Start game - When a new game was started, the game generates secret code. The code should have 4 items.

Code-breaker submits guess - The code-breaker propose a guess, and the system replies by marking the guess according to the marking algorithm.

Code-breaker wins game - The code-breaker propose a guess that matches the secret code exactly. The system responds by marking the guess with four + signs.

Code-breaker loses game - After some number of turns, the game tells the code-breaker that the game is over (need to decide how many turns and whether to reveal the code).

Code-breaker plays again - After the game is won or lost, the system prompts the code-breaker to play again. If the code-breaker indicates yes, a new game begins. If the code-breaker indicates no, the system shuts down.

Code-breaker requests hint - At any time during a game, the code-breaker can request a hint, at which point the system reveals one of the numbers in the secret code.

Code-breaker saves score - After the game is won or lost, the code-breaker can opt to save information about the game: who (initials?), how many turns, and so on.