feat: add states

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

@@ -0,0 +1,41 @@
+package sudoku.structure;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Stack;
+
+//TODO: doc
+public class StateManager {
+
+	private final Stack<Map<Cell, Integer>> states;
+	private final MultiDoku doku;
+
+	public StateManager(MultiDoku doku) {
+		this.states = new Stack<>();
+		this.doku = doku;
+	}
+
+	public void pushState() {
+		states.add(new HashMap<>());
+		saveState();
+	}
+
+	public void popState() {
+		assert (states.size() > 0);
+		restoreState();
+		states.pop();
+	}
+
+	private void restoreState() {
+		for (var entry : this.states.getLast().entrySet()) {
+			entry.getKey().setSymbolIndex(entry.getValue());
+		}
+	}
+
+	private void saveState() {
+		for (Cell cell : this.doku.getCells()) {
+			states.getLast().put(cell, cell.getSymbolIndex());
+		}
+	}
+
+}