Recursion limit for bailing on unsolvable, more extensive testing and additions to interface

app/src/main/java/sudoku/Solver.java

@@ -2,6 +2,7 @@ package sudoku;
 
 public class Solver implements SudokuSolver {
     private int[][] board = null;
+    private int tries = 0;
 
     public Solver() {
         board = new int[9][9];
@@ -10,6 +11,7 @@ public class Solver implements SudokuSolver {
     /**
      * {@inheritDoc}
      */
+    @Override
     public void setBoard(int[][] board) {
         this.board = board;
     }
@@ -17,6 +19,7 @@ public class Solver implements SudokuSolver {
     /**
      * {@inheritDoc}
      */
+    @Override
     public int[][] getBoard() {
         return board;
     }
@@ -24,6 +27,7 @@ public class Solver implements SudokuSolver {
     /**
      * Resets the board to all zeros
      */
+    @Override
     public void clear() {
         board = new int[9][9];
     }
@@ -31,6 +35,7 @@ public class Solver implements SudokuSolver {
     /**
      * {@inheritDoc}
      */
+    @Override
     public boolean solve() {
         return solve(0, 0);
     }
@@ -43,6 +48,12 @@ public class Solver implements SudokuSolver {
      * @return true if solved
      */
     private boolean solve(int row, int col) {
+        if (++tries >= 10000) {
+            if (tries == 10000)
+                System.out.println(String.format("Likely unsolvable. Tries: %d", tries));
+            return false;
+        }
+
         if (row < 0 || row > 9 || col < 0 || col > 9) {
             return false;
         }
@@ -57,6 +68,7 @@ public class Solver implements SudokuSolver {
 
         // If we have a "number" in the current cell
         // recursively call solve() on the next cell
+        // until we find a zero (empty cell)
         if (board[row][col] != 0) {
             return solve(row + 1, col);
         }
@@ -72,16 +84,28 @@ public class Solver implements SudokuSolver {
             }
         }
 
+        // Reset the current cell to zero and backtrack
         board[row][col] = 0;
         return false;
     }
 
     /**
-     * Randomizes the board. This guarantees a solvable board.
+     * {@inheritDoc}
+     * <p>
+     * Default difficulty is 3
      */
     public void randomizeBoard() {
+        randomizeBoard(3);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void randomizeBoard(int difficulty) {
+        int amount_prefilled = (difficulty * 9) + 1;
         this.clear();
-        for (int i = 0; i < 9; ++i) {
+        for (int i = 0; i < amount_prefilled; ++i) {
             int row = (int) (Math.random() * 9);
             int col = (int) (Math.random() * 9);
             int val = (int) (Math.random() * 9) + 1;
@@ -89,11 +113,18 @@ public class Solver implements SudokuSolver {
                 board[row][col] = val;
             }
         }
+
+        // Recursively call randomizeBoard() until we get a solvable board
+        // This is expensive, and there should be some voodoo magic that computes this in n^2 time
+        if (!isSolvable()) {
+            randomizeBoard(difficulty);
+        }
     }
 
     /**
      * {@inheritDoc}
      */
+    @Override
     public void set(int row, int col, int val) {
         if (row < 9 && col < 9) {
             board[row][col] = val;
@@ -103,6 +134,7 @@ public class Solver implements SudokuSolver {
     /**
      * {@inheritDoc}
      */
+    @Override
     public int get(int row, int col) {
         if (row < 9 && col < 9) {
             return board[row][col];
@@ -113,32 +145,26 @@ public class Solver implements SudokuSolver {
     /**
      * {@inheritDoc}
      */
-    public boolean isLegal(int row, int col, int val) {
-        if (row < 0 || row >= 9 || col < 0 || col >= 9 || val < 1 || val > 9) {
+    @Override
+    public boolean isLegal(int row, int col, int num) {
+        // Sanity check
+        if (row < 0 || row >= 9 || col < 0 || col >= 9 || num < 1 || num > 9) {
             return false;
         }
 
-        // Check if val is already in col
-        for (int i = 0; i < 9; ++i) {
-            if (val == board[i][col]) {
-                return false;
-            }
-        }
-
-        // Check if val is already in row
-        for (int j = 0; j < 9; ++j) {
-            if (val == board[row][j]) {
-                return false;
+        // Check both the row and column
+        for (int i = 0; i < 9; i++) {
+            if (board[row][i] == num || board[i][col] == num) {
+                return false; // 'num' is already in the row or column
             }
         }
 
         // Check the 3x3 box
         int boxRowOffset = (row / 3) * 3;
         int boxColOffset = (col / 3) * 3;
-
         for (int k = 0; k < 3; ++k) {
             for (int m = 0; m < 3; ++m) {
-                if (val == board[boxRowOffset + k][boxColOffset + m]) {
+                if (num == board[boxRowOffset + k][boxColOffset + m]) {
                     return false;
                 }
             }
@@ -148,11 +174,28 @@ public class Solver implements SudokuSolver {
         return true;
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    public boolean isSolvable() {
+        // We want to work on a copy
+        int[][] copy = new int[9][9];
+        for (int row = 0; row < 9; row++) {
+            System.arraycopy(board[row], 0, copy[row], 0, 9);
+        }
+
+        Solver copyModel = new Solver();
+        copyModel.setBoard(copy);
+
+        return copyModel.solve();
+    }
+
     /**
      * Checks if the board is solved
      * 
      * @return true if solved
      */
+    @Override
     public boolean isSolved() {
         return isSolved(0, 0);
     }
@@ -204,7 +247,7 @@ public class Solver implements SudokuSolver {
             }
         }
 
-        return true;
+        return isSolved(row + 1, col);
     }
 
     /**

app/src/main/java/sudoku/SudokuSolver.java

@@ -7,7 +7,7 @@ public interface SudokuSolver {
      * @param board a board to copy values from
      * @throws IllegalArgumentException if board is invalid, e.g. not 9x9
      */
-    void setBoard(int[][] board);
+    void setBoard(int[][] board) throws IllegalArgumentException;
 
     /**
      * Get a copy of the sudoku board
@@ -49,6 +49,32 @@ public interface SudokuSolver {
      */
     void set(int row, int col, int nbr);
 
+    /**
+     * Randomize the board. Guaranteed to be solvable.
+     */
+    public void randomizeBoard();
+
+    /**
+     * Randomize the board. Guaranteed to be solvable.
+     * 
+     * @param difficulty 0-9, 0 being easiest
+     */
+    void randomizeBoard(int difficulty);
+
+    /**
+     * Check if the board is solved
+     * 
+     * @return true if solved
+     */
+    boolean isSolved();
+
+    /**
+     * Checks if the board is solvable
+     * 
+     * @return true if solvable
+     */
+    boolean isSolvable();
+
     /**
      * Clear the board
      */

app/src/test/java/sudoku/SolverTest.java

@@ -1,6 +1,6 @@
 package sudoku;
 
-import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.RepeatedTest;
 import org.junit.jupiter.api.Test;
 import static org.junit.jupiter.api.Assertions.*;
 import java.util.stream.IntStream;
@@ -45,6 +45,7 @@ class SolverTest {
     }
 
     @Test
+    @RepeatedTest(100)
     void solverTest() {
         Solver solver = new Solver();
         assertFalse(solver.isSolved());
@@ -52,12 +53,16 @@ class SolverTest {
         assertTrue(solver.isSolved());
 
         solver.clear();
-        solver.randomizeBoard();
         assertFalse(solver.isSolved());
         assertTrue(solver.solve());
-        solver.clear();
+    }
+
+    @Test
+    @RepeatedTest(100)
+    void randomizeBoardGuaranteeSolvableTest() {
+        Solver solver = new Solver();
         solver.randomizeBoard();
-        assertFalse(solver.isSolved());
+        assertTrue(solver.solve());
     }
 
     @Test
@@ -79,11 +84,26 @@ class SolverTest {
     }
 
     @Test
-    @Disabled
     void unsolvableTest() {
         Solver solver = new Solver();
+        
+        // Simple example
+        solver.clear();
         solver.set(0, 0, 1);
         solver.set(0, 1, 1);
         assertFalse(solver.solve());
+
+        // More complex example
+        solver.clear();
+        solver.set(0, 5, 7);
+        solver.set(0, 6, 8);
+        solver.set(0, 7, 2);
+        solver.set(1, 5, 3);
+        solver.set(2, 7, 1);
+        solver.set(3, 1, 4);
+        solver.set(5, 2, 8);
+        solver.set(5, 8, 6);
+        solver.set(7, 1, 8);
+        assertFalse(solver.solve());
     }
 }