Finding the values we need to xor with value at ( x2 , y2 ) in prefix - xor matrix
if ( x1 != 0 ) xor_1 = prefix_xor [ x1 - 1 , y2 ] ; if ( y1 != 0 ) xor_2 = prefix_xor [ x2 , y1 - 1 ] ; if ( x1 != 0 && y1 != 0 ) xor_3 = prefix_xor [ x1 - 1 , y1 - 1 ] ;