Basic undo Mememto Pattern

3 files changed, 102 insertions, 0 deletions

diff --git a/src/main/java/design/model/undo/Memento.java b/src/main/java/design/model/undo/Memento.java
new file mode 100644
index 0000000..162c3f3
--- /dev/null
+++ b/src/main/java/design/model/undo/Memento.java
@@ -0,0 +1,4 @@
+package design.model.undo;
+
+public interface Memento {
+}
diff --git a/src/main/java/design/model/undo/Originator.java b/src/main/java/design/model/undo/Originator.java
new file mode 100644
index 0000000..63eac38
--- /dev/null
+++ b/src/main/java/design/model/undo/Originator.java
@@ -0,0 +1,7 @@
+package design.model.undo;
+
+public interface Originator {
+    Memento createMemento();
+
+    void restore(Memento memento);
+}
\ No newline at end of file
diff --git a/src/main/java/design/model/undo/UndoManager.java b/src/main/java/design/model/undo/UndoManager.java
new file mode 100644
index 0000000..5c6a4a0
--- /dev/null
+++ b/src/main/java/design/model/undo/UndoManager.java
@@ -0,0 +1,91 @@
+package design.model.undo;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+import java.util.List;
+import java.util.stream.Collectors;
+
+// Singleton caretaker that handles undo and redo stacks.
+public final class UndoManager {
+
+    private static final UndoManager INSTANCE = new UndoManager();
+
+    public static UndoManager instance() {
+        return INSTANCE;
+    }
+
+    private UndoManager() {
+    }
+
+    // Stack entry
+    private static class Entry {
+        final Originator originator;
+        final Memento memento;
+        final String label;
+
+        Entry(Originator originator, Memento memento, String label) {
+            this.originator = originator;
+            this.memento = memento;
+            this.label = label;
+        }
+    }
+
+    private final Deque<Entry> undoStack = new ArrayDeque<>();
+    private final Deque<Entry> redoStack = new ArrayDeque<>();
+
+    // Capture state
+    public void capture(Originator originator, String label) {
+        undoStack.push(new Entry(originator, originator.createMemento(), label));
+        redoStack.clear();
+    }
+
+    // Undo / Redo ops
+    public boolean canUndo() {
+        return !undoStack.isEmpty();
+    }
+
+    public boolean canRedo() {
+        return !redoStack.isEmpty();
+    }
+
+    public String peekUndoLabel() {
+        return canUndo() ? undoStack.peek().label : null;
+    }
+
+    public String peekRedoLabel() {
+        return canRedo() ? redoStack.peek().label : null;
+    }
+
+    public List<String> getUndoHistoryLabels() {
+        return undoStack.stream()
+                .map(e -> e.label)
+                .collect(Collectors.toList());
+    }
+
+    public void undo() {
+        if (!canUndo()) {
+            System.out.println("Nothing to undo.");
+            return;
+        }
+
+        Entry entry = undoStack.pop();
+        redoStack.push(new Entry(entry.originator, entry.originator.createMemento(), entry.label));
+        entry.originator.restore(entry.memento);
+    }
+
+    public void redo() {
+        if (!canRedo()) {
+            System.out.println("Nothing to redo.");
+            return;
+        }
+
+        Entry entry = redoStack.pop();
+        undoStack.push(new Entry(entry.originator, entry.originator.createMemento(), entry.label));
+        entry.originator.restore(entry.memento);
+    }
+
+    public void purge() {
+        undoStack.clear();
+        redoStack.clear();
+    }
+}