%
% Killer Sudoku puzzle in ASP.
%
% http://en.wikipedia.org/wiki/Killer_Sudoku
% """
% Killer sudoku (also killer su doku, sumdoku, sum doku, addoku, or
% samunamupure) is a puzzle that combines elements of sudoku and kakuro.
% Despite the name, the simpler killer sudokus can be easier to solve
% than regular sudokus, depending on the solver's skill at mental arithmetic;
% the hardest ones, however, can take hours to crack.
%
% ...
%
% The objective is to fill the grid with numbers from 1 to 9 in a way that
% the following conditions are met:
%
%   * Each row, column, and nonet contains each number exactly once.
%   * The sum of all numbers in a cage must match the small number printed
%     in its corner.
%   * No number appears more than once in a cage. (This is the standard rule
%     for killer sudokus, and implies that no cage can include more
%     than 9 cells.)
%
% In 'Killer X', an additional rule is that each of the long diagonals
% contains each number once.
% """
%
% Here we solve the problem from the Wikipedia page, also shown here
% http://en.wikipedia.org/wiki/File:Killersudoku_color.svg
%
% The solution is:
%   2 1 5 6 4 7 3 9 8
%   3 6 8 9 5 2 1 7 4
%   7 9 4 3 8 1 6 5 2
%   5 8 6 2 7 4 9 3 1
%   1 4 2 5 9 3 8 6 7
%   9 7 3 8 1 6 4 2 5
%   8 2 1 7 3 9 5 4 6
%   6 5 9 4 2 8 7 1 3
%   4 3 7 1 6 5 2 8 9
%
%
% This was created by Hakan Kjellerstrand, hakank@gmail.com
%

% slighly faster:
%    clingo --stat --heuristic=Vmtf killer_sudoku.lp 0   Choices: 875 Conflicts: 483 Restarts: 0
%    clingo --stat --heuristic=Vsids killer_sudoku.lp 0   Choices: 291 Conflicts: 210 Restarts: 0

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

#const n=9.

%
% Hints:
% p(Result,  index,....)
%
p(3,   1,1,  1,2).
p(15,  1,3,  1,4, 1,5).
p(22,  1,6,  2,5, 2,6, 3,5).
p(4,   1,7,  2,7).
p(16,  1,8,  2,8).
p(15,  1,9,  2,9, 3,9, 4,9).
p(25,  2,1,  2,2, 3,1, 3,2).
p(17,  2,3,  2,4).
p(9,   3,3,  3,4, 4,4).
p(8,   3,6,  4,6, 5,6).
p(20,  3,7,  3,8, 4,7).
p(6,   4,1,  5,1).
p(14,  4,2,  4,3).
p(17,  4,5,  5,5, 6,5).
p(17,  4,8,  5,7, 5,8).
p(13,  5,2,  5,3, 6,2).
p(20,  5,4,  6,4, 7,4).
p(12,  5,9,  6,9).
p(27,  6,1,  7,1, 8,1, 9,1).
p(6,   6,3,  7,2, 7,3).
p(20,  6,6,  7,6, 7,7).
p(6,   6,7,  6,8).
p(10,  7,5,  8,4, 8,5, 9,4).
p(14,  7,8,  7,9, 8,8, 8,9).
p(8,   8,2,  9,2).
p(16,  8,3,  9,3).
p(15,  8,6,  8,7).
p(13,  9,5,  9,6, 9,7).
p(17,  9,8,  9,9).

%
% domains
%
val(1..n).
border(1;4;7).

% alldifferent rows, columns, values
1 { x(X,Y,N) : val(N) } 1 :- val(X;Y).
1 { x(X,Y,N) : val(X) } 1 :- val(N;Y).
1 { x(X,Y,N) : val(Y) } 1 :- val(N;X).

% alldifferent boxes
1 { x(X,Y,N) : val(X;Y),
X1<=X, X<=X1+2, Y1<=Y, Y<=Y1+2 } 1 :- val(N), border(X1;Y1).

% 2 arguments
:- p(Res, X1,X2, Y1,Y2),
x(X1,X2, X),
x(Y1,Y2, Y),
X+Y != Res.

% 3 arguments
:- p(Res, X1,X2, Y1,Y2, Z1,Z2),
x(X1,X2, X),
x(Y1,Y2, Y),
x(Z1,Z2, Z),
X+Y+Z != Res.

% 4 arguments
:- p(Res, X1,X2, Y1,Y2, Z1,Z2, A1,A2),
x(X1,X2, X),
x(Y1,Y2, Y),
x(Z1,Z2, Z),
x(A1,A2, A),
X+Y+Z+A != Res.

#show x/3.