function strategy1()
%
% Strategy #1:
%
% If the only possiblities for a value are all 
% in the same row (or column), then we can mark off
% that row (or col) in the other regions
%
%
% Author: Greg Coombe
% Date: June 5, 2006
%

% the board and the choices
global B;
global Valid;

% Get the row and col number for regions
rrow = inline('mod((x-1),3)+1');
rcol = inline('floor((x-1)/3)+1');

% Loop over every region, and check whether the only valid choices
% are in the same row or column. If so, then that 
for i=1:3:9,
    for j=1:3:9,
        region_valid = Valid(i:i+2,j:j+2,:);
        for v=1:9,
            % Find the valid choices in this region
            pts = find(region_valid(:,:,v));
            if ( length(pts) >= 2 )
                % Convert to rows and columns
                rr = rrow(pts);
                rc = rcol(pts);
                % If the only possiblities for this value are all 
                % in the same row (or column), then we can mark off
                % that row (or col) in the other regions
                if ( all(rr == rr(1)) )
                    % Mark off the valid choices from the other 
                    % regions for this row
                    Valid(i-1+rr(1),:,v) = 0;
                    Valid(i-1+rr(1),j-1+rc, v) = 1;
                elseif ( all(rc == rc(1)) )
                    % Mark off the valid choices from the other 
                    % regions for this column
                    Valid(:,j-1+rc(1),v) = 0;
                    Valid(i-1+rr,j-1+rc(1),v) = 1;
                end
            end
        end
    end
end