💾 Memento Design Pattern

// Memento — immutable snapshot
public class EditorMemento {
    private final String content;
    private final int cursorPosition;
    private final String fontFamily;
    private final int fontSize;
    private final LocalDateTime timestamp;

    // Package-private constructor — only Originator should create
    EditorMemento(String content, int cursorPosition,
                  String fontFamily, int fontSize) {
        this.content = content;
        this.cursorPosition = cursorPosition;
        this.fontFamily = fontFamily;
        this.fontSize = fontSize;
        this.timestamp = LocalDateTime.now();
    }

    // Package-private getters — only Originator should access
    String getContent() { return content; }
    int getCursorPosition() { return cursorPosition; }
    String getFontFamily() { return fontFamily; }
    int getFontSize() { return fontSize; }

    // Public metadata for Caretaker display purposes
    public LocalDateTime getTimestamp() { return timestamp; }
    public String getDescription() {
        return "Snapshot at " + timestamp.format(
            DateTimeFormatter.ofPattern("HH:mm:ss"));
    }
}

// Originator — creates and restores from mementos
public class TextEditor {
    private String content = "";
    private int cursorPosition = 0;
    private String fontFamily = "Arial";
    private int fontSize = 12;

    public void type(String text) {
        content = content.substring(0, cursorPosition) +
                  text +
                  content.substring(cursorPosition);
        cursorPosition += text.length();
    }

    public void setFont(String family, int size) {
        this.fontFamily = family;
        this.fontSize = size;
    }

    public void moveCursor(int position) {
        this.cursorPosition = Math.min(position, content.length());
    }

    // Create memento
    public EditorMemento save() {
        return new EditorMemento(content, cursorPosition, fontFamily, fontSize);
    }

    // Restore from memento
    public void restore(EditorMemento memento) {
        this.content = memento.getContent();
        this.cursorPosition = memento.getCursorPosition();
        this.fontFamily = memento.getFontFamily();
        this.fontSize = memento.getFontSize();
    }

    public String getContent() { return content; }

    @Override
    public String toString() {
        return String.format("Editor[content='%s', cursor=%d, font=%s %d]",
            content, cursorPosition, fontFamily, fontSize);
    }
}

// Caretaker — manages memento history
public class EditorHistory {
    private final Deque<EditorMemento> undoStack = new ArrayDeque<>();
    private final Deque<EditorMemento> redoStack = new ArrayDeque<>();
    private final TextEditor editor;
    private static final int MAX_HISTORY = 50;

    public EditorHistory(TextEditor editor) {
        this.editor = editor;
    }

    public void save() {
        if (undoStack.size() >= MAX_HISTORY) {
            // Remove oldest to prevent memory bloat
            ((ArrayDeque<EditorMemento>) undoStack).removeLast();
        }
        undoStack.push(editor.save());
        redoStack.clear();
    }

    public void undo() {
        if (undoStack.isEmpty()) {
            System.out.println("⚠️ Nothing to undo");
            return;
        }
        redoStack.push(editor.save()); // Save current for redo
        EditorMemento memento = undoStack.pop();
        editor.restore(memento);
        System.out.println("↩️ Undo → " + memento.getDescription());
    }

    public void redo() {
        if (redoStack.isEmpty()) {
            System.out.println("⚠️ Nothing to redo");
            return;
        }
        undoStack.push(editor.save());
        EditorMemento memento = redoStack.pop();
        editor.restore(memento);
        System.out.println("↪️ Redo → " + memento.getDescription());
    }

    public void showHistory() {
        System.out.println("📜 History (" + undoStack.size() + " snapshots):");
        undoStack.forEach(m -> System.out.println("  • " + m.getDescription()));
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        TextEditor editor = new TextEditor();
        EditorHistory history = new EditorHistory(editor);

        // Type and save
        history.save();
        editor.type("Hello ");
        System.out.println(editor); // content='Hello '

        history.save();
        editor.type("World!");
        System.out.println(editor); // content='Hello World!'

        history.save();
        editor.setFont("Courier", 14);
        editor.type(" How are you?");
        System.out.println(editor); // content='Hello World! How are you?'

        // Undo
        history.undo(); // Back to "Hello World!"
        System.out.println(editor);

        history.undo(); // Back to "Hello "
        System.out.println(editor);

        // Redo
        history.redo(); // Forward to "Hello World!"
        System.out.println(editor);
    }
}

// Originator with nested Memento — strictest encapsulation
public class GameCharacter {
    private int health;
    private int mana;
    private int x, y;
    private List<String> inventory;

    public GameCharacter(int health, int mana) {
        this.health = health;
        this.mana = mana;
        this.inventory = new ArrayList<>();
    }

    public void takeDamage(int damage) { health -= damage; }
    public void heal(int amount) { health += amount; }
    public void move(int x, int y) { this.x = x; this.y = y; }
    public void addItem(String item) { inventory.add(item); }

    // Memento as inner class — only GameCharacter can access internals
    public class SaveState {
        private final int health;
        private final int mana;
        private final int x, y;
        private final List<String> inventory;

        private SaveState() {
            this.health = GameCharacter.this.health;
            this.mana = GameCharacter.this.mana;
            this.x = GameCharacter.this.x;
            this.y = GameCharacter.this.y;
            this.inventory = new ArrayList<>(GameCharacter.this.inventory);
        }
    }

    public SaveState saveState() {
        return new SaveState();
    }

    public void loadState(SaveState state) {
        this.health = state.health;
        this.mana = state.mana;
        this.x = state.x;
        this.y = state.y;
        this.inventory = new ArrayList<>(state.inventory);
    }

    @Override
    public String toString() {
        return String.format("Character[HP=%d, MP=%d, pos=(%d,%d), items=%s]",
            health, mana, x, y, inventory);
    }
}