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 guess -3 bulls 0 cows, guess -0 bulls 4 cows. 2) Wizard mode: Now you think of a four-digit number and let the wizard guess.