function pred = regress_pred(price, quantity)

	days = size(price, 1) - size(quantity, 1);
	product_count = size(price, 2);
	test_count = size(quantity, 1);
	test_set = 1:test_count;
	pred_set = test_count+1:size(price, 1);

	pred = zeros(days, product_count);
	for i = 1:product_count
		[m,n] = regress(price(test_set, i), quantity(test_set, i));
		pred(:, i) = m .* price(pred_set, i) + n;
	end

endfunction

function [m, n] = regress(x, y)
	mn = [ x ones(length(x), 1) ] \ y;
	m = mn(1);
	n = mn(2);
endfunction