory patterns as mentioned above, and as a workaround for a
problem with @samp{B} patterns in the followup influence.

To see why this workaround is necessary, consider the follwoing situation:

@example

..XXX
.a*.O
.X.O.
..XXO

@end example

(Imagine that there is @samp{X} territory on the left.)

The move by @samp{O} at @samp{*} has a natural followup move at @samp{a}.
So, in the computation of the followup influence for @samp{*}, there would
be an extra influence source for @samp{O} at @samp{a} which would destroy
a lot of black territory on the left. This would give a big followup value,
and in effect the move @samp{*} would be treated as sente. 

But of course it is gote, since @samp{X} will answer at @samp{a}, which
both stops the possible intrusion and  threatens to capture @samp{*}. This
situation is in fact quite common.

Hence we need an additional constraint that can tell when an intrusion
pattern can be used in followup influence. This is done by misusing the
action line: An additional line

@example
>return <condition>;
@end example

gets added to the pattern. The @code{condition} should be true if the
intrusion cannot be stopped in sente. In the above example, the relevant
intrusion pattern will have an action line of the form

@example
>return (!xplay_attack(a,b));
@end example

where @samp{b} refers to the stone at @samp{*}. In fact, almost all 
followup-specific constraints look similar to this.


@node Influential Display
@section Colored display and debugging of influence

There are various ways to obtain detailed information about the influence
computations. Colored diagrams showing influence are possible from
a colored xterm or rxvt window. 

There are two options controlling when to generate diagrams:

@itemize @bullet
@item @option{-m 0x08} or @option{-m 8}
@quotation
Show diagrams for the initial influence computation. This is done
twice, the first time before @code{make_dragons()} is run and the second time
after. The difference is that dead dragons are taken into account the
second time. Tactically captured worms are taken into account both
times. 
@end quotation
@item @option{--debug-influence @var{location}}
@quotation
Show influence diagrams after the move at the given location. An
important limitation of this option is that it's only effective for
moves that the move generation is considering.
@end quotation
@end itemize

The other options control which diagrams should be generated in these
situations. You have to specify at least one of the options above and
at least one of the options below to generate any output.

@strong{
The options below must be combined with one of the two previous
ones, or the diagram will not be printed. For example to print
the influence diagram, you may combine 0x08 and 0x010, and use
the option @option{-m 0x018}.}

@itemize @bullet
@item @option{-m 0x010} or @option{-m 16}
@quotation
Show colored display of territory/moyo/area regions.
@itemize @minus
@item territory: cyan
@item moyo: yellow
@item area: red
@end itemize
This feature is very useful to get an immediate impression of the influence
regions as GNU Go sees them.
@end quotation
@item @option{-m 0x20} or @option{-m 32}
@quotation
Show numerical influence values for white and black. These come in
two separate diagrams, the first one for white, the second one for
black. Notice that the influence values are represented by floats and
thus have been rounded in these diagrams.
@end quotation
@item @option{-m 0x40} or @option{-m 64}
@quotation
This generates two diagrams showing the permeability for black and white
influence on the board.
@end quotation
@item @option{-m 0x80} or @option{-m 128}
@quotation
This shows the strength of the influence sources for black and white 
across the board. You will see sources at each lively stone (with strength
depending on the strength of this stone), and sources contributed by
patterns.
@end quotation
@item @option{-m 0x100} or @option{-m 256}
@quotation
This shows the attenuation with which the influence sources spread
influence across the board. Low attenuation indicates far-reaching
influence sources.
@end quotation
@item @option{-m 0x200} or @option{-m 512}
@quotation
This shows the territory valuation of GNU Go. Each intersection is
shown with a value between -1.0 and +1.0 (or -2 resp. +2 if there is
a dead stone on this intersection). Positive values indicate territory
for white. A value of -0.5 thus indicates a point where black has a
50% chance of getting territory.
@end quotation
@end itemize

Finally, there is the debug option @option{-d 0x1} which turns on
on @code{DEBUG_INFLUENCE}. This gives a message for each influence pattern
that gets matched. Unfortunately, these are way too many messages making
it tedious to navigate the output. However, if you discover an influence
source with @option{-m 0x80} that looks wrong, the debug output can
help you to quickly find out the responsible pattern.

@node Influence Tuning
@section Influence Tuning with @code{view.pike}

A useful program in the regression directory is @code{view.pike}.
To run it, you need Pike, which you may download from
@url{http://pike.ida.liu.se/}.

The test case @file{endgame:920} fails in GNU Go 3.6. We will
explain how to fix it.

Start by firing up view.pike on testcase endgame:920, e.g. by running
@command{pike view.pike endgame:920} in the regression directory.

We see from the first view of move values that filling dame at P15 is
valued highest with 0.17 points while the correct move at C4 is valued
slightly lower with 0.16. The real problem is of course that C4 is
worth a full point and thus should be valued about 1.0.

Now click on C4 to get a list of move reasons and move valuation
information. Everything looks okay except that change in territory is
0.00 rather than 1.00 as it ought to be.

We can confirm this by choosing the ``delta territory for...'' button
and again clicking C4. Now B5 should have been marked as one point of
change in territory, but it's not.

Next step is to enter the influence debug tool. Press the ``influence''
button, followed by ``black influence, dragons known,'' and ``territory
value.'' This shows the expected territory if black locally moves first
everywhere (thus ``black influence''). Here we can see that B5 is
incorrectly considered as 1.0 points of white territory.

We can compare this with the territory after a white move at C4 (still
assuming that black locally moves first everywhere after that) by
pressing ``after move influence for...'' and clicking C4. This looks
identical, as expected since delta territory was 0, but here it is
correct that B5 is 1.0 points of territory for white.

The most straightforward solution to this problem is to add a
non-territory pattern, saying that white can't get territory on B5 if
black moves first. The nonterritory patterns are in @file{barriers.db}.

@example
Pattern Nonterritory56

...
X.O
?O.

:8,t

eac
XbO
?Od

;oplay_attack(a,b,c,d,d)

>non_xterritory(e);
@end example

In these patterns it's always assumed that @samp{O} moves first and thus it
says that @samp{X} can't get territory at @code{B5} (@samp{e} in the
pattern). Now we need to be a bit careful however since after @samp{O} plays
at @samp{a} and @samp{X} cuts in at @samp{b}, it may well happen that @samp{O}
needs to defend around @samp{d}, allowing @samp{X} to cut at @samp{c}, possibly
making the nonterritory assumption invalid. It's difficult to do this entirely
accurate, but the constraint above is fairly conservative and should guarantee
that @samp{a} is safe in most, although not all, cases.