Sudoku.java

// Copyright 2011 Hakan Kjellerstrand hakank@gmail.com
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
import com.google.ortools.constraintsolver.DecisionBuilder;
import com.google.ortools.constraintsolver.IntVar;
import com.google.ortools.constraintsolver.Solver;
import java.io.*;
import java.text.*;
import java.util.*;

public class Sudoku {

  static {
    System.loadLibrary("jniortools");
  }

  /** Solves a Sudoku problem. */
  private static void solve() {

    Solver solver = new Solver("Sudoku");

    int cell_size = 3;
    int n = cell_size * cell_size;

    // 0 marks an unknown value
    int[][] initial_grid =
        new int[][] {
          {0, 6, 0, 0, 5, 0, 0, 2, 0},
          {0, 0, 0, 3, 0, 0, 0, 9, 0},
          {7, 0, 0, 6, 0, 0, 0, 1, 0},
          {0, 0, 6, 0, 3, 0, 4, 0, 0},
          {0, 0, 4, 0, 7, 0, 1, 0, 0},
          {0, 0, 5, 0, 9, 0, 8, 0, 0},
          {0, 4, 0, 0, 0, 1, 0, 0, 6},
          {0, 3, 0, 0, 0, 8, 0, 0, 0},
          {0, 2, 0, 0, 4, 0, 0, 5, 0}
        };

    //
    // variables
    //
    IntVar[][] grid = new IntVar[n][n];
    IntVar[] grid_flat = new IntVar[n * n];

    for (int i = 0; i < n; i++) {
      for (int j = 0; j < n; j++) {
        grid[i][j] = solver.makeIntVar(1, 9, "grid[" + i + "," + j + "]");
        grid_flat[i * n + j] = grid[i][j];
      }
    }

    //
    // constraints
    //

    // init and rows
    for (int i = 0; i < n; i++) {
      IntVar[] row = new IntVar[n];
      for (int j = 0; j < n; j++) {
        if (initial_grid[i][j] > 0) {
          solver.addConstraint(solver.makeEquality(grid[i][j], initial_grid[i][j]));
        }
        row[j] = grid[i][j];
      }
      solver.addConstraint(solver.makeAllDifferent(row));
    }

    // columns
    for (int j = 0; j < n; j++) {
      IntVar[] col = new IntVar[n];
      for (int i = 0; i < n; i++) {
        col[i] = grid[i][j];
      }
      solver.addConstraint(solver.makeAllDifferent(col));
    }

    // cells
    for (int i = 0; i < cell_size; i++) {
      for (int j = 0; j < cell_size; j++) {
        IntVar[] cell = new IntVar[n];
        for (int di = 0; di < cell_size; di++) {
          for (int dj = 0; dj < cell_size; dj++) {
            cell[di * cell_size + dj] = grid[i * cell_size + di][j * cell_size + dj];
          }
        }
        solver.addConstraint(solver.makeAllDifferent(cell));
      }
    }

    //
    // Search
    //
    DecisionBuilder db =
        solver.makePhase(grid_flat, solver.INT_VAR_SIMPLE, solver.INT_VALUE_SIMPLE);

    solver.newSearch(db);

    while (solver.nextSolution()) {
      for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
          System.out.print(grid[i][j].value() + " ");
        }
        System.out.println();
      }
      System.out.println();
    }
    solver.endSearch();

    // Statistics
    System.out.println();
    System.out.println("Solutions: " + solver.solutions());
    System.out.println("Failures: " + solver.failures());
    System.out.println("Branches: " + solver.branches());
    System.out.println("Wall time: " + solver.wallTime() + "ms");
  }

  public static void main(String[] args) throws Exception {
    Sudoku.solve();
  }
}