Cows Bulls

It is classical logic game Cows&Bulls. The Computer make a 4 digit number. The digits are all different. Then, in turn, the player try to guess the number. If the matching digits are in different positions, they are "cows", if in their right positions, they are "bulls". Example: Secret number: 4271. Opponent's try: 1234. Answer: 2 cows and 1 bull.