gistfile1.pl

%%
% Authors:
% - Ioannis Gakos, 3634
% - Alkis Livathinos, 3690
%%

% Place disk on a pole
%
:- dynamic disk_state/2.

% Retract all predicates first
%
:- retractall(disk_state(_,_)).

% Fixed initialization
%
:- forall(
  member(X, [1, 2, 3, 4, 5]), assert(disk_state(X, 1))
).

% Use the following predicate to initialize the game
%
% N: disks
% P: pole to place the disks given
%
% Example: state_initialization([1,2,3,4,5], 3)
%
state_initialization(N, P) :-
  forall(
    member(X, N), assert(disk_state(X, P))
  ).

% Show in which state the game is by printing to std
% out, on which pole each disk has been placed
%
% OUT: D:1 on P:2, means disk 1 is placed on pole 2
%
show_state(N) :-
  forall(
    member(X, N), (
      disk_state(X, Y),
      write('D:'), write(X),
      write(' on '),
      write('P:'), write(Y),
      nl
    )
  ).

% The following minimum related predicates are used
% to find the minimum element of a list
%
minimum([Min], Min). % minimum found

minimum([H,K|T],M) :-
  H =< K, % H is less than or equal to K
  minimum([H|T],M). % so use H

minimum([H,K|T],M) :-
  H > K, % H is greater than K
  minimum([K|T],M). % so use K

% Check if legal move
legal_move(Disk, Dest) :-
  is_ontop(Disk), % check if on top
  smaller_than(Disk, Dest). % check if smaller disk exists

% Checks if the top disk on the destination pole
% is bigger than the one to be placed
%
smaller_than(Disk, Dest) :-
  findall(X, (member(Pole, [Dest]), disk_state(X, Pole)), Disks),
  minimum(Disks, Min) -> Disk<Min ; \+ fail.

% Checks if the disk to be moved is on top of the pole
%
is_ontop(Disk) :-
  disk_state(Disk, S), % Find where the disk is located
  findall(X, (member(Pole, [S]), disk_state(X, Pole)), Disks),
  minimum(Disks, Min),
  write(Min), write('-'), write(Disk), nl,
  Min =:= Disk.

% Check if problem solved by evaluating all disk_state
% predicates
%
validate :-
  forall(
    member(X, [1,2,3,4,5]), disk_state(X, 3)
  ).

% Implement the move of a disk, by calling place predicate.
% For example, if you like to move disk 1, to pole 3,
% call place(1, 3)
%
place(Disk, Dest) :-
  legal_move(Disk, Dest) -> write('Legal move'), nl,
  disk_state(Disk, S), % Find where the disk is located
  retract(disk_state(Disk, S)), % retract from previous state
  assert(disk_state(Disk, Dest)), % store new state
  show_state([1,2,3,4,5]),
  (validate -> write('Problem solved'); write('Not yet solved'));
  write('Illegal move'), fail.