Synthesising a Game Solver in Imandra¶
In this notebook we introduce a simple game called "Les Bâtonnets Géants", and show how imandra can be exploited to synthesise a strategy which always wins. The game itself consists of 16 pegs, and opponents take turns in taking 1,2 or 3 pegs from the end. The loser is the player with 1 remaining peg a their turn.
Game specific rules¶
Let us first set up a very simple representation of this game consisting of a state which is either in play with n pieces or ended with a winner:
type choice = int ;; type player = | Imandra | Opponent ;; type state = | Inplay of int | Terminal of player ;; let create_initial_state n = Inplay n;; let final_state n = n=1 ;; let remove_pins num_pins state = match state with | Terminal p -> Terminal p | Inplay n -> if n-num_pins >= 1 then Inplay (n - num_pins) else Inplay n ;;
type choice = int type player = Imandra | Opponent type state = Inplay of choice | Terminal of player val create_initial_state : choice -> state = <fun> val final_state : choice -> bool = <fun> val remove_pins : choice -> state -> state = <fun>
Now let us also set up simple functions which describe a "move" in the game, given by the function
step, and two functions which find all the valid choices for a given state, and determine if a move is valid for a given state.
let step choice state = if choice <=0 || choice >3 then state else remove_pins choice state ;; let find_all_available_choices state = match state with | Inplay n -> if n>3 then [1;2;3] else if n>2 then [2;1] else if n>1 then  else  | Terminal _ -> ;; let is_valid_choice choice state = choice <=3 && choice >=1 && match state with | Inplay s -> s > choice | Terminal _ -> false;;
val step : choice -> state -> state = <fun> val find_all_available_choices : state -> choice list = <fun> val is_valid_choice : choice -> state -> bool = <fun>
Above are the specific functions we need for this game. In what follows a generalised architecture for solving adversarial games is given - this could be viewed as similar to a Functor structure in OCaml where the specific functions given for game of Batonnets Géants are those given above.
General solver synthesis functions¶
We introduce first a function called
one_step which assumes the player of the game is
Imandra. The function takes a list of possible states as well as a map between states and choices. For each list of states the choice is played, resulting in a new list of states. If any of these states are in a final state they become "annealed" to the
Terminal variant of state declaring
Imandra as the winner. For any non-terminal states, every possible opponent play is calcuated using the function
find_all_available_choices to calculate all the next possible states.
let one_step (choice_map: (state*choice) list) (states:state list): state list = let new_states = List.fold_left (fun acc el -> match List.find (fun (x,_) -> x=el) choice_map with | None -> el::acc | Some (_,choice) -> let a = step choice el in if List.mem a acc then acc else (step choice el)::acc)  states in let annealed_states = List.map (fun x -> match x with | Inplay l -> if final_state l then Terminal Imandra else Inplay l | Terminal p -> Terminal p ) new_states in List.fold_left (fun acc el -> match el with | Terminal p -> (Terminal p)::acc | Inplay l -> let next_states = List.fold_left (fun acc el -> (step el (Inplay l))::acc )  (find_all_available_choices (Inplay l)) in List.fold_left (fun acc el -> if List.mem el acc then acc else el::acc) acc next_states )  annealed_states ;;
val one_step : (state * choice) list -> state list -> state list = <fun>
Using Imandra to synthesise a solver¶
We now introduce a function which takes an initial state and a set of steps and returns true if every resulting list of states is a winning state for Imandra.
let init_state = create_initial_state 16;; let instance_function init_state steps = let states,validity_cond = List.fold_left (fun acc el -> match acc with | first,second -> (one_step el first, second && ( let fsts = List.map fst el in fsts=first && List.for_all ( fun (s,c) -> match List.find (fun (x,_) -> x = s) el with | None -> false | Some (s,c) -> is_valid_choice c s ) el ) )) ([init_state],true) steps in validity_cond && List.for_all (fun x -> match x with | Terminal Imandra -> true | _ -> false) states;;
val init_state : state = Inplay 16 val instance_function : state -> (state * choice) list list -> bool = <fun>
Now we can exploit Imandra's technology to find a solution for the game - in this case using
[@@blast] to find the solution:
instance (fun steps -> instance_function init_state steps) [@@blast];;
- : (state * choice) list list -> bool = <fun> module CX : sig val steps : (state * choice) list list end
Instance (after 431 steps, 8.092s): let steps = [[(Inplay 16, 3)]; [(Inplay 12, 3); (Inplay 11, 2); (Inplay 10, 1)]; [(Inplay 8, 3); (Inplay 7, 2); (Inplay 6, 1)]; [(Inplay 4, 3); (Inplay 3, 2); (Inplay 2, 1)]]
Playing against Imandra¶
Now this is a strategy for the game, we can write a simple game player to play against.
[@@@program] let rec gather_inputs max = let user_input = read_line () in if user_input = "" then gather_inputs max else let n = String.to_nat user_input in match n with | None -> gather_inputs max | Some n -> if n <=0 || n >max then gather_inputs max else n;; let winner_message p = match p with Imandra -> "imandra wins " | Opponent -> "you win";; let print_state state = let rec print_state_aux l = if l <=0 then "\n" else "|"^(print_state_aux (l-1)) in match state with | Terminal p -> winner_message p | Inplay l -> print_state_aux l;; let print_choice choice = String.of_int choice;; let rec play_against_imandra state solver = match state with | Terminal p -> print_endline (winner_message p) | Inplay l -> if final_state l then print_endline (winner_message Imandra) else begin match solver with |  -> () | h::t -> begin match List.find (fun (x,_) -> x = state) h with | None -> print_endline "Solver error"; () | Some (_,choice) -> begin let next_state = step choice state in print_endline ("Imandra plays: "^ (print_choice choice)^"\n"); print_endline (print_state next_state); if (match next_state with |Inplay l -> final_state l | _ -> false)then print_endline "Imandra wins\n"else print_endline "Enter your choices"; let user_choice = gather_inputs (match next_state with Terminal _ -> 0 | Inplay n -> n-1 ) in if is_valid_choice user_choice next_state then let next_state = step user_choice next_state in print_endline ("You played: "^ (print_choice user_choice)^"\n"); print_endline (print_state next_state); if (match next_state with |Inplay l -> final_state l | _ -> false)then print_endline "You win\n"else play_against_imandra next_state t else print_endline "invalid choices - replaying..."; play_against_imandra state solver end end end ;;
val gather_inputs : choice -> choice = <fun> val winner_message : player -> string = <fun> val print_state : state -> string = <fun> val print_choice : choice -> string = <fun> val play_against_imandra : state -> (state * choice) list list -> unit = <fun>
By invoking the following code in program mode it is possible to play against imandra, but never win:
let play () = print_endline (print_state init_state); play_against_imandra init_state CX.steps ;; play ();;
An example trace is:
play ();; |||||||||||||||| Imandra plays:  ||||||||||||| Enter your choices 1 You played:  |||||||||||| Imandra plays:  ||||||||| Enter your choices 2 You played:  ||||||| Imandra plays:  ||||| Enter your choices 1 You played:  |||| Imandra plays:  | Imandra wins