toothpick_game.pl

%% -*- Prolog -*-

% A very simple "toothpicks" game in Prolog. Two players take turns
% drawing one, two, or three toothpicks from a stack. If you take the
% last one, then you lose. I made this late one night, mainly to prove
% to myself that game programming is actually possible in Prolog.

:-
    use_module(library(readutil)).

player(player1).
player(player2).

other_player(player1, player2).
other_player(player2, player1).

game_state(game_state(Toothpicks, Turn_Player)) :-
    integer(Toothpicks),
    player(Turn_Player).

starting_state(N, game_state(N, player1)).

state_player(game_state(_, Turn_Player), Turn_Player).

state_toothpicks(game_state(Toothpicks, _), Toothpicks).

next_player(game_state(T, Player0), game_state(T, Player1)) :-
    other_player(Player0, Player1).

remove_toothpicks(game_state(Toothpicks0, P), game_state(Toothpicks1, P), N) :-
    Toothpicks1 is max(Toothpicks0 - N, 0).

read_number(Amount) :-
    current_input(Stdin),
    read_line_to_string(Stdin, String),
    (number_string(Amount, String), Amount >= 1, Amount =< 3) ; read_number(Amount),
    !.

ask_for_player_input(State, Amount) :-
    state_player(State, Turn_Player),
    state_toothpicks(State, Toothpicks),
    writef("%w, it is your turn. There are %w toothpicks. How many would you like to take (up to 3?)\n", [Turn_Player, Toothpicks]),
    read_number(Amount).

play_turn(State0, State2) :-
    ask_for_player_input(State0, Amount),
    remove_toothpicks(State0, State1, Amount),
    next_player(State1, State2),
    !.

end_of_game(State) :-
    state_toothpicks(State, Toothpicks),
    Toothpicks =:= 0.

play_game(State0, Winner) :-
    end_of_game(State0) ->
        state_player(State0, Winner) ;
        (
            play_turn(State0, State1),
            play_game(State1, Winner)
        ).

:-
    starting_state(15, State),
    play_game(State, Winner),
    writef("%w wins! Congratulations!\n", [Winner]),
    halt.