The game 247 is played as follows:
- The first player says "2, 4, 7".
- The next player makes a move by subtracting 1 or more from one of these numbers, potentially eliminating one of the numbers.
- Players take turns repeating this step until someone is forced to eliminate the last remaining number. The player who does this loses.
Assuming both players play optimally, every game state is either a winning or losing move. For example, the player who says "1" must win, as this forces the other player to eliminate the number. This solver brute forces the game to tell you whether a game state wins or loses.
If x is a winning state (meaning the player who says it will win),
then defeat x will return None indicating x can't be defeated.
If x is a losing state, defeat x will return Some y, where y is a move
one can make to defeat state x.