%
% Code to check for a win in a connect4 board.
%

%
% valpos(X,Y,B,W,M1,M2) := M1 represents a sum so far. M2 represents sum
%                          after checking position (X,Y). M2 is reset to 0
%                          if (X,Y) is not on board or not of type W and
%                          M1 is less than 4. Otherwise if it is of type W
%                          M2:=M1+1. else if M1 >=4 then M2:=M1.
%
valpos(X,Y,B,_,M1,M2) :-ith(X,B,Col), 
                        llength(Col,N),
                        N <Y,
                        (M1 >=4 -> M2 is M1; M2 is 0).

valpos(X,Y,B,W,M1,M2) :- ith(X,B,Col),
                         ith(Y,Col,W),
                         M2 is M1+1.

valpos(_,_,_,_,M1,M2) :- (M1 >=4 -> M2 is M1; M2 is 0).

%
% psum(I,Pos,Sgn,B,W,PSum,Sum) :=
% Sum along a line of slope Sgn through the point (Pos,N) in board B
% where W's are being check for four in a row. PSum
% is the accumulating value. Sum is the final value.
%

psum(8,_,_,_,_,_,Sum,Sum). 
psum(I,Pos,Sgn,N,B,W,PSum,Sum) :-
                         Z is I-Pos,
                         Y is Sgn*Z+N,
                         ((Y =<6, Y>0) -> valpos(I,Y,B,W,PSum,PSum2)
                                          ;PSum2 is PSum),             
                         J is I+1,
                         psum(J,Pos,Sgn,N,B,W,PSum2,Sum).

%
% winvert(Pos,B,W) := checks if move Pos in board B produces a win vertically
%
winvert(Pos,B,W) :- !,
                    ith(Pos,B,Col),
                    llength(Col,N),
                    N >= 4,
                    N1 is N-1,
                    ith(N1,Col,W),
                    N2 is N-2,
                    ith(N2,Col,W),
                    N3 is N-3,
                    ith(N3,Col,W).

%
% winhorurdr(Pos,B,W) :=
% check if there is a win horizontally, or up to the right, or up to the
% left in board B at position Pos playing W.
%

winhorurdr(Pos,B,W) :- ith(Pos,B,Col),
                   llength(Col,N),
                   psum(1,1,0,N,B,W,0,Sumh), %compute horizontal sum
                   psum(1,Pos,-1,N,B,W,0,Sumdr), %downright diagonal sum
                   psum(1,Pos,1,N,B,W,0,Sumur), %upright diagonal sum
                   !,
                   (Sumh >= 4 ; Sumdr >= 4 ; Sumur >=4 ).


% The following code checks for a win
win(Pos,B,c,W,winc) :- winvert(Pos,B,W).
win(Pos,B,c,W,winc) :- winhorurdr(Pos,B,W).

win(Pos,B,p,W,winp) :- winvert(Pos,B,W).
win(Pos,B,p,W,winp) :- winhorurdr(Pos,B,W).

% if no win then continue
win(_,_,_,_,cont).