Cows & Bulls -- Number game

Cows & Bulls is an old code-breaking mind game.

The game may be played by app vs player OR 2 players OR two teams of 2–3 players. Here we are playing app vs player.

* With each 'New Game' the app will calculate a 4-digit secret number.

* The digits will non-repetative.

* Then the player tries to guess the secret number.

* If the matching digits are in their right positions, they are "Bulls", if in different positions, they are "Cows".

* Example:

- Secret number: 9573

- Your guess: 3569

- Answer: 1 bull and 2 cows. (The bull is "5", the cows are "9" and "3".)