core-difficulty

Difficulty levels. A ruleset declares its levels as subclasses of Difficulty, each with a static level() (0 is the easiest), and registers them in the AvailableDifficultyRegistry. A game keeps the one it is played at in its GameDifficultyRegistry, which holds a single GameDifficulty entity so that the level is saved with the game.

Rules read the level from the game's registry (get()), which is null until a level is chosen.