Prolog/chess960

% Using SWI-Prolog 7.2.3
% see https://en.wikipedia.org/wiki/Chess960 to understant what is this about.

:- use_module(library(random)).

even(N) :- 0 is N mod 2.
odd(N) :- \+even(N).

index_of(Xs, X, N) :- nth0(N, Xs, X).

start_position(P) :-
  Pieces = ['N','N','K','B','b','T','t','Q'],
  permutation(Pieces, P),
  index_of(P, 'T', IT),
  index_of(P, 't', It),
  index_of(P, 'K', IK),
  IT < IK,
  IK < It,   % The King must be between the two towers
  index_of(P, 'B', IB),
  index_of(P, 'b', Ib),
  even(Ib),
  odd(IB).   % Bishops must be in different colored squares

turn(t, 'T') :- !.
turn(b, 'B') :- !.
turn(X,   X).

% All valid Fischer Random Chess positions
all_pos(Ps) :-
  setof(P, start_position(P), Ps1),
  maplist(maplist(turn), Ps1, Ps).

% Picks randomly, one of the 960 different positions.
random_960_position(P) :-
  all_pos(Ps),
  random_member(P, Ps).

/*
   | Example:
     | -?  all_pos(Ps), length(Ps, L).
     | Ps = ... L = 960
     |
     | ?- random_960_position(X).
     | X = ['N', 'B', 'N', 'T', 'B', 'K', 'Q', 'T'].
*/