Fixes #25

app/src/main/java/sudoku/solver/HintHelper.java

@@ -0,0 +1,54 @@
+package sudoku.solver;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Random;
+
+import sudoku.structure.Cell;
+import sudoku.structure.MultiDoku;
+
+public class HintHelper {
+
+	public static record Hint(Cell cell, int newValue) {
+
+	}
+
+	public static Hint getHint(MultiDoku doku, Solver solver) {
+		doku.getStateManager().pushState();
+		doku.clearMutableCells();
+		if (!solver.solve(doku))
+			return null;
+		var stateSolved = doku.getStateManager().popAndGetState();
+		// find differences
+		Map<Cell, Integer> newValues = new HashMap<>();
+		for (var entry : stateSolved.entrySet()) {
+			Cell cell = entry.getKey();
+			// we only want the cells that can be filled
+			if (!cell.isMutable())
+				continue;
+			int oldValue = cell.getSymbolIndex();
+			int newValue = stateSolved.get(cell);
+			if (oldValue == newValue)
+				continue;
+			// we have to clear the cell
+			if (newValue == Cell.NOSYMBOL)
+				return new Hint(cell, newValue);
+			// we have to change the cell value
+			if (oldValue != Cell.NOSYMBOL && newValue != oldValue)
+				return new Hint(cell, newValue);
+
+			// there is a valid move
+			newValues.put(cell, newValue);
+		}
+
+		// this is too complex just for fetching a random entry, but whatever ...
+		Random r = new Random();
+		List<Cell> cells = new ArrayList<>(newValues.keySet());
+		Cell randomCell = cells.get(r.nextInt(cells.size()));
+		int randomCellValue = newValues.get(randomCell);
+		return new Hint(randomCell, randomCellValue);
+	}
+
+}

app/src/main/java/sudoku/structure/StateManager.java

@@ -15,27 +15,35 @@ public class StateManager {
 		this.doku = doku;
 	}
 
-	public void pushState() {
-		states.add(new HashMap<>());
-		saveState();
+	public Map<Cell, Integer> pushState() {
+		states.add(saveState());
+		return states.getLast();
 	}
 
 	public void popState() {
 		assert (states.size() > 0);
-		restoreState();
-		states.pop();
+		restoreState(states.pop());
 	}
 
-	private void restoreState() {
-		for (var entry : this.states.getLast().entrySet()) {
+	public Map<Cell, Integer> popAndGetState() {
+		assert (states.size() > 0);
+		var currentState = saveState();
+		restoreState(states.pop());
+		return currentState;
+	}
+
+	private void restoreState(Map<Cell, Integer> state) {
+		for (var entry : state.entrySet()) {
 			entry.getKey().setSymbolIndex(entry.getValue());
 		}
 	}
 
-	private void saveState() {
+	private Map<Cell, Integer> saveState() {
+		Map<Cell, Integer> state = new HashMap<>();
 		for (Cell cell : this.doku.getCells()) {
-			states.getLast().put(cell, cell.getSymbolIndex());
+			state.put(cell, cell.getSymbolIndex());
 		}
+		return state;
 	}
 
 }