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[sgk-go] / doc / utils.texi

In this Chapter, we document some of the utilities which may be
called from the GNU Go engine.

@menu
* General Utilities::              Utilities from @file{engine/utils.c}
* Print Utilities::                Utilities from @file{engine/printutils.c}
* Board Utilities::                Utilities from @file{engine/board.c}
* Influence Utilities::		   Utilities from @file{engine/influence.c}
@end menu

@node General Utilities
@section General Utilities

Utility functions from @file{engine/utils.c}. Many of these
functions underlie autohelper functions (@pxref{Autohelper Functions}).


@itemize @bullet
@item @code{void change_dragon_status(int dr, int status)}
@findex change_dragon_status
@quotation
Change the status of all the stones in the dragon at @code{dr}.
@end quotation
@item @code{int defend_against(int move, int color, int apos)}
@findex defend_against
@quotation
Check whether a move at @code{move} stops the enemy from playing at (apos).
@end quotation
@item @code{int cut_possible(int pos, int color)}
@quotation
Returns true if @code{color} can cut at @code{pos}, or if connection through
@code{pos} is inhibited. This information is collected by @code{find_cuts()},
using the B patterns in the connections database.
@end quotation
@item @code{int does_attack(int move, int str)}
@findex does_attack
@quotation
returns true if the move at @code{move} attacks @code{str}. This means that it captures
the string, and that @code{str} is not already dead.  
@end quotation
@item @code{int does_defend(int move, int str)}
@findex does_defend
@quotation
@code{does_defend(move, str)} returns true if the move at @code{move}
defends @code{str}. This means that it defends the string, and that
@code{str} can be captured if no defense is made.
@end quotation
@item @code{int somewhere(int color, int last_move, ...)}
@findex somewhere
@quotation
Example: @code{somewhere(WHITE, 2, apos, bpos, cpos)}.
Returns true if one of the vertices listed satisfies
@code{board[pos]==color}. Here num_moves is the number of moves minus one.
If the check is true the dragon is not allowed to be dead. This
check is only valid if @code{stackp==0}.
@end quotation
@item @code{int visible_along_edge(int color, int apos, int bpos)}
@quotation
Search along the edge for the first visible stone. Start at apos
and move in the direction of bpos. Return 1 if the first visible
stone is of the given color. It is required that apos and bpos are
at the same distance from the edge.
@end quotation
@item @code{int test_symmetry_after_move(int move, int color, int strict)}
@findex test_symmetry_after_move
@quotation
Is the board symmetric (or rather antisymmetric) with respect to
mirroring in tengen after a specific move has been played? If the
move is PASS_MOVE, check the current board.
If strict is set we require that each stone is matched by a stone
of the opposite color at the mirrored vertex. Otherwise we only
require that each stone is matched by a stone of either color.
@end quotation
@item @code{int play_break_through_n(int color, int num_moves, ...)}
@findex play_break_through_n
@quotation
The function @code{play_break_through_n()} plays a sequence of moves,
alternating between the players and starting with color. After
having played through the sequence, the three last coordinate pairs
gives a position to be analyzed by @code{break_through()}, to see whether
either color has managed to enclose some stones and/or connected
his own stones. If any of the three last positions is empty, it's
assumed that the enclosure has failed, as well as the attempt to
connect. If one or more of the moves to play turns out to be illegal for
some reason, the rest of the sequence is played anyway, and
@code{break_through()} is called as if nothing special happened.
Like @code{break_through()}, this function returns 1 if the attempt to
break through was succesful and 2 if it only managed to cut
through.
@end quotation
@item @code{int play_attack_defend_n(int color, int do_attack, int num_moves, ...)}
@item @code{int play_attack_defend2_n(int color, int do_attack, int num_moves, ...)}
@findex play_attack_defend2_n
@findex play_attack_defend_n
@quotation
The function @code{play_attack_defend_n()} plays a sequence of moves,
alternating between the players and starting with @code{color}. After
having played through the sequence, the last coordinate pair gives
a target to attack or defend, depending on the value of do_attack.
If there is no stone present to attack or defend, it is assumed
that it has already been captured. If one or more of the moves to
play turns out to be illegal for some reason, the rest of the
sequence is played anyway, and attack/defense is tested as if
nothing special happened. Conversely, 
@code{play_attack_defend2_n()} plays a sequence of moves,
alternating between the players and starting with @code{color}. After
having played through the sequence, the two last coordinate pairs
give two targets to simultaneously attack or defend, depending on
the value of do_attack. If there is no stone present to attack or
defend, it is assumed that it has already been captured. If one or
more of the moves to play turns out to be illegal for some reason,
the rest of the sequence is played anyway, and attack/defense is
tested as if nothing special happened.  A typical use of these functions is to
set up a ladder in an autohelper and see whether it works or not.  
@end quotation
@item @code{int play_connect_n(int color, int do_connect, int num_moves, ...)}
@findex play_connect_n
@quotation
Plays a sequence of moves, alternating between the players and starting
with @code{color}. After having played through the sequence, the two last
coordinates give two targets that should be connected or disconnected,
depending on the value of do_connect. If there is no stone present to
connect or disconnect, it is assumed that the connection has failed. If
one or more of the moves to play turns out to be illegal for some
reason, the rest of the sequence is played anyway, and
connection/disconnection is tested as if nothing special happened.
Ultimately the connection is decided by the functions 
@code{string_connect} and @code{disconnect} (@pxref{Connection Reading}).
@end quotation
@item @code{void set_depth_values(int level)}
@findex set_depth_values
@quotation
It is assumed in reading a ladder if @code{stackp >= depth} that
as soon as a bounding stone is in atari, the string is safe.
Similar uses are made of the other depth parameters such
as @code{backfill_depth} and so forth. In short, simplifying
assumptions are made when @code{stackp} is large. Unfortunately any such
scheme invites the ``horizon effect,'' in which a stalling move is perceived
as a win, by pushing the refutation past the ``horizon''---the value of
@code{stackp} in which the reading assumptions are relaxed. To avoid the depth
it is sometimes necessary to increase the depth parameters. This
function can be used to set the various reading depth parameters. If
@code{mandated_depth_value} is not -1 that value is used; otherwise the depth
values are set as a function of level. The parameter
@code{mandated_depth_value} can be set at the command line to force a
particular value of depth; normally it is -1.
@end quotation
@item @code{void modify_depth_values(int n)}
@findex modify_depth_values
@quotation
Modify the various tactical reading depth parameters. This is
typically used to avoid horizon effects. By temporarily increasing
the depth values when trying some move, one can avoid that an
irrelevant move seems effective just because the reading hits a
depth limit earlier than it did when reading only on relevant
moves.
@end quotation
@item @code{void increase_depth_values(void)}
@findex increase_depth_values
@quotation
@code{modify_depth_values(1)}.
@end quotation
@item @code{void decrease_depth_values(void)}
@findex decrease_depth_values
@quotation
@code{modify_depth_values(-1)}.
@end quotation
@item @code{void restore_depth_values()}
@findex restore_depth_values
@quotation
Sets @code{depth} and so forth to their saved values.
@end quotation
@item @code{void set_temporary_depth_values(int d, int b, int b2, int bc, int ss, int br, int f, int k)}
@quotation
Explicitly set the depth values. This function is currently never
called.
@end quotation
@item @code{int confirm_safety(int move, int color, int *defense_point, char safe_stones[BOARDMAX])}
@findex confirm_safety
@quotation
Check that the move at color doesn't involve any kind of blunder,
regardless of size.
@end quotation
@item @code{float blunder_size(int move, int color, int *defense_point, char safe_stones[BOARDMAX])}
@findex blunder_size
@quotation
This function will detect some blunders. If the move reduces the number of
liberties of an adjacent friendly string, there is a danger that the move
could backfire, so the function checks that no friendly worm which was
formerly not attackable becomes attackable, and it checks that no opposing
worm which was not defendable becomes defendable.  It returns the estimated
size of the blunder, or 0.0 if nothing bad has happened.  The array
@code{safe_stones[]} contains the stones that are supposedly safe after
@code{move}. It may be @code{NULL}. For use when called from
@code{fill_liberty()}, this function may optionally return a point of defense,
which, if taken, will presumably make the move at @code{move} safe on a
subsequent turn.
@end quotation
@item @code{int double_atari(int move, int color, float *value, char safe_stones[BOARDMAX])}
@findex double_atari
@quotation
Returns true if a move by (color) fits the following shape:
@example
    X*        (O=color)
    OX
@end example
capturing one of the two @samp{X} strings. The name is a slight misnomer since
this includes attacks which are not necessarily double ataris, though the
common double atari is the most important special case. If @code{safe_stones
!= NULL}, then only attacks on stones marked as safe are tried. The value of
the double atari attack is returned in value (unless value is @code{NULL}),
and the attacked stones are marked unsafe.
@end quotation
@item @code{void unconditional_life(int unconditional_territory[BOARDMAX], int color)}
@findex unconditional_life
@quotation
Find those worms of the given color that can never be captured, even if the
opponent is allowed an arbitrary number of consecutive moves. The coordinates
of the origins of these worms are written to the worm arrays and the number of
non-capturable worms is returned.  The algorithm is to cycle through the worms
until none remains or no more can be captured. A worm is removed when it is
found to be capturable, by letting the opponent try to play on all its
liberties. If the attack fails, the moves are undone. When no more worm can be
removed in this way, the remaining ones are unconditionally alive.  After
this, unconditionally dead opponent worms and unconditional territory are
identified. To find these, we continue from the position obtained at the end
of the previous operation (only unconditionally alive strings remain for
color) with the following steps:

@enumerate
@item Play opponent stones on all liberties of the unconditionally
alive strings except where illegal. (That the move order may
determine exactly which liberties can be played legally is not
important. Just pick an arbitrary order).
@item
Recursively extend opponent strings in atari, except where this
would be suicide.
@item
Play an opponent stone anywhere it can get two empty
neighbors. (I.e. split big eyes into small ones).
@item
an opponent stone anywhere it can get one empty
neighbor. (I.e. reduce two space eyes to one space eyes.)
Remaining opponent strings in atari and remaining liberties of the
unconditionally alive strings constitute the unconditional
territory.
Opponent strings from the initial position placed on
unconditional territory are unconditionally dead.
On return, @code{unconditional_territory[][]} is 1 where color has
unconditionally alive stones, 2 where it has unconditional
territory, and 0 otherwise.
@end enumerate
@end quotation
@item @code{void who_wins(int color, FILE *outfile)}
@quotation
Score the game and determine the winner
@end quotation
@item @code{void find_superstring(int str, int *num_stones, int *stones)}
@findex find_superstring
@cindex superstring
@quotation
Find the stones of an extended string, where the extensions are
through the following kinds of connections:
@enumerate
@item Solid connections (just like ordinary string).
@example
    OO
@end example
@item Diagonal connection or one space jump through an intersection
where an opponent move would be suicide or self-atari.
@example
    ...
    O.O
    XOX
    X.X
@end example
@item
Bamboo joint.
@example
    OO
    ..
    OO
@end example
@item Diagonal connection where both adjacent intersections are empty.
@example
    .O
    O.
@end example
@item Connection through adjacent or diagonal tactically captured stones.
Connections of this type are omitted when the superstring code is
called from reading.c, but included when the superstring code is
called from owl.c
@end enumerate
@end quotation
@item @code{void find_superstring_liberties(int str, int *num_libs, int *libs, int liberty_cap)}
@findex find_superstring_liberties
@quotation
This function computes the superstring at @code{str} as described above, but
omitting connections of type 5. Then it constructs a list of liberties of the
superstring which are not already liberties of @code{str}.  If
@code{liberty_cap} is nonzero, only liberties of substrings of the superstring
which have fewer than @code{liberty_cap} liberties are generated.
@end quotation
@item @code{void find_proper_superstring_liberties(int str, int *num_libs, int *libs, int liberty_cap)}
@findex find_proper_superstring_liberties
@quotation
This function is the same as find_superstring_liberties, but it omits those
liberties of the string @code{str}, presumably since those have already been
treated elsewhere.  If @code{liberty_cap} is nonzero, only liberties of
substrings of the superstring which have at most @code{liberty_cap} liberties
are generated.
@end quotation
@item @code{void find_superstring_stones_and_liberties(int str, int *num_stones, int *stones, int *num_libs, int *libs, int liberty_cap)}
@findex find_superstring_stones_and_liberties
@quotation
This function computes the superstring at @code{str} as described above,
but omitting connections of type 5. Then it constructs a list of
liberties of the superstring which are not already liberties of
@code{str}. If liberty_cap is nonzero, only liberties of substrings of the
superstring which have fewer than liberty_cap liberties are
generated.
@end quotation
@item @code{void superstring_chainlinks(int str, int *num_adj, int adjs[MAXCHAIN], int liberty_cap)}
@findex superstring_chainlinks
@quotation
analogous to chainlinks, this function finds boundary chains of the
superstring at @code{str}, including those which are boundary chains of
@code{str} itself. If @code{liberty_cap != 0}, only those boundary chains with
@code{<= liberty_cap} liberties are reported.
@end quotation
@item @code{void proper_superstring_chainlinks(int str, int *num_adj, int adjs[MAXCHAIN], int liberty_cap)}
@findex proper_superstring_chainlingks
@quotation
analogous to chainlinks, this function finds boundary chains of the
superstring at @code{str}, omitting those which are boundary chains of
@code{str} itself. If @code{liberty_cap != 0}, only those boundary chains with
@code{<= liberty_cap} liberties are reported.
@end quotation
@item @code{void start_timer(int n)}
@findex start_timer
@cindex timers
@quotation
Start a timer. GNU Go has four internal timers available for
assessing the time spent on various tasks.
@end quotation
@item @code{double time_report(int n, const char *occupation, int move, double mintime)}
@findex time_report
@quotation
Report time spent and restart the timer. Make no report if elapsed
time is less than mintime.
@end quotation
@end itemize

@node Print Utilities
@section Print Utilities
@cindex formatted printing

Functions in @file{engine/printutils.c} do formatted printing similar to
@code{printf} and its allies.  The following formats are recognized:

@itemize @bullet
@item @code{%c}, @code{%d}, @code{%f}, @code{%s}, @code{%x}
@quotation
These have their usual meaning in formatted output, printing
a character, integer, float, string or hexadecimal, respectively.
@end quotation
@item @code{%o}
@quotation
`Outdent.' Normally output is indented by @code{2*stackp} spaces,
so that the depth can be seen at a glance in traces. At the
beginning of a format, this @code{%o} inhibits the indentation.
@end quotation
@item @code{%H}
@quotation
Print a hashvalue.
@end quotation
@item @code{%C}
@quotation
Print a color as a string.
@end quotation
@item @code{%m}, @code{%2m} (synonyms)
@quotation
Takes 2 integers and writes a move, using the two dimensional
board representation (@pxref{The Board Array})
@end quotation
@item @code{%1m}
@quotation
Takes 1 integers and writes a move, using the one dimensional
board representation (@pxref{The Board Array})
@end quotation
@end itemize

We list the non statically declared functions in @file{printutils.c}.

@itemize @bullet
@item @code{void gfprintf(FILE *outfile, const char *fmt, ...)}
@findex gfprintf
@quotation
Formatted output to @file{outfile}.
@end quotation
@item @code{int gprintf(const char *fmt, ...)}
@findex gprintf
@quotation
Formatted output to stderr. Always returns 1 to allow use in short-circuit
logical expressions.
@end quotation
@item @code{int mprintf(const char *fmt, ...)}
@findex mprintf
@quotation
Formatted output to stdout. 
@end quotation
@item @code{DEBUG(level, fmt, args...)}
@findex DEBUG
@quotation
If @code{level & debug}, do formatted output to stderr. Otherwise, ignore.
@end quotation
@item @code{void abortgo(const char *file, int line, const char *msg, int pos)}
@findex abortgo
@quotation
Print debugging output in an error situation, then exit.
@end quotation
@item @code{const char * color_to_string(int color)}
@findex color_to_string
@quotation
Convert a color value to a string
@end quotation
@item @code{const char * location_to_string(int pos)}
@findex location_to_string
@quotation
Convert a location to a string
@end quotation
@item @code{void location_to_buffer(int pos, char *buf)}
@findex location_to_buffer
@quotation
Convert a location to a string, writing to a buffer.
@end quotation
@item @code{int string_to_location(int boardsize, char *str, int *m, int *n)}
@findex string_to_location
@quotation
Get the @code{(m, n)} coordinates in the standard GNU Go coordinate system
from the string @code{str}.  This means that @samp{m} is the nth row from the
top and @samp{n} is the column. Both coordinates are between 0 and
@code{boardsize-1}, inclusive. Return 1 if ok, otherwise return 0;
@end quotation
@item @code{int is_hoshi_point(int m, int n)}
@findex is_hoshi_point
True if the coordinate is a hoshi point.
@item @code{void draw_letter_coordinates(FILE *outfile)}
@findex draw_letter_coordinates
Print a line with coordinate letters above the board.
@item @code{void simple_showboard(FILE *outfile)}
@findex simple_showboard
@quotation
Bare bones version of @code{showboard(0)}. No fancy options, no hint of
color, and you can choose where to write it.
@end quotation
@end itemize

The following functions are in @file{showbord.c}. Not all public
functions in that file are listed here.

@itemize
@item @code{void showboard(int xo)}
@findex showboard
@quotation
Show go board.
@example
xo=0:      black and white XO board for ascii game
xo=1:      colored dragon display
xo=2:      colored eye display
xo=3:      colored owl display
xo=4:      colored matcher status display
@end example
@end quotation
@item @code{const char * status_to_string(int status)}
@findex status_to_string
@quotation
Convert a status value to a string.
@end quotation
@item @code{const char * safety_to_string(int status)}
@findex safety_to_string
@quotation
Convert a safety value to a string.
@end quotation
@item @code{const char * result_to_string(int result)}
@findex result_to_string
@quotation
Convert a read result to a string
@end quotation
@end itemize

@node Board Utilities
@section Board Utilities

The functions documented in this section are from @file{board.c}. Other
functions in @file{board.c} are described in @xref{Some Board Functions}.

@itemize @bullet
@item @code{void store_board(struct board_state *state)}
@findex store_board
@quotation
Save board state.
@end quotation
@item @code{void restore_board(struct board_state *state)}
@findex restore_board
@quotation
Restore a saved board state.
@end quotation
@item @code{void clear_board(void)}
@findex clear_board
@quotation
Clear the internal board.
@end quotation
@item @code{void dump_stack(void)}
@findex dump_stack
@quotation
for use under GDB prints the move stack.
@end quotation
@item @code{void add_stone(int pos, int color)}
@findex add_stone
@quotation
Place a stone on the board and update the board_hash. This operation
destroys all move history.
@end quotation
@item @code{void remove_stone(int pos)}
@findex remove_stone
@quotation
Remove a stone from the board and update the board_hash. This
operation destroys the move history.
@end quotation
@item @code{int is_pass(int pos)}
@findex is_pass
@quotation
Test if the move is a pass or not.  Return 1 if it is.
@end quotation
@item @code{int  is_legal(int pos, int color)}
@findex is_legal
@quotation
Determines whether the move @code{color} at @code{pos} is legal.
@end quotation
@item @code{int  is_suicide(int pos, int color)}
@findex is_suicide
@quotation
Determines whether the move @code{color} at @code{pos} would be a suicide.
This is the case if
@enumerate
@item There is no neighboring empty intersection.
@item There is no neighboring opponent string with exactly one liberty.
@item There is no neighboring friendly string with more than one liberty.
@end enumerate
@end quotation
@item @code{int is_illegal_ko_capture(int pos, int color)}
@findex is_illegal_ko_capture
@quotation
Determines whether the move @code{color} at @code{pos} would be an illegal ko
capture.
@end quotation
@item @code{int is_edge_vertex(int pos)}
@findex is_edge_vertex
@quotation
Determine whether vertex is on the edge.
@end quotation
@item @code{int edge_distance(int pos)}
@findex edge_distance
@quotation
Distance to the edge.
@end quotation
@item @code{int is_corner_vertex(int pos)}
@findex is_corner_vertex
@quotation
Determine whether vertex is a corner.
@end quotation
@item @code{int get_komaster()}
@findex get_komaster
@item @code{int get_kom_pos()}
@findex get_kom_pos
@quotation
Public functions to access the variable @code{komaster} and @code{kom_pos},
which are static in @file{board.c}.
@end quotation
@end itemize

Next we come to @code{countlib()} and its allies, which
address the problem of determining how many liberties a
string has. Although @code{countlib()} addresses this
basic question, other functions can often get the needed
information more quickly, so there are a number of
different functions in this family.

@itemize @bullet
@item @code{int countlib(int str)}
@findex countlib
@quotation
Count the number of liberties of the string at @code{pos}. There
must be a stone at this location.
@end quotation
@item @code{int findlib(int str, int maxlib, int *libs)}
@findex findlib
@quotation
Find the liberties of the string at @code{str}. This location must not be
empty. The locations of up to maxlib liberties are written into
@code{libs[]}. The full number of liberties is returned.  If you want the
locations of all liberties, whatever their number, you should pass
@code{MAXLIBS} as the value for @code{maxlib} and allocate space for
@code{libs[]} accordingly.
@end quotation
@item @code{int fastlib(int pos, int color, int ignore_captures)}
@findex fastlib
@quotation
Count the liberties a stone of the given color would get if played
at @code{pos}. The intent of this function is to be as fast as possible, not
necessarily complete. But if it returns a positive value (meaning
it has succeeded), the value is guaranteed to be correct. Captures are ignored
based if the @code{ignore_captures} field is nonzero. The location @code{pos}
must be empty.  The function fails if there are more than two neighbor strings
of the same color.  In this case, the return value is -1.  Captures are
handled in a very limited way, so if ignore_capture is 0, and a capture is
required, it will often return -1.
@end quotation
@item @code{int approxlib(int pos, int color, int maxlib, int *libs)}
@findex approxlib
@quotation
Find the liberties a stone of the given color would get if played at
@code{pos}, ignoring possible captures of opponent stones.  The location
@code{pos} must be empty. If @code{libs != NULL}, the locations of up to
@code{maxlib} liberties are written into @code{libs[]}. The counting of
liberties may or may not be halted when @code{maxlib} is reached. The number
of liberties found is returned, which may be less than the total number of
liberties if @code{maxlib} is small. If you want the number or the locations
of all liberties, however many they are, you should pass @code{MAXLIBS} as the
value for maxlib and allocate space for @code{libs[]} accordingly.
@end quotation
@item @code{int accuratelib(int pos, int color, int maxlib, int *libs)}
@findex accuratelib
@quotation
Find the liberties a stone of the given color would get if played at
@code{pos}. This function takes into consideration all captures. Its return
value is exact in that sense it counts all the liberties, unless @code{maxlib}
allows it to stop earlier. The location @code{pos} must be empty. If
@code{libs != NULL}, the locations of up to @code{maxlib} liberties are
written into @code{libs[]}. The counting of liberties may or may not be halted
when @code{maxlib} is reached. The number of found liberties is returned.
This function guarantees that liberties which are not results of captures come
first in @code{libs[]} array. To find whether all the liberties starting from
a given one are results of captures, one may use @code{if (board[libs[k]] !=
EMPTY)} construction.  If you want the number or the locations of all
liberties, however many they are, you should pass @code{MAXLIBS} as the value
for @code{maxlib} and allocate space for @code{libs[]} accordingly.
@end quotation
@end itemize

Next we have some general utility functions.

@itemize @bullet
@item @code{int count_common_libs(int str1, int str2)}
@findex count_common_libs
@quotation
Find the number of common liberties of the two strings.
@end quotation
@item @code{int find_common_libs(int str1, int str2, int maxlib, int *libs)}
@findex find_common_libs
@quotation
Find the common liberties of the two strings. The locations of up to
@code{maxlib} common liberties are written into @code{libs[]}.  The full
number of common liberties is returned.  If you want the locations of all
common liberties, whatever their number, you should pass @code{MAXLIBS} as the
value for @code{maxlib} and allocate space for @code{libs[]} accordingly.
@end quotation
@item @code{int have_common_lib(int str1, int str2, int *lib)}
@findex have_common_lib
@quotation
Determine whether two strings have at least one common liberty.
If they do and @code{lib != NULL}, one common liberty is returned in 
@code{*lib}.
@end quotation
@item @code{int countstones(int str)}
@findex countstones
@quotation
Report the number of stones in a string.
@end quotation
@item @code{int findstones(int str, int maxstones, int *stones)}
@findex findstones
@quotation
Find the stones of the string at @code{str}. The location must not be
empty. The locations of up to maxstones stones are written into
@code{stones[]}. The full number of stones is returned.
@end quotation
@item @code{int  chainlinks(int str, int adj[MAXCHAIN])}
@findex chainlinks
@quotation
This very useful function returns (in the @code{adj} array) the chains
surrounding the string at @code{str}. The number of chains is returned.
@end quotation
@item @code{int chainlinks2(int str, int adj[MAXCHAIN], int lib)}
@findex chainlinks2
@quotation
Returns (in @code{adj} array) those chains surrounding the string at
@code{str}, which has exactly @code{lib} liberties. The number of such chains
is returned.
@end quotation
@item @code{int chainlinks3(int str, int adj[MAXCHAIN], int lib)}
@findex chainlinks3
@quotation
Returns (in @code{adj} array) the chains surrounding
the string at @code{str}, which have less or equal @code{lib} liberties.
The number of such chains is returned.
@end quotation
@item @code{int extended_chainlinks(int str, int adj[MAXCHAIN], int both_colors)}
@findex extended_chainlinks
@quotation
Returns (in the @code{adj} array) the opponent strings being directly adjacent
to @code{str} or having a common liberty with @code{str}. The number of such
strings is returned.  If the both_colors parameter is true, also own strings
sharing a liberty are returned.
@end quotation
@item @code{int find_origin(int str)}
@findex find_origin
@quotation
Find the origin of a string, i.e. the point with the smallest 1D board
coordinate. The idea is to have a canonical reference point for a
string.
@end quotation
@item @code{int is_self_atari(int pos, int color)}
@findex is_self_atari
@quotation
Determine whether a move by color at @code{pos} would be a self atari,
i.e. whether it would get more than one liberty. This function
returns true also for the case of a suicide move.
@end quotation
@item @code{int liberty_of_string(int pos, int str)}
@findex liberty_of_string
@quotation
Returns true if @code{pos} is a liberty of the string at @code{str}.
@end quotation
@item @code{int second_order_liberty_of_string(int pos, int str)}
@findex second_order_liberty_of_string
@quotation
Returns true if @code{pos} is a second order liberty of the string at str.
@end quotation
@item @code{int neighbor_of_string(int pos, int str)}
@findex neighbor_of_string
@quotation
Returns true if @code{pos} is adjacent to the string at @code{str}.
@end quotation
@item @code{int has_neighbor(int pos, int color)}
@findex has_neighbor
@quotation
Returns true if @code{pos} has a neighbor of @code{color}.
@end quotation
@item @code{int same_string(int str1, int str2)}
@findex same_string
@quotation
Returns true if @code{str1} and @code{str2} belong to the same string.
@end quotation
@item @code{int adjacent_strings(int str1, int str2)}
@findex adjacent_strings
@quotation
Returns true if the strings at @code{str1} and @code{str2} are adjacent.
@end quotation
@item @code{int is_ko(int pos, int color, int *ko_pos)}
@findex is_ko
@quotation
Return true if the move @code{pos} by @code{color} is a ko capture
(whether capture is legal on this move or not). If so,
and if @code{ko_pos} is not a @code{NULL} pointer, then
@code{*ko_pos} returns the location of the captured ko stone.
If the move is not a ko capture, @code{*ko_pos} is set to 0.
A move is a ko capture if and only if
@enumerate
@item All neighbors are opponent stones.
@item The number of captured stones is exactly one.
@end enumerate
@end quotation
@item @code{int is_ko_point(int pos)}
@findex is_ko_point
@quotation
Return true if @code{pos} is either a stone, which if captured would give
ko, or if @code{pos} is an empty intersection adjacent to a ko stone.
@end quotation
@item @code{int does_capture_something(int pos, int color)}
@findex does_capture_something
@quotation
Returns 1 if at least one string is captured when color plays at @code{pos}.
@end quotation
@item @code{void mark_string(int str, char mx[BOARDMAX], char mark)}
@findex mark_string
@quotation
For each stone in the string at pos, set @code{mx} to value mark. If
some of the stones in the string are marked prior to calling this
function, only the connected unmarked stones starting from pos
are guaranteed to become marked. The rest of the string may or may
not become marked. (In the current implementation, it will.)
@end quotation
@item @code{int move_in_stack(int pos, int cutoff)}
@findex move_in_stack
@quotation
Returns true if at least one move has been played at pos
at deeper than level @code{cutoff} in the reading tree.
@end quotation
@item @code{int stones_on_board(int color)}
@findex stones_on_board
@quotation
Return the number of stones of the indicated color(s) on the board.
This only counts stones in the permanent position, not stones placed
by @code{trymove()} or @code{tryko()}. Use 
@code{stones_on_board(BLACK | WHITE)} to get
the total number of stones on the board.
@end quotation
@end itemize

@node Influence Utilities
@section Utilities from @file{engine/influence.c}

We will only list here a portion of the public functions in @code{influence.c}.
The influence code is invoked through the function @code{compute_influence}
(@pxref{Influence Usage}). It is invoked as follows.

@itemize @bullet
@item @code{void compute_influence(int color, const char safe_stones[BOARDMAX], const float strength[BOARDMAX], struct influence_data *q, int move, const char *trace_message)}
@findex compute_influence
@quotation
Compute the influence values for both colors.
The caller must
@itemize @minus
@item set up the @code{board[]} state
@item mark safe stones with @code{INFLUENCE_SAFE_STONE}, dead stones with 0
@item mark stones newly saved by a move with @code{INFLUENCE_SAVED_STONE}
(this is relevant if the influence_data *q is reused to compute
a followup value for this move).
@end itemize
Results will be stored in q.
@code{move} has no effects except toggling debugging. Set it to -1
for no debug output at all (otherwise it will be controlled by
the @option{-m} command line option). It is assumed that @code{color} is in turn to move. (This affects the
barrier patterns (class A, D) and intrusions (class B)). Color
@end quotation
@end itemize

Other functions in @file{influence.c} are of the nature of utilities
which may be useful throughout the engine. We list the most useful
ones here.

@itemize @bullet
@item @code{void influence_mark_non_territory(int pos, int color)}
@findex influence_mark_non_territory
@quotation
Called from actions for @samp{t} patterns in @file{barriers.db}.
Marks @code{pos} as not being territory for @code{color}.
@end quotation
@item @code{int whose_territory(const struct influence_data *q, int pos)}
@findex whose_territory
@quotation
Return the color of the territory at @code{pos}. If it's territory for
neither color, @code{EMPTY} is returned.
@end quotation
@item @code{int whose_moyo(const struct influence_data *q, int pos)}
@findex whose_moyo
@quotation
Return the color who has a moyo at @code{pos}. If neither color has a
moyo there, @code{EMPTY} is returned. The definition of moyo in terms of the
influences is totally ad hoc.
@end quotation
@item @code{int whose_area(const struct influence_data *q, int pos)}
@findex whose_area
@quotation
Return the color who has dominating influence (``area'') at @code{pos}.
If neither color dominates the influence there, EMPTY is returned.
The definition of area in terms of the influences is totally ad hoc.
@end quotation
@end itemize