Thus, it does not matter whether the barcode is scanned from the left or from the right : the scanning software can determine what is the beginning and end of the barcode, with the help of the fact that the first digit should have odd parity and the last digit should have even parity.
32.
Note that while a naive enumeration of states would list 25 states, many are impossible either because the mouse can never have a lower index than the cat ( as that would mean the mouse occupied the cat's box and survived to move past it ), or because the sum of the two indices will always have even parity.
33.
After seeing a S ( start ) or E ( end ) guard pattern ( they are the same, " bar-space-bar ", whichever direction they are read ), the scanner will first see odd parity digits, if scanning left-to-right, or even parity digits, if scanning right-to-left . With the parity / direction information, an upside-down symbol will not confuse the scanner.
