/* 

  queens_clpdf benchmark in Picat v3.

  bench/queens_clpfd.pl
  https://github.com/SWI-Prolog/bench

  Changes:
  - added "import cp" instead of ":- use_module(library(clpfd))"
  - replaced "ins" to "::" (for the domain)

  Note:
  This benchmark don't actually solve the 200-queens problems.
  Using solve (or label in the SWI-Prolog model) to just show the first
  solution is slow. Showing _all_ solutions (using fail/0) is even slower.
  From https://en.wikipedia.org/wiki/Eight_queens_puzzle#Counting_solutions
  The total number of solutions for just 27-queens is 234,907,967,154,122,528.

  Thus, this model behaves as the SWI Prolog model.

  Solving for first solutions of N=200 takes 2.6s which is not especially fast.


  This Picat model was created by Hakan Kjellerstrand, hakank@gmail.com
  See also my Picat page: http://www.hakank.org/picat/

*/

import cp.

main => go.


% Solving first solution: 2.588s
go ?=>
   n_queens(200,Q),
   solve($[ff],Q),
   println(Q),
   % fail,
   nl.

go => true.

% """
% Copied from https://swish.swi-prolog.org/example/clpfd_queens.pl
% Author: Markus Triska
% """

top :- n_queens(8,Q),solve($[ff,split],Q), fail.
top.

n_queens(N, Qs) :-
	bp.length(Qs, N),
	Qs :: 1..N,
	safe_queens(Qs).

safe_queens([]).
safe_queens([Q|Qs]) :-
	safe_queens(Qs, Q, 1),
	safe_queens(Qs).

safe_queens([], _, _).
safe_queens([Q|Qs], Q0, D0) :-
	Q0 #!= Q,
	abs(Q0 - Q) #!= D0,
	D1 #= D0 + 1,
	safe_queens(Qs, Q0, D1).