function computeValid()
% 
% This function computes the valid matrix, which is a data structure that 
% we use to keep track of the valid values
% for this cell. For every cell on the board, there is a vector of
% 8 values (representing numerals 1-9). It is 1 if valid to write this
% number in this cell, 0 if it is not valid, and -1 if unassigned
%
% Author: Greg Coombe
% Date: Nov 29, 2005
%

global B;
global Valid;
global RegionTable;

% For each element on the board, mark off all of the cells in 
% that rows and columns. For example, if there is a 5 in cell 3,6,
% then we mark off all of the valid 5's in row 3, and all of the 
% valid 5's in column 6.

for i=1:9,
    for j=1:9,
        % if there is an assigned value here
        if ( B(i,j) ~= 0 )
            value = B(i,j);
            % mark off row
            Valid( :, j, value ) = 0;
            % mark off column
            Valid( i, :, value ) = 0;
            % mark off region
            region_i = RegionTable(i,:);
            region_j = RegionTable(j,:);
            Valid( region_i, region_j, value ) = 0;
            % Also mark off all the possibilites for this cell
            Valid( i, j, :) = zeros(1,9);
        end
    end %for
end %for