....

%%%%%%%%%%%%%%%%%%%%%%%%%%%%validmove(Plyr,State,Proposed)%%%%%%%%%%%%%%%%%%%%
%% 
%% define validmove(Plyr,State,Proposed). 
%   - true if Proposed move by Plyr is valid at State.
%   holds if the Proposed position is empty ('.'), and
%   if in any of the 8 directions
%   there is a sequence of other player's stones
%   followed by this player's stone without empty spaces
%
%north
validmove(Plyr,State,Proposed) :-
	get(State,Proposed,.),
	northLine(State,Plyr,Proposed,Line),
	last(Line,LastPoint),
	north(LastPoint,AfterLastPoint),
	get(State,AfterLastPoint,Plyr).
%south
validmove(Plyr,State,Proposed) :-
	get(State,Proposed,.),
	southLine(State,Plyr,Proposed,Line),
	last(Line,LastPoint),
	south(LastPoint,AfterLastPoint),
	get(State,AfterLastPoint,Plyr).
%west
validmove(Plyr,State,Proposed) :-
	get(State,Proposed,.),
	westLine(State,Plyr,Proposed,Line),
	last(Line,LastPoint),
	west(LastPoint,AfterLastPoint),
	get(State,AfterLastPoint,Plyr).
%east
validmove(Plyr,State,Proposed) :-
	get(State,Proposed,.),
	eastLine(State,Plyr,Proposed,Line),
	last(Line,LastPoint),
	east(LastPoint,AfterLastPoint),
	get(State,AfterLastPoint,Plyr).
%northWest
validmove(Plyr,State,Proposed) :-
	get(State,Proposed,.),
	northWestLine(State,Plyr,Proposed,Line),
	last(Line,LastPoint),
	northWest(LastPoint,AfterLastPoint),
	get(State,AfterLastPoint,Plyr).
%northEast
validmove(Plyr,State,Proposed) :-
	get(State,Proposed,.),
	northEastLine(State,Plyr,Proposed,Line),
	last(Line,LastPoint),
	northEast(LastPoint,AfterLastPoint),
	get(State,AfterLastPoint,Plyr).
%southWest
validmove(Plyr,State,Proposed) :-
	get(State,Proposed,.),
	southWestLine(State,Plyr,Proposed,Line),
	last(Line,LastPoint),
	southWest(LastPoint,AfterLastPoint),
	get(State,AfterLastPoint,Plyr).
%southEast
validmove(Plyr,State,Proposed) :-
	get(State,Proposed,.),
	southEastLine(State,Plyr,Proposed,Line),
	last(Line,LastPoint),
	southEast(LastPoint,AfterLastPoint),
	get(State,AfterLastPoint,Plyr).


....


% predicates that return adjacent positions along 4 major directions
% and 4 minor, diagonal, directions
%
% major directions
% north(Pos,NorthOfPos)
north([X,Y], [X, YN]) :- YN is Y - 1.

% south(Pos,SouthOfPos)
south([X,Y], [X, YS]) :- YS is Y + 1.

% west(Pos,WestOfPos)
west([X,Y], [XW, Y]) :- XW is X - 1.

% east(Pos,WestOfPos)
east([X,Y], [XW, Y]) :- XW is X + 1.

% minor, diagonal, directions are obtained from the major predicates
% using DCG expansion
% northEast(Pos,NorthEastOfPos)
northEast --> north,east.

% northWest(Pos,NorthWestOfPos)
northWest --> north,west.

% southEast(Pos,SouthEastOfPos)
southEast --> south,east.

% southWest(Pos,SouthWestOfPos)
southWest --> south,west.

% +---------------list of lines-----------------------------------------------+
% | +-------line of positions--------+                                        |
% | | position                       |                                        |
% [ [ [X1, Y1], [X2,Y2], [X3,Y3], ...], [[X4, Y4], [X5,Y5], [X6,Y6], ...], ...]


% find a line of positions with other player's stones
% followed by this player's stones in the North direction
% from the given position
% do not check the state of the given position
% (it can be empty, or contain either player's stones)
%

% return a line of consecutive other player's stones
% in the 8 directions
%
% example:
% given
% S = ([[.,.,.,.,.,.], 
%       [.,.,.,.,.,.],
%       [.,.,2,1,.,.],
%       [.,.,1,2,.,.],
%       [.,.,.,2,.,.],
%       [.,.,.,x,.,.] ]).
%
%       x is at [3,5]
%
% northLine(S,1,[3,5],[[3,4],[3,3]]) holds
%
% logic:
% if get holds on the adjacent position
% and contents are other player's stone
% add this position to the list,
% otherwise return an empty list
northLine(State,Plyr,Pos,LineOfPositions) :-
	otherPlyr(Plyr,OtherPlyr),
	north(Pos,NextPos),
	(  get(State,NextPos,OtherPlyr)
	-> northLine(State,Plyr,NextPos,FollowingPositions),
	   LineOfPositions = [NextPos|FollowingPositions]
	; LineOfPositions = []
	).

% the following prediates are the same as above except in a different
% direction

southLine(State,Plyr,Pos,LineOfPositions) :-
	otherPlyr(Plyr,OtherPlyr),
	south(Pos,NextPos),
	(  get(State,NextPos,OtherPlyr)
	-> southLine(State,Plyr,NextPos,FollowingPositions),
	   LineOfPositions = [NextPos|FollowingPositions]
	; LineOfPositions = []
	).
westLine(State,Plyr,Pos,LineOfPositions) :-
	otherPlyr(Plyr,OtherPlyr),
	west(Pos,NextPos),
	(  get(State,NextPos,OtherPlyr)
	-> westLine(State,Plyr,NextPos,FollowingPositions),
	   LineOfPositions = [NextPos|FollowingPositions]
	; LineOfPositions = []
	).
eastLine(State,Plyr,Pos,LineOfPositions) :-
	otherPlyr(Plyr,OtherPlyr),
	east(Pos,NextPos),
	(  get(State,NextPos,OtherPlyr)
	-> eastLine(State,Plyr,NextPos,FollowingPositions),
	   LineOfPositions = [NextPos|FollowingPositions]
	; LineOfPositions = []
	).
northWestLine(State,Plyr,Pos,LineOfPositions) :-
	otherPlyr(Plyr,OtherPlyr),
	northWest(Pos,NextPos),
	(  get(State,NextPos,OtherPlyr)
	-> northWestLine(State,Plyr,NextPos,FollowingPositions),
	   LineOfPositions = [NextPos|FollowingPositions]
	; LineOfPositions = []
	).
northEastLine(State,Plyr,Pos,LineOfPositions) :-
	otherPlyr(Plyr,OtherPlyr),
	northEast(Pos,NextPos),
	(  get(State,NextPos,OtherPlyr)
	-> northEastLine(State,Plyr,NextPos,FollowingPositions),
	   LineOfPositions = [NextPos|FollowingPositions]
	; LineOfPositions = []
	).
southWestLine(State,Plyr,Pos,LineOfPositions) :-
	otherPlyr(Plyr,OtherPlyr),
	southWest(Pos,NextPos),
	(  get(State,NextPos,OtherPlyr)
	-> southWestLine(State,Plyr,NextPos,FollowingPositions),
	   LineOfPositions = [NextPos|FollowingPositions]
	; LineOfPositions = []
	).
southEastLine(State,Plyr,Pos,LineOfPositions) :-
	otherPlyr(Plyr,OtherPlyr),
	southEast(Pos,NextPos),
	(  get(State,NextPos,OtherPlyr)
	-> southEastLine(State,Plyr,NextPos,FollowingPositions),
	   LineOfPositions = [NextPos|FollowingPositions]
	; LineOfPositions = []
	).