032: Maximum density still life

/*

  Maximum density still life in Picat.

  CSPLib 032: http://www.csplib.org/prob/prob032

  This model (or rather my earlier MiniZinc and Comet models) was 
  inspired by the OPL model from
  Toni Mancini, Davide Micaletto, Fabio Patrizi, Marco Cadoli: 
  "Evaluating ASP and commercial solvers on the CSPLib"
  http://www.dis.uniroma1.it/~tmancini/index.php?problemid=032&solver=OPL&spec=BASE&currItem=research.publications.webappendices.csplib2x.problemDetails#listing

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

*/

% Licenced under CC-BY-4.0 : http://creativecommons.org/licenses/by/4.0/

import cp.

main => go.

go =>
   Size = 6,
   life(Size, Grid, Z),
   print_grid(Grid),
   writeln(z=Z),
   nl.

print_grid(Grid) =>
   foreach(G in Grid) 
      foreach(V in G) printf("%d", V) end, nl
   end.


life(Size, Grid, Z) =>

   GridSize = Size+4,

   Grid = new_array(GridSize,GridSize),
   Vars = vars(Grid),
   Vars :: 0..1,

   Z #= sum(Vars),
  
   % C1: Cells in the first/last two rows/columns are all 0 (dead)
   foreach(X in 1..GridSize)
          Grid[1,X] #= 0,
          Grid[2,X] #= 0,
          Grid[Size+3,X] #= 0,  
          Grid[Size+4,X] #= 0,
          Grid[X,1] #= 0,       
          Grid[X,2] #= 0,
          Grid[X,Size+3] #= 0,  
          Grid[X,Size+4] #= 0 
   end,
 
   foreach(R in 2..Size+3, C in 2..Size+3)
      % C2: Each cell of the board (except those of the first/last row/column) 
      %     that has exactly three live neighbors is alive. 
      %     Together with constraint C1, this implies that cells in the
      %     second/last-but-one row/column cannot have three live neighbors.
      ( ( Grid[R-1,C-1] + Grid[R-1,C] + Grid[R-1,C+1] + 
          Grid[R,C-1] + Grid[R,C+1] + 
          Grid[R+1,C-1] + Grid[R+1,C] + Grid[R+1,C+1]
        ) #= 3 
      ) #=> (Grid[R,C] #= 1)
      ,
           
      % C3: Each live cell must have 2 or 3 live neighbors (cells of the first/last 
      % row/column may be ignored by this constraint)
      (Grid[R,C] #= 1) #=> 
            2 #=< 
            ( Grid[R-1,C-1] + Grid[R-1,C] + Grid[R-1,C+1] +
              Grid[R,C-1] + Grid[R,C+1] +
              Grid[R+1,C-1] + Grid[R+1,C] + Grid[R+1,C+1] 
            )
            #/\
            ( Grid[R-1,C-1] + Grid[R-1,C] + Grid[R-1,C+1] +
              Grid[R,C-1] + Grid[R,C+1] +
              Grid[R+1,C-1] + Grid[R+1,C] + Grid[R+1,C+1] 
            ) #=< 3
   end,

   % SBSO: Symmetry-breaking by selective ordering
   % The assignment is forced to respect an ordering on the values that occur in corner entries
   % of the board. In particular:  
   % - if the NW-corner cell is dead, the SE-corner cell
   % must be dead too 
   % - if the NE-corner cell is dead, the SW-corner cell must be dead too
   % 
   Grid[2,2] #>= Grid[Size+1,Size+1],
   Grid[2,Size+1] #>= Grid[Size+1,2],

   solve([$max(Z),min,updown], Vars).