/*

Killer Sudoku in B-Prolog.

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 output 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

Model created by Hakan Kjellerstrand, hakank@gmail.com

*/

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

go :-
solve(1).

solve(ProblemName) :-
problem(ProblemName, Board),
killer_sudoku(Board,X),
print_board(X).

killer_sudoku(Hints,X) :-

N = 3,
N2 is N*N,

new_array(X,[N2,N2]),
array_to_list(X,Vars),

% The hints (from kakuro.pl)
foreach([Sum|List] in Hints,[XLine],
(XLine @= [X[R,C] : [R,C] in List, X[R,C] #> 0],
sum(XLine) #= Sum,
alldifferent(XLine))
),

sudoku(N, X),

labeling(Vars).

%
% Ensure a Latin square,
% i.e. all rows and all columns are different
%
alldifferent_matrix(Board) :-
Rows @= Board^rows,
foreach(Row in Rows, alldifferent(Row)),
Columns @= Board^columns,
foreach(Column in Columns, alldifferent(Column)).

% This is from sudoku.pl
sudoku(N, Board) :-
N2 is N*N,

BoardVar @= [BB : I in 1..N2, J in 1..N2, [BB], BB @= Board[I,J]],
BoardVar :: 1..N2,

alldifferent_matrix(Board),
foreach(I in 1..N..N2, J in 1..N..N2,
[SubSquare],
(
SubSquare @= [X : K in 0..N-1, L in 0..N-1,
[X], X @= Board[I+K,J+L]],
alldifferent(SubSquare)
)
),
labeling([ff,down], BoardVar).

print_board(Board) :-
N @= Board^length,
foreach(I in 1..N,
(foreach(J in 1..N,
[X],
(X @= Board[I,J],
(var(X) -> write('  _') ; format('  ~q', [X]))
)
),
nl
)
),
nl.

problem(1,
[% The hints:
%  [Sum, [list of indices in X]]
[ 3, [1,1], [1,2]],
[15, [1,3], [1,4], [1,5]],
[22, [1,6], [2,5], [2,6], [3,5]],
[ 4, [1,7], [2,7]],
[16, [1,8], [2,8]],
[15, [1,9], [2,9], [3,9], [4,9]],
[25, [2,1], [2,2], [3,1], [3,2]],
[17, [2,3], [2,4]],
[ 9, [3,3], [3,4], [4,4]],
[ 8, [3,6], [4,6],[5,6]],
[20, [3,7], [3,8],[4,7]],
[ 6, [4,1], [5,1]],
[14, [4,2], [4,3]],
[17, [4,5], [5,5],[6,5]],
[17, [4,8], [5,7],[5,8]],
[13, [5,2], [5,3],[6,2]],
[20, [5,4], [6,4],[7,4]],
[12, [5,9], [6,9]],
[27, [6,1], [7,1],[8,1],[9,1]],
[ 6, [6,3], [7,2],[7,3]],
[20, [6,6], [7,6], [7,7]],
[ 6, [6,7], [6,8]],
[10, [7,5], [8,4],[8,5],[9,4]],
[14, [7,8], [7,9],[8,8],[8,9]],
[ 8, [8,2], [9,2]],
[16, [8,3], [9,3]],
[15, [8,6], [8,7]],
[13, [9,5], [9,6],[9,7]],
[17, [9,8], [9,9]]
]).