import java.util.*;
import javax.swing.*;

import java.io.IOException;
import java.io.FileReader;
import java.io.Reader;

class Sudoku {

  int[] fieldValues = new int[81];
  int[] emptyFields;
  int currentField;

  void printField() {
    String fieldOut = "";
    int i;
    for(i=0; i<81; i++) {
      if(i%27 == 0 && i>0) {
        fieldOut += "\n---------------------\n";
      }
      else if(i%9 == 0) {
        fieldOut += "\n";
      }
      else if(i%3 == 0) {
        fieldOut += "| ";
      }
      if(fieldValues[i] != 0) {
        fieldOut += fieldValues[i] + " ";
      }
      else {
        fieldOut += "  ";
      }
    }
    System.out.println(fieldOut);
  }

  /** Function asks for an file name which
   * should contain a sudoku and this sudoku
   * into fieldValues, Empty fields should
   * be marked by a '0'
   */
  void loadFieldFromFile() {
    String fileName = "";
    Reader reader = null;
    int i = 0;

    fileName = JOptionPane.showInputDialog( "Enter filename" );
    System.out.println(fileName);

    try
    {
      reader = new FileReader (fileName);
      for ( int c; ( c = reader.read() ) != -1; ) {
        int v = 0;
        v = c - (int) '0';
        if( v >= 0 && v <= 9 ) {
          fieldValues[i] = v;
          i++;
        }
        if( i > 80 ) break;
      }
    }
    catch ( IOException e) {
      System.err.println( "Could not open file " + fileName );
      return;
    }
    finally {
      try { reader.close(); } catch ( Exception e ) { e.printStackTrace(); }
    }

    System.out.println("File loaded");
  }

  /** Function checks if "number" may be inserted
   * in field "index"
   */
  boolean isNumberAllowed(int number, int index) {
    int i = 0, j = 0;
    /* check col */
    for(i=index%9; i<81; i+=9) {
      if(fieldValues[i] == number)
        return false;
    }
    /* check row */
    for(i=index/9*9; i<(index/9*9+9); i++) {
      if(fieldValues[i] == number)
        return false;
    }
    /* check field */
    int lu = (index/9/3*27) + (index%9/3*3); //first index of field
    for(j = 0; j<3; j++) {
      for(i = lu+(j*9); i<lu+(j*9)+3; i++) {
        if(fieldValues[i] == number)
          return false;
      }
    }
    return true;
  }

  /** Function trys to solve the current sudoku
   * using backtracking algorithm
   */
  void solveSudoku() {
    currentField = 0;
    stepAhead();
  }

  boolean stepAhead() {
    if(currentField==81) {
      return true;
    }

    if(fieldValues[currentField] != 0) {
      currentField++;
      if(stepAhead()) {
        return true;
      }
    }
    else {
      for(int i = 1; i<=9; i++) {
        if(isNumberAllowed(i, currentField)) {
          fieldValues[currentField] = i;
          currentField++;
          if(stepAhead())
            return true;
        }
      }
      fieldValues[currentField] = 0;
    }
    currentField--;

    return false;
  }

}

/** This class is to test the
 * Sudoku class and it's functions
 */
class SudokuDemo {
  public static void main(String[] args) {
    System.out.println("Sudoku demo loading...");
    Sudoku s = new Sudoku();
    s.loadFieldFromFile();
    s.printField();
    //s.testAllowed();
    s.solveSudoku();
    s.printField();
  }
}