Bulls and Cows Puzzle

Classic code-breaking game. Three modes of play.

1) Solo mode: Figure out the four-digit code through the given hints. A bull means right number and right position. A cow means right number but wrong position.
For example: answer(1234), guess(1230)--3 bulls 0 cows, guess(4321)--0 bulls 4 cows

2) Wizard mode: Now you think of a four-digit number and let the wizard guess. After each of the wizard's guesses, you tell him how many bulls and cows for your number.

3) You Vs. Wizard: You compete with the wizard to see who cracks the other's code first.