---
title: "Memento"
type: "design-pattern"
slug: "memento"
url: "http://localhost:3000/en/design-patterns/memento.md"
category: "Behavioral Patterns"
description: "Memento is a behavioral design pattern that lets you save and restore the previous state of an object without revealing the details of its implementation."
languages: ["java", "csharp", "cpp", "go", "php", "python", "ruby", "rust", "swift", "typescript"]
---
# Memento

> Memento is a behavioral design pattern that lets you save and restore the previous state of an object without revealing the details of its implementation.

## Intent

**Memento** is a behavioral design pattern that lets you save and restore the previous state of an object without revealing the details of its implementation.

## Problem

Imagine that you’re creating a text editor app. In addition to simple text editing, your editor can format text, insert inline images, etc.

At some point, you decided to let users undo any operations carried out on the text. This feature has become so common over the years that nowadays people expect every app to have it. For the implementation, you chose to take the direct approach. Before performing any operation, the app records the state of all objects and saves it in some storage. Later, when a user decides to revert an action, the app fetches the latest snapshot from the history and uses it to restore the state of all objects.

Before executing an operation, the app saves a snapshot of the objects’ state, which can later be used to restore objects to their previous state.

Let’s think about those state snapshots. How exactly would you produce one? You’d probably need to go over all the fields in an object and copy their values into storage. However, this would only work if the object had quite relaxed access restrictions to its contents. Unfortunately, most real objects won’t let others peek inside them that easily, hiding all significant data in private fields.

Ignore that problem for now and let’s assume that our objects behave like hippies: preferring open relations and keeping their state public. While this approach would solve the immediate problem and let you produce snapshots of objects’ states at will, it still has some serious issues. In the future, you might decide to refactor some of the editor classes, or add or remove some of the fields. Sounds easy, but this would also require changing the classes responsible for copying the state of the affected objects.

How to make a copy of the object’s private state?

But there’s more. Let’s consider the actual “snapshots” of the editor’s state. What data does it contain? At a bare minimum, it must contain the actual text, cursor coordinates, current scroll position, etc. To make a snapshot, you’d need to collect these values and put them into some kind of container.

Most likely, you’re going to store lots of these container objects inside some list that would represent the history. Therefore the containers would probably end up being objects of one class. The class would have almost no methods, but lots of fields that mirror the editor’s state. To allow other objects to write and read data to and from a snapshot, you’d probably need to make its fields public. That would expose all the editor’s states, private or not. Other classes would become dependent on every little change to the snapshot class, which would otherwise happen within private fields and methods without affecting outer classes.

It looks like we’ve reached a dead end: you either expose all internal details of classes, making them too fragile, or restrict access to their state, making it impossible to produce snapshots. Is there any other way to implement the "undo"?

## Solution

All problems that we’ve just experienced are caused by broken encapsulation. Some objects try to do more than they are supposed to. To collect the data required to perform some action, they invade the private space of other objects instead of letting these objects perform the actual action.

The Memento pattern delegates creating the state snapshots to the actual owner of that state, the _originator_ object. Hence, instead of other objects trying to copy the editor’s state from the “outside,” the editor class itself can make the snapshot since it has full access to its own state.

The pattern suggests storing the copy of the object’s state in a special object called _memento_. The contents of the memento aren’t accessible to any other object except the one that produced it. Other objects must communicate with mementos using a limited interface which may allow fetching the snapshot’s metadata (creation time, the name of the performed operation, etc.), but not the original object’s state contained in the snapshot.

The originator has full access to the memento, whereas the caretaker can only access the metadata.

Such a restrictive policy lets you store mementos inside other objects, usually called _caretakers_. Since the caretaker works with the memento only via the limited interface, it’s not able to tamper with the state stored inside the memento. At the same time, the originator has access to all fields inside the memento, allowing it to restore its previous state at will.

In our text editor example, we can create a separate history class to act as the caretaker. A stack of mementos stored inside the caretaker will grow each time the editor is about to execute an operation. You could even render this stack within the app’s UI, displaying the history of previously performed operations to a user.

When a user triggers the undo, the history grabs the most recent memento from the stack and passes it back to the editor, requesting a roll-back. Since the editor has full access to the memento, it changes its own state with the values taken from the memento.

## Structure

#### Implementation based on nested classes

The classic implementation of the pattern relies on support for nested classes, available in many popular programming languages (such as C++, C#, and Java).

1. The **Originator** class can produce snapshots of its own state, as well as restore its state from snapshots when needed.
2. The **Memento** is a value object that acts as a snapshot of the originator’s state. It’s a common practice to make the memento immutable and pass it the data only once, via the constructor.
3. The **Caretaker** knows not only “when” and “why” to capture the originator’s state, but also when the state should be restored.
A caretaker can keep track of the originator’s history by storing a stack of mementos. When the originator has to travel back in history, the caretaker fetches the topmost memento from the stack and passes it to the originator’s restoration method.
4. In this implementation, the memento class is nested inside the originator. This lets the originator access the fields and methods of the memento, even though they’re declared private. On the other hand, the caretaker has very limited access to the memento’s fields and methods, which lets it store mementos in a stack but not tamper with their state.

#### Implementation based on an intermediate interface

There’s an alternative implementation, suitable for programming languages that don’t support nested classes (yeah, PHP, I’m talking about you).

1. In the absence of nested classes, you can restrict access to the memento’s fields by establishing a convention that caretakers can work with a memento only through an explicitly declared intermediary interface, which would only declare methods related to the memento’s metadata.
2. On the other hand, originators can work with a memento object directly, accessing fields and methods declared in the memento class. The downside of this approach is that you need to declare all members of the memento public.

#### Implementation with even stricter encapsulation

There’s another implementation which is useful when you don’t want to leave even the slightest chance of other classes accessing the state of the originator through the memento.

1. This implementation allows having multiple types of originators and mementos. Each originator works with a corresponding memento class. Neither originators nor mementos expose their state to anyone.
2. Caretakers are now explicitly restricted from changing the state stored in mementos. Moreover, the caretaker class becomes independent from the originator because the restoration method is now defined in the memento class.
3. Each memento becomes linked to the originator that produced it. The originator passes itself to the memento’s constructor, along with the values of its state. Thanks to the close relationship between these classes, a memento can restore the state of its originator, given that the latter has defined the appropriate setters.

## Pseudocode

This example uses the Memento pattern alongside the [Command](/design-patterns/command) pattern for storing snapshots of the complex text editor’s state and restoring an earlier state from these snapshots when needed.

Saving snapshots of the text editor’s state.

The command objects act as caretakers. They fetch the editor’s memento before executing operations related to commands. When a user attempts to undo the most recent command, the editor can use the memento stored in that command to revert itself to the previous state.

The memento class doesn’t declare any public fields, getters or setters. Therefore no object can alter its contents. Mementos are linked to the editor object that created them. This lets a memento restore the linked editor’s state by passing the data via setters on the editor object. Since mementos are linked to specific editor objects, you can make your app support several independent editor windows with a centralized undo stack.

// The originator holds some important data that may change over
// time. It also defines a method for saving its state inside a
// memento and another method for restoring the state from it.
class Editor is
    private field text, curX, curY, selectionWidth

    method setText(text) is
        this.text = text

    method setCursor(x, y) is
        this.curX = x
        this.curY = y

    method setSelectionWidth(width) is
        this.selectionWidth = width

    // Saves the current state inside a memento.
    method createSnapshot():Snapshot is
        // Memento is an immutable object; that's why the
        // originator passes its state to the memento's
        // constructor parameters.
        return new Snapshot(this, text, curX, curY, selectionWidth)

// The memento class stores the past state of the editor.
class Snapshot is
    private field editor: Editor
    private field text, curX, curY, selectionWidth

    constructor Snapshot(editor, text, curX, curY, selectionWidth) is
        this.editor = editor
        this.text = text
        this.curX = x
        this.curY = y
        this.selectionWidth = selectionWidth

    // At some point, a previous state of the editor can be
    // restored using a memento object.
    method restore() is
        editor.setText(text)
        editor.setCursor(curX, curY)
        editor.setSelectionWidth(selectionWidth)

// A command object can act as a caretaker. In that case, the
// command gets a memento just before it changes the
// originator's state. When undo is requested, it restores the
// originator's state from a memento.
class Command is
    private field backup: Snapshot

    method makeBackup() is
        backup = editor.createSnapshot()

    method undo() is
        if (backup != null)
            backup.restore()
    // ...

## Applicability

Use the Memento pattern when you want to produce snapshots of the object’s state to be able to restore a previous state of the object.

 The Memento pattern lets you make full copies of an object’s state, including private fields, and store them separately from the object. While most people remember this pattern thanks to the “undo” use case, it’s also indispensable when dealing with transactions (i.e., if you need to roll back an operation on error).

 Use the pattern when direct access to the object’s fields/getters/setters violates its encapsulation.

 The Memento makes the object itself responsible for creating a snapshot of its state. No other object can read the snapshot, making the original object’s state data safe and secure.

## How to Implement

1. Determine what class will play the role of the originator. It’s important to know whether the program uses one central object of this type or multiple smaller ones.
2. Create the memento class. One by one, declare a set of fields that mirror the fields declared inside the originator class.
3. Make the memento class immutable. A memento should accept the data just once, via the constructor. The class should have no setters.
4. If your programming language supports nested classes, nest the memento inside the originator. If not, extract a blank interface from the memento class and make all other objects use it to refer to the memento. You may add some metadata operations to the interface, but nothing that exposes the originator’s state.
5. Add a method for producing mementos to the originator class. The originator should pass its state to the memento via one or multiple arguments of the memento’s constructor.
The return type of the method should be of the interface you extracted in the previous step (assuming that you extracted it at all). Under the hood, the memento-producing method should work directly with the memento class.
6. Add a method for restoring the originator’s state to its class. It should accept a memento object as an argument. If you extracted an interface in the previous step, make it the type of the parameter. In this case, you need to typecast the incoming object to the memento class, since the originator needs full access to that object.
7. The caretaker, whether it represents a command object, a history, or something entirely different, should know when to request new mementos from the originator, how to store them and when to restore the originator with a particular memento.
8. The link between caretakers and originators may be moved into the memento class. In this case, each memento must be connected to the originator that had created it. The restoration method would also move to the memento class. However, this would all make sense only if the memento class is nested into originator or the originator class provides sufficient setters for overriding its state.

## Pros

* You can produce snapshots of the object’s state without violating its encapsulation.
* You can simplify the originator’s code by letting the caretaker maintain the history of the originator’s state.

## Cons

* The app might consume lots of RAM if clients create mementos too often.
* Caretakers should track the originator’s lifecycle to be able to destroy obsolete mementos.
* Most dynamic programming languages, such as PHP, Python and JavaScript, can’t guarantee that the state within the memento stays untouched.

## Relations with Other Patterns

* You can use [Command](/design-patterns/command) and [Memento](/design-patterns/memento) together when implementing “undo”. In this case, commands are responsible for performing various operations over a target object, while mementos save the state of that object just before a command gets executed.
* You can use [Memento](/design-patterns/memento) along with [Iterator](/design-patterns/iterator) to capture the current iteration state and roll it back if necessary.
* Sometimes [Prototype](/design-patterns/prototype) can be a simpler alternative to [Memento](/design-patterns/memento). This works if the object, the state of which you want to store in the history, is fairly straightforward and doesn’t have links to external resources, or the links are easy to re-establish.
## Relations

**Related patterns**

- [Command](/en/design-patterns/command.md)
- [Iterator](/en/design-patterns/iterator.md)
- [Prototype](/en/design-patterns/prototype.md)

## Code Examples

### java

```java
package refactoring_guru.memento.example.editor;

import refactoring_guru.memento.example.commands.Command;
import refactoring_guru.memento.example.history.History;
import refactoring_guru.memento.example.history.Memento;
import refactoring_guru.memento.example.shapes.CompoundShape;
import refactoring_guru.memento.example.shapes.Shape;

import javax.swing.*;
import java.io.*;
import java.util.Base64;

public class Editor extends JComponent {
    private Canvas canvas;
    private CompoundShape allShapes = new CompoundShape();
    private History history;

    public Editor() {
        canvas = new Canvas(this);
        history = new History();
    }

    public void loadShapes(Shape... shapes) {
        allShapes.clear();
        allShapes.add(shapes);
        canvas.refresh();
    }

    public CompoundShape getShapes() {
        return allShapes;
    }

    public void execute(Command c) {
        history.push(c, new Memento(this));
        c.execute();
    }

    public void undo() {
        if (history.undo())
            canvas.repaint();
    }

    public void redo() {
        if (history.redo())
            canvas.repaint();
    }

    public String backup() {
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(baos);
            oos.writeObject(this.allShapes);
            oos.close();
            return Base64.getEncoder().encodeToString(baos.toByteArray());
        } catch (IOException e) {
            return "";
        }
    }

    public void restore(String state) {
        try {
            byte[] data = Base64.getDecoder().decode(state);
            ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(data));
            this.allShapes = (CompoundShape) ois.readObject();
            ois.close();
        } catch (ClassNotFoundException e) {
            System.out.print("ClassNotFoundException occurred.");
        } catch (IOException e) {
            System.out.print("IOException occurred.");
        }
    }
}

package refactoring_guru.memento.example.editor;

import refactoring_guru.memento.example.commands.ColorCommand;
import refactoring_guru.memento.example.commands.MoveCommand;
import refactoring_guru.memento.example.shapes.Shape;

import javax.swing.*;
import javax.swing.border.Border;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;

class Canvas extends java.awt.Canvas {
    private Editor editor;
    private JFrame frame;
    private static final int PADDING = 10;

    Canvas(Editor editor) {
        this.editor = editor;
        createFrame();
        attachKeyboardListeners();
        attachMouseListeners();
        refresh();
    }

    private void createFrame() {
        frame = new JFrame();
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);

        JPanel contentPanel = new JPanel();
        Border padding = BorderFactory.createEmptyBorder(PADDING, PADDING, PADDING, PADDING);
        contentPanel.setBorder(padding);
        contentPanel.setLayout(new BoxLayout(contentPanel, BoxLayout.Y_AXIS));
        frame.setContentPane(contentPanel);

        contentPanel.add(new JLabel("Select and drag to move."), BorderLayout.PAGE_END);
        contentPanel.add(new JLabel("Right click to change color."), BorderLayout.PAGE_END);
        contentPanel.add(new JLabel("Undo: Ctrl+Z, Redo: Ctrl+R"), BorderLayout.PAGE_END);
        contentPanel.add(this);
        frame.setVisible(true);
        contentPanel.setBackground(Color.LIGHT_GRAY);
    }

    private void attachKeyboardListeners() {
        addKeyListener(new KeyAdapter() {
            @Override
            public void keyPressed(KeyEvent e) {
                if ((e.getModifiers() & KeyEvent.CTRL_MASK) != 0) {
                    switch (e.getKeyCode()) {
                        case KeyEvent.VK_Z:
                            editor.undo();
                            break;
                        case KeyEvent.VK_R:
                            editor.redo();
                            break;
                    }
                }
            }
        });
    }

    private void attachMouseListeners() {
        MouseAdapter colorizer = new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                if (e.getButton() != MouseEvent.BUTTON3) {
                    return;
                }
                Shape target = editor.getShapes().getChildAt(e.getX(), e.getY());
                if (target != null) {
                    editor.execute(new ColorCommand(editor, new Color((int) (Math.random() * 0x1000000))));
                    repaint();
                }
            }
        };
        addMouseListener(colorizer);

        MouseAdapter selector = new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                if (e.getButton() != MouseEvent.BUTTON1) {
                    return;
                }

                Shape target = editor.getShapes().getChildAt(e.getX(), e.getY());
                boolean ctrl = (e.getModifiers() & ActionEvent.CTRL_MASK) == ActionEvent.CTRL_MASK;

                if (target == null) {
                    if (!ctrl) {
                        editor.getShapes().unSelect();
                    }
                } else {
                    if (ctrl) {
                        if (target.isSelected()) {
                            target.unSelect();
                        } else {
                            target.select();
                        }
                    } else {
                        if (!target.isSelected()) {
                            editor.getShapes().unSelect();
                        }
                        target.select();
                    }
                }
                repaint();
            }
        };
        addMouseListener(selector);


        MouseAdapter dragger = new MouseAdapter() {
            MoveCommand moveCommand;

            @Override
            public void mouseDragged(MouseEvent e) {
                if ((e.getModifiersEx() & MouseEvent.BUTTON1_DOWN_MASK) != MouseEvent.BUTTON1_DOWN_MASK) {
                    return;
                }
                if (moveCommand == null) {
                    moveCommand = new MoveCommand(editor);
                    moveCommand.start(e.getX(), e.getY());
                }
                moveCommand.move(e.getX(), e.getY());
                repaint();
            }

            @Override
            public void mouseReleased(MouseEvent e) {
                if (e.getButton() != MouseEvent.BUTTON1 || moveCommand == null) {
                    return;
                }
                moveCommand.stop(e.getX(), e.getY());
                editor.execute(moveCommand);
                this.moveCommand = null;
                repaint();
            }
        };
        addMouseListener(dragger);
        addMouseMotionListener(dragger);
    }

    public int getWidth() {
        return editor.getShapes().getX() + editor.getShapes().getWidth() + PADDING;
    }

    public int getHeight() {
        return editor.getShapes().getY() + editor.getShapes().getHeight() + PADDING;
    }

    void refresh() {
        this.setSize(getWidth(), getHeight());
        frame.pack();
    }

    public void update(Graphics g) {
        paint(g);
    }

    public void paint(Graphics graphics) {
        BufferedImage buffer = new BufferedImage(this.getWidth(), this.getHeight(), BufferedImage.TYPE_INT_RGB);
        Graphics2D ig2 = buffer.createGraphics();
        ig2.setBackground(Color.WHITE);
        ig2.clearRect(0, 0, this.getWidth(), this.getHeight());

        editor.getShapes().paint(buffer.getGraphics());

        graphics.drawImage(buffer, 0, 0, null);
    }
}

package refactoring_guru.memento.example.history;

import refactoring_guru.memento.example.commands.Command;

import java.util.ArrayList;
import java.util.List;

public class History {
    private List<Pair> history = new ArrayList<Pair>();
    private int virtualSize = 0;

    private class Pair {
        Command command;
        Memento memento;
        Pair(Command c, Memento m) {
            command = c;
            memento = m;
        }

        private Command getCommand() {
            return command;
        }

        private Memento getMemento() {
            return memento;
        }
    }

    public void push(Command c, Memento m) {
        if (virtualSize != history.size() && virtualSize > 0) {
            history = history.subList(0, virtualSize - 1);
        }
        history.add(new Pair(c, m));
        virtualSize = history.size();
    }

    public boolean undo() {
        Pair pair = getUndo();
        if (pair == null) {
            return false;
        }
        System.out.println("Undoing: " + pair.getCommand().getName());
        pair.getMemento().restore();
        return true;
    }

    public boolean redo() {
        Pair pair = getRedo();
        if (pair == null) {
            return false;
        }
        System.out.println("Redoing: " + pair.getCommand().getName());
        pair.getMemento().restore();
        pair.getCommand().execute();
        return true;
    }

    private Pair getUndo() {
        if (virtualSize == 0) {
            return null;
        }
        virtualSize = Math.max(0, virtualSize - 1);
        return history.get(virtualSize);
    }

    private Pair getRedo() {
        if (virtualSize == history.size()) {
            return null;
        }
        virtualSize = Math.min(history.size(), virtualSize + 1);
        return history.get(virtualSize - 1);
    }
}

package refactoring_guru.memento.example.history;

import refactoring_guru.memento.example.editor.Editor;

public class Memento {
    private String backup;
    private Editor editor;

    public Memento(Editor editor) {
        this.editor = editor;
        this.backup = editor.backup();
    }

    public void restore() {
        editor.restore(backup);
    }
}

package refactoring_guru.memento.example.commands;

public interface Command {
    String getName();
    void execute();
}

package refactoring_guru.memento.example.commands;

import refactoring_guru.memento.example.editor.Editor;
import refactoring_guru.memento.example.shapes.Shape;

import java.awt.*;

public class ColorCommand implements Command {
    private Editor editor;
    private Color color;

    public ColorCommand(Editor editor, Color color) {
        this.editor = editor;
        this.color = color;
    }

    @Override
    public String getName() {
        return "Colorize: " + color.toString();
    }

    @Override
    public void execute() {
        for (Shape child : editor.getShapes().getSelected()) {
            child.setColor(color);
        }
    }
}

package refactoring_guru.memento.example.commands;

import refactoring_guru.memento.example.editor.Editor;
import refactoring_guru.memento.example.shapes.Shape;

public class MoveCommand implements Command {
    private Editor editor;
    private int startX, startY;
    private int endX, endY;

    public MoveCommand(Editor editor) {
        this.editor = editor;
    }

    @Override
    public String getName() {
        return "Move by X:" + (endX - startX) + " Y:" + (endY - startY);
    }

    public void start(int x, int y) {
        startX = x;
        startY = y;
        for (Shape child : editor.getShapes().getSelected()) {
            child.drag();
        }
    }

    public void move(int x, int y) {
        for (Shape child : editor.getShapes().getSelected()) {
            child.moveTo(x - startX, y - startY);
        }
    }

    public void stop(int x, int y) {
        endX = x;
        endY = y;
        for (Shape child : editor.getShapes().getSelected()) {
            child.drop();
        }
    }

    @Override
    public void execute() {
        for (Shape child : editor.getShapes().getSelected()) {
            child.moveBy(endX - startX, endY - startY);
        }
    }
}

package refactoring_guru.memento.example.shapes;

import java.awt.*;
import java.io.Serializable;

public interface Shape extends Serializable {
    int getX();
    int getY();
    int getWidth();
    int getHeight();
    void drag();
    void drop();
    void moveTo(int x, int y);
    void moveBy(int x, int y);
    boolean isInsideBounds(int x, int y);
    Color getColor();
    void setColor(Color color);
    void select();
    void unSelect();
    boolean isSelected();
    void paint(Graphics graphics);
}

package refactoring_guru.memento.example.shapes;

import java.awt.*;

public abstract class BaseShape implements Shape {
    int x, y;
    private int dx = 0, dy = 0;
    private Color color;
    private boolean selected = false;

    BaseShape(int x, int y, Color color) {
        this.x = x;
        this.y = y;
        this.color = color;
    }

    @Override
    public int getX() {
        return x;
    }

    @Override
    public int getY() {
        return y;
    }

    @Override
    public int getWidth() {
        return 0;
    }

    @Override
    public int getHeight() {
        return 0;
    }

    @Override
    public void drag() {
        dx = x;
        dy = y;
    }

    @Override
    public void moveTo(int x, int y) {
        this.x = dx + x;
        this.y = dy + y;
    }

    @Override
    public void moveBy(int x, int y) {
        this.x += x;
        this.y += y;
    }

    @Override
    public void drop() {
        this.x = dx;
        this.y = dy;
    }

    @Override
    public boolean isInsideBounds(int x, int y) {
        return x > getX() && x < (getX() + getWidth()) &&
                y > getY() && y < (getY() + getHeight());
    }

    @Override
    public Color getColor() {
        return color;
    }

    @Override
    public void setColor(Color color) {
        this.color = color;
    }

    @Override
    public void select() {
        selected = true;
    }

    @Override
    public void unSelect() {
        selected = false;
    }

    @Override
    public boolean isSelected() {
        return selected;
    }

    void enableSelectionStyle(Graphics graphics) {
        graphics.setColor(Color.LIGHT_GRAY);

        Graphics2D g2 = (Graphics2D) graphics;
        float[] dash1 = {2.0f};
        g2.setStroke(new BasicStroke(1.0f,
                BasicStroke.CAP_BUTT,
                BasicStroke.JOIN_MITER,
                2.0f, dash1, 0.0f));
    }

    void disableSelectionStyle(Graphics graphics) {
        graphics.setColor(color);
        Graphics2D g2 = (Graphics2D) graphics;
        g2.setStroke(new BasicStroke());
    }

    @Override
    public void paint(Graphics graphics) {
        if (isSelected()) {
            enableSelectionStyle(graphics);
        }
        else {
            disableSelectionStyle(graphics);
        }

        // ...
    }
}

package refactoring_guru.memento.example.shapes;

import java.awt.*;

public class Circle extends BaseShape {
    private int radius;

    public Circle(int x, int y, int radius, Color color) {
        super(x, y, color);
        this.radius = radius;
    }

    @Override
    public int getWidth() {
        return radius * 2;
    }

    @Override
    public int getHeight() {
        return radius * 2;
    }

    @Override
    public void paint(Graphics graphics) {
        super.paint(graphics);
        graphics.drawOval(x, y, getWidth() - 1, getHeight() - 1);
    }
}

package refactoring_guru.memento.example.shapes;

import java.awt.*;

public class Dot extends BaseShape {
    private final int DOT_SIZE = 3;

    public Dot(int x, int y, Color color) {
        super(x, y, color);
    }

    @Override
    public int getWidth() {
        return DOT_SIZE;
    }

    @Override
    public int getHeight() {
        return DOT_SIZE;
    }

    @Override
    public void paint(Graphics graphics) {
        super.paint(graphics);
        graphics.fillRect(x - 1, y - 1, getWidth(), getHeight());
    }
}

package refactoring_guru.memento.example.shapes;

import java.awt.*;

public class Rectangle extends BaseShape {
    private int width;
    private int height;

    public Rectangle(int x, int y, int width, int height, Color color) {
        super(x, y, color);
        this.width = width;
        this.height = height;
    }

    @Override
    public int getWidth() {
        return width;
    }

    @Override
    public int getHeight() {
        return height;
    }

    @Override
    public void paint(Graphics graphics) {
        super.paint(graphics);
        graphics.drawRect(x, y, getWidth() - 1, getHeight() - 1);
    }
}

package refactoring_guru.memento.example.shapes;

import java.awt.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class CompoundShape extends BaseShape {
    private List<Shape> children = new ArrayList<>();

    public CompoundShape(Shape... components) {
        super(0, 0, Color.BLACK);
        add(components);
    }

    public void add(Shape component) {
        children.add(component);
    }

    public void add(Shape... components) {
        children.addAll(Arrays.asList(components));
    }

    public void remove(Shape child) {
        children.remove(child);
    }

    public void remove(Shape... components) {
        children.removeAll(Arrays.asList(components));
    }

    public void clear() {
        children.clear();
    }

    @Override
    public int getX() {
        if (children.size() == 0) {
            return 0;
        }
        int x = children.get(0).getX();
        for (Shape child : children) {
            if (child.getX() < x) {
                x = child.getX();
            }
        }
        return x;
    }

    @Override
    public int getY() {
        if (children.size() == 0) {
            return 0;
        }
        int y = children.get(0).getY();
        for (Shape child : children) {
            if (child.getY() < y) {
                y = child.getY();
            }
        }
        return y;
    }

    @Override
    public int getWidth() {
        int maxWidth = 0;
        int x = getX();
        for (Shape child : children) {
            int childsRelativeX = child.getX() - x;
            int childWidth = childsRelativeX + child.getWidth();
            if (childWidth > maxWidth) {
                maxWidth = childWidth;
            }
        }
        return maxWidth;
    }

    @Override
    public int getHeight() {
        int maxHeight = 0;
        int y = getY();
        for (Shape child : children) {
            int childsRelativeY = child.getY() - y;
            int childHeight = childsRelativeY + child.getHeight();
            if (childHeight > maxHeight) {
                maxHeight = childHeight;
            }
        }
        return maxHeight;
    }

    @Override
    public void drag() {
        for (Shape child : children) {
            child.drag();
        }
    }

    @Override
    public void drop() {
        for (Shape child : children) {
            child.drop();
        }
    }

    @Override
    public void moveTo(int x, int y) {
        for (Shape child : children) {
            child.moveTo(x, y);
        }
    }

    @Override
    public void moveBy(int x, int y) {
        for (Shape child : children) {
            child.moveBy(x, y);
        }
    }

    @Override
    public boolean isInsideBounds(int x, int y) {
        for (Shape child : children) {
            if (child.isInsideBounds(x, y)) {
                return true;
            }
        }
        return false;
    }

    @Override
    public void setColor(Color color) {
        super.setColor(color);
        for (Shape child : children) {
            child.setColor(color);
        }
    }

    @Override
    public void unSelect() {
        super.unSelect();
        for (Shape child : children) {
            child.unSelect();
        }
    }

    public Shape getChildAt(int x, int y) {
        for (Shape child : children) {
            if (child.isInsideBounds(x, y)) {
                return child;
            }
        }
        return null;
    }

    public boolean selectChildAt(int x, int y) {
        Shape child = getChildAt(x,y);
        if (child != null) {
            child.select();
            return true;
        }
        return false;
    }

    public List<Shape> getSelected() {
        List<Shape> selected = new ArrayList<>();
        for (Shape child : children) {
            if (child.isSelected()) {
                selected.add(child);
            }
        }
        return selected;
    }

    @Override
    public void paint(Graphics graphics) {
        if (isSelected()) {
            enableSelectionStyle(graphics);
            graphics.drawRect(getX() - 1, getY() - 1, getWidth() + 1, getHeight() + 1);
            disableSelectionStyle(graphics);
        }

        for (Shape child : children) {
            child.paint(graphics);
        }
    }
}

package refactoring_guru.memento.example;

import refactoring_guru.memento.example.editor.Editor;
import refactoring_guru.memento.example.shapes.Circle;
import refactoring_guru.memento.example.shapes.CompoundShape;
import refactoring_guru.memento.example.shapes.Dot;
import refactoring_guru.memento.example.shapes.Rectangle;

import java.awt.*;

public class Demo {
    public static void main(String[] args) {
        Editor editor = new Editor();
        editor.loadShapes(
                new Circle(10, 10, 10, Color.BLUE),

                new CompoundShape(
                        new Circle(110, 110, 50, Color.RED),
                        new Dot(160, 160, Color.RED)
                ),

                new CompoundShape(
                        new Rectangle(250, 250, 100, 100, Color.GREEN),
                        new Dot(240, 240, Color.GREEN),
                        new Dot(240, 360, Color.GREEN),
                        new Dot(360, 360, Color.GREEN),
                        new Dot(360, 240, Color.GREEN)
                )
        );
    }
}
```

### csharp

```csharp
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;

namespace RefactoringGuru.DesignPatterns.Memento.Conceptual
{
    // The Originator holds some important state that may change over time. It
    // also defines a method for saving the state inside a memento and another
    // method for restoring the state from it.
    class Originator
    {
        // For the sake of simplicity, the originator's state is stored inside a
        // single variable.
        private string _state;

        public Originator(string state)
        {
            this._state = state;
            Console.WriteLine("Originator: My initial state is: " + state);
        }

        // The Originator's business logic may affect its internal state.
        // Therefore, the client should backup the state before launching
        // methods of the business logic via the save() method.
        public void DoSomething()
        {
            Console.WriteLine("Originator: I'm doing something important.");
            this._state = this.GenerateRandomString(30);
            Console.WriteLine($"Originator: and my state has changed to: {_state}");
        }

        private string GenerateRandomString(int length = 10)
        {
            string allowedSymbols = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
            string result = string.Empty;

            while (length > 0)
            {
                result += allowedSymbols[new Random().Next(0, allowedSymbols.Length)];

                Thread.Sleep(12);

                length--;
            }

            return result;
        }

        // Saves the current state inside a memento.
        public IMemento Save()
        {
            return new ConcreteMemento(this._state);
        }

        // Restores the Originator's state from a memento object.
        public void Restore(IMemento memento)
        {
            if (!(memento is ConcreteMemento))
            {
                throw new Exception("Unknown memento class " + memento.ToString());
            }

            this._state = memento.GetState();
            Console.Write($"Originator: My state has changed to: {_state}");
        }
    }

    // The Memento interface provides a way to retrieve the memento's metadata,
    // such as creation date or name. However, it doesn't expose the
    // Originator's state.
    public interface IMemento
    {
        string GetName();

        string GetState();

        DateTime GetDate();
    }

    // The Concrete Memento contains the infrastructure for storing the
    // Originator's state.
    class ConcreteMemento : IMemento
    {
        private string _state;

        private DateTime _date;

        public ConcreteMemento(string state)
        {
            this._state = state;
            this._date = DateTime.Now;
        }

        // The Originator uses this method when restoring its state.
        public string GetState()
        {
            return this._state;
        }
        
        // The rest of the methods are used by the Caretaker to display
        // metadata.
        public string GetName()
        {
            return $"{this._date} / ({this._state.Substring(0, 9)})...";
        }

        public DateTime GetDate()
        {
            return this._date;
        }
    }

    // The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
    // doesn't have access to the originator's state, stored inside the memento.
    // It works with all mementos via the base Memento interface.
    class Caretaker
    {
        private List<IMemento> _mementos = new List<IMemento>();

        private Originator _originator = null;

        public Caretaker(Originator originator)
        {
            this._originator = originator;
        }

        public void Backup()
        {
            Console.WriteLine("\nCaretaker: Saving Originator's state...");
            this._mementos.Add(this._originator.Save());
        }

        public void Undo()
        {
            if (this._mementos.Count == 0)
            {
                return;
            }

            var memento = this._mementos.Last();
            this._mementos.Remove(memento);

            Console.WriteLine("Caretaker: Restoring state to: " + memento.GetName());

            try
            {
                this._originator.Restore(memento);
            }
            catch (Exception)
            {
                this.Undo();
            }
        }

        public void ShowHistory()
        {
            Console.WriteLine("Caretaker: Here's the list of mementos:");

            foreach (var memento in this._mementos)
            {
                Console.WriteLine(memento.GetName());
            }
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            // Client code.
            Originator originator = new Originator("Super-duper-super-puper-super.");
            Caretaker caretaker = new Caretaker(originator);

            caretaker.Backup();
            originator.DoSomething();

            caretaker.Backup();
            originator.DoSomething();

            caretaker.Backup();
            originator.DoSomething();

            Console.WriteLine();
            caretaker.ShowHistory();

            Console.WriteLine("\nClient: Now, let's rollback!\n");
            caretaker.Undo();

            Console.WriteLine("\n\nClient: Once more!\n");
            caretaker.Undo();

            Console.WriteLine();
        }
    }
}

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: oGyQIIatlDDWNgYYqJATTmdwnnGZQj

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: jBtMDDWogzzRJbTTmEwOOhZrjjBULe

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: exoHyyRkbuuNEXOhhArKccUmexPPHZ

Caretaker: Here's the list of mementos:
12.06.2018 15:52:45 / (Super-dup...)
12.06.2018 15:52:46 / (oGyQIIatl...)
12.06.2018 15:52:46 / (jBtMDDWog...)

Client: Now, let's rollback!

Caretaker: Restoring state to: 12.06.2018 15:52:46 / (jBtMDDWog...)
Originator: My state has changed to: jBtMDDWogzzRJbTTmEwOOhZrjjBULe

Client: Once more!

Caretaker: Restoring state to: 12.06.2018 15:52:46 / (oGyQIIatl...)
Originator: My state has changed to: oGyQIIatlDDWNgYYqJATTmdwnnGZQj
```

### cpp

```cpp
/**
 * The Memento interface provides a way to retrieve the memento's metadata, such
 * as creation date or name. However, it doesn't expose the Originator's state.
 */
class Memento {
 public:
  virtual ~Memento() {}
  virtual std::string GetName() const = 0;
  virtual std::string date() const = 0;
  virtual std::string state() const = 0;
};

/**
 * The Concrete Memento contains the infrastructure for storing the Originator's
 * state.
 */
class ConcreteMemento : public Memento {
 private:
  std::string state_;
  std::string date_;

 public:
  ConcreteMemento(std::string state) : state_(state) {
    this->state_ = state;
    std::time_t now = std::time(0);
    this->date_ = std::ctime(&now);
  }
  /**
   * The Originator uses this method when restoring its state.
   */
  std::string state() const override {
    return this->state_;
  }
  /**
   * The rest of the methods are used by the Caretaker to display metadata.
   */
  std::string GetName() const override {
    return this->date_ + " / (" + this->state_.substr(0, 9) + "...)";
  }
  std::string date() const override {
    return this->date_;
  }
};

/**
 * The Originator holds some important state that may change over time. It also
 * defines a method for saving the state inside a memento and another method for
 * restoring the state from it.
 */
class Originator {
  /**
   * @var string For the sake of simplicity, the originator's state is stored
   * inside a single variable.
   */
 private:
  std::string state_;

  std::string GenerateRandomString(int length = 10) {
    const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";
    int stringLength = sizeof(alphanum) - 1;

    std::string random_string;
    for (int i = 0; i < length; i++) {
      random_string += alphanum[std::rand() % stringLength];
    }
    return random_string;
  }

 public:
  Originator(std::string state) : state_(state) {
    std::cout << "Originator: My initial state is: " << this->state_ << "\n";
  }
  /**
   * The Originator's business logic may affect its internal state. Therefore,
   * the client should backup the state before launching methods of the business
   * logic via the save() method.
   */
  void DoSomething() {
    std::cout << "Originator: I'm doing something important.\n";
    this->state_ = this->GenerateRandomString(30);
    std::cout << "Originator: and my state has changed to: " << this->state_ << "\n";
  }

  /**
   * Saves the current state inside a memento.
   */
  Memento *Save() {
    return new ConcreteMemento(this->state_);
  }
  /**
   * Restores the Originator's state from a memento object.
   */
  void Restore(Memento *memento) {
    this->state_ = memento->state();
    std::cout << "Originator: My state has changed to: " << this->state_ << "\n";
    delete memento;
  }
};

/**
 * The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
 * doesn't have access to the originator's state, stored inside the memento. It
 * works with all mementos via the base Memento interface.
 */
class Caretaker {
  /**
   * @var Memento[]
   */
 private:
  std::vector<Memento *> mementos_;

  /**
   * @var Originator
   */
  Originator *originator_;

 public:
     Caretaker(Originator* originator) : originator_(originator) {
     }

     ~Caretaker() {
         for (auto m : mementos_) delete m;
     }

  void Backup() {
    std::cout << "\nCaretaker: Saving Originator's state...\n";
    this->mementos_.push_back(this->originator_->Save());
  }
  void Undo() {
    if (!this->mementos_.size()) {
      return;
    }
    Memento *memento = this->mementos_.back();
    this->mementos_.pop_back();
    std::cout << "Caretaker: Restoring state to: " << memento->GetName() << "\n";
    try {
      this->originator_->Restore(memento);
    } catch (...) {
      this->Undo();
    }
  }
  void ShowHistory() const {
    std::cout << "Caretaker: Here's the list of mementos:\n";
    for (Memento *memento : this->mementos_) {
      std::cout << memento->GetName() << "\n";
    }
  }
};
/**
 * Client code.
 */

void ClientCode() {
  Originator *originator = new Originator("Super-duper-super-puper-super.");
  Caretaker *caretaker = new Caretaker(originator);
  caretaker->Backup();
  originator->DoSomething();
  caretaker->Backup();
  originator->DoSomething();
  caretaker->Backup();
  originator->DoSomething();
  std::cout << "\n";
  caretaker->ShowHistory();
  std::cout << "\nClient: Now, let's rollback!\n\n";
  caretaker->Undo();
  std::cout << "\nClient: Once more!\n\n";
  caretaker->Undo();

  delete originator;
  delete caretaker;
}

int main() {
  std::srand(static_cast<unsigned int>(std::time(NULL)));
  ClientCode();
  return 0;
}

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: uOInE8wmckHYPwZS7PtUTwuwZfCIbz

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: te6RGmykRpbqaWo5MEwjji1fpM1t5D

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: hX5xWDVljcQ9ydD7StUfbBt5Z7pcSN

Caretaker: Here's the list of mementos:
Sat Oct 19 18:09:37 2019
 / (Super-dup...)
Sat Oct 19 18:09:37 2019
 / (uOInE8wmc...)
Sat Oct 19 18:09:37 2019
 / (te6RGmykR...)

Client: Now, let's rollback!

Caretaker: Restoring state to: Sat Oct 19 18:09:37 2019
 / (te6RGmykR...)
Originator: My state has changed to: te6RGmykRpbqaWo5MEwjji1fpM1t5D

Client: Once more!

Caretaker: Restoring state to: Sat Oct 19 18:09:37 2019
 / (uOInE8wmc...)
Originator: My state has changed to: uOInE8wmckHYPwZS7PtUTwuwZfCIbz
```

### go

```go
package main

type Originator struct {
	state string
}

func (e *Originator) createMemento() *Memento {
	return &Memento{state: e.state}
}

func (e *Originator) restoreMemento(m *Memento) {
	e.state = m.getSavedState()
}

func (e *Originator) setState(state string) {
	e.state = state
}

func (e *Originator) getState() string {
	return e.state
}

package main

type Memento struct {
	state string
}

func (m *Memento) getSavedState() string {
	return m.state
}

package main

type Caretaker struct {
	mementoArray []*Memento
}

func (c *Caretaker) addMemento(m *Memento) {
	c.mementoArray = append(c.mementoArray, m)
}

func (c *Caretaker) getMemento(index int) *Memento {
	return c.mementoArray[index]
}

package main

import "fmt"

func main() {

	caretaker := &Caretaker{
		mementoArray: make([]*Memento, 0),
	}

	originator := &Originator{
		state: "A",
	}

	fmt.Printf("Originator Current State: %s\n", originator.getState())
	caretaker.addMemento(originator.createMemento())

	originator.setState("B")
	fmt.Printf("Originator Current State: %s\n", originator.getState())
	caretaker.addMemento(originator.createMemento())

	originator.setState("C")
	fmt.Printf("Originator Current State: %s\n", originator.getState())
	caretaker.addMemento(originator.createMemento())

	originator.restoreMemento(caretaker.getMemento(1))
	fmt.Printf("Restored to State: %s\n", originator.getState())

	originator.restoreMemento(caretaker.getMemento(0))
	fmt.Printf("Restored to State: %s\n", originator.getState())

}

originator Current State: A
originator Current State: B
originator Current State: C
Restored to State: B
Restored to State: A
```

### php

```php
<?php

namespace RefactoringGuru\Memento\Conceptual;

/**
 * The Originator holds some important state that may change over time. It also
 * defines a method for saving the state inside a memento and another method for
 * restoring the state from it.
 */
class Originator
{
    /**
     * @var string For the sake of simplicity, the originator's state is stored
     * inside a single variable.
     */
    private $state;

    public function __construct(string $state)
    {
        $this->state = $state;
        echo "Originator: My initial state is: {$this->state}\n";
    }

    /**
     * The Originator's business logic may affect its internal state. Therefore,
     * the client should backup the state before launching methods of the
     * business logic via the save() method.
     */
    public function doSomething(): void
    {
        echo "Originator: I'm doing something important.\n";
        $this->state = $this->generateRandomString(30);
        echo "Originator: and my state has changed to: {$this->state}\n";
    }

    private function generateRandomString(int $length = 10): string
    {
        return substr(
            str_shuffle(
                str_repeat(
                    $x = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',
                    ceil($length / strlen($x))
                )
            ),
            1,
            $length,
        );
    }

    /**
     * Saves the current state inside a memento.
     */
    public function save(): Memento
    {
        return new ConcreteMemento($this->state);
    }

    /**
     * Restores the Originator's state from a memento object.
     */
    public function restore(Memento $memento): void
    {
        $this->state = $memento->getState();
        echo "Originator: My state has changed to: {$this->state}\n";
    }
}

/**
 * The Memento interface provides a way to retrieve the memento's metadata, such
 * as creation date or name. However, it doesn't expose the Originator's state.
 */
interface Memento
{
    public function getName(): string;

    public function getDate(): string;
}

/**
 * The Concrete Memento contains the infrastructure for storing the Originator's
 * state.
 */
class ConcreteMemento implements Memento
{
    private $state;

    private $date;

    public function __construct(string $state)
    {
        $this->state = $state;
        $this->date = date('Y-m-d H:i:s');
    }

    /**
     * The Originator uses this method when restoring its state.
     */
    public function getState(): string
    {
        return $this->state;
    }

    /**
     * The rest of the methods are used by the Caretaker to display metadata.
     */
    public function getName(): string
    {
        return $this->date . " / (" . substr($this->state, 0, 9) . "...)";
    }

    public function getDate(): string
    {
        return $this->date;
    }
}

/**
 * The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
 * doesn't have access to the originator's state, stored inside the memento. It
 * works with all mementos via the base Memento interface.
 */
class Caretaker
{
    /**
     * @var Memento[]
     */
    private $mementos = [];

    /**
     * @var Originator
     */
    private $originator;

    public function __construct(Originator $originator)
    {
        $this->originator = $originator;
    }

    public function backup(): void
    {
        echo "\nCaretaker: Saving Originator's state...\n";
        $this->mementos[] = $this->originator->save();
    }

    public function undo(): void
    {
        if (!count($this->mementos)) {
            return;
        }
        $memento = array_pop($this->mementos);

        echo "Caretaker: Restoring state to: " . $memento->getName() . "\n";
        try {
            $this->originator->restore($memento);
        } catch (\Exception $e) {
            $this->undo();
        }
    }

    public function showHistory(): void
    {
        echo "Caretaker: Here's the list of mementos:\n";
        foreach ($this->mementos as $memento) {
            echo $memento->getName() . "\n";
        }
    }
}

/**
 * Client code.
 */
$originator = new Originator("Super-duper-super-puper-super.");
$caretaker = new Caretaker($originator);

$caretaker->backup();
$originator->doSomething();

$caretaker->backup();
$originator->doSomething();

$caretaker->backup();
$originator->doSomething();

echo "\n";
$caretaker->showHistory();

echo "\nClient: Now, let's rollback!\n\n";
$caretaker->undo();

echo "\nClient: Once more!\n\n";
$caretaker->undo();

Originator: My initial state is: Super-duper-super-puper-super. 

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: srGIngezAEboNPDjBkuvymJKUtMSFX

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: UwCZQaHJOiERLlchyVuMbXNtpqTgWF

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: incqsdoJXkbDUuVOvRFYyKBgfzwZCQ

Caretaker: Here's the list of mementos:
2018-06-04 14:50:39 / (Super-dup...)
2018-06-04 14:50:39 / (srGIngezA...)
2018-06-04 14:50:39 / (UwCZQaHJO...)

Client: Now, let's rollback!

Caretaker: Restoring state to: 2018-06-04 14:50:39 / (UwCZQaHJO...)
Originator: My state has changed to: UwCZQaHJOiERLlchyVuMbXNtpqTgWF

Client: Once more!

Caretaker: Restoring state to: 2018-06-04 14:50:39 / (srGIngezA...)
Originator: My state has changed to: srGIngezAEboNPDjBkuvymJKUtMSFX

<?php

namespace RefactoringGuru\Memento\RealWorld;

/**
 * The Originator holds some important state that may change over time. It also
 * defines a method for saving the state inside a memento and another method for
 * restoring the state from it.
 *
 * In this example, the ConfigManager acts as the Originator. It manages
 * application configuration settings that can be modified during runtime. The
 * configuration might include database settings, feature flags, UI themes, SEO
 * settings, and other application parameters.
 *
 * The ConfigManager's business logic may affect its internal state. Therefore,
 * the client should backup the state before launching methods of the business
 * logic via the save() method.
 */
class ConfigManager
{
    /**
     * @var array For simplicity, the configuration state is stored inside an
     * array. In real applications, this might be a more complex structure with
     * validation, type checking, and nested configurations.
     */
    private $config;

    /**
     * Constructor initializes the configuration manager with default settings.
     *
     * @param array $initialConfig The initial configuration values
     */
    public function __construct(array $initialConfig)
    {
        $this->config = $initialConfig;
        echo "ConfigManager: Initialized with " . count($initialConfig) . " config items.\n";
    }

    /**
     * The ConfigManager's business logic may affect its internal state.
     * Therefore, the client should backup the state before launching methods of
     * the business logic via the save() method.
     *
     * This method simulates updating configuration values, which is a common
     * operation in web applications (admin panels, user preferences, etc.).
     *
     * @param array $newValues New configuration values to merge with existing
     * config
     */
    public function updateConfig(array $newValues): void
    {
        echo "ConfigManager: Updating configuration with new values...\n";
        $this->config = array_merge($this->config, $newValues);
        echo "ConfigManager: Configuration updated. Current config has " . count($this->config) . " items.\n";
    }

    /**
     * Retrieves the current configuration state.
     *
     * @return array The current configuration
     */
    public function getConfig(): array
    {
        return $this->config;
    }

    /**
     * Saves the current state inside a memento.
     *
     * This method creates a snapshot of the current configuration state and
     * returns it wrapped in a memento object. The memento contains all the
     * information needed to restore the configuration to its current state
     * later.
     *
     * @return ConfigSnapshot A memento containing the current config state
     */
    public function save(): ConfigSnapshot
    {
        echo "ConfigManager: Saving current configuration state...\n";
        return new ConfigSnapshot($this->config);
    }

    /**
     * Restores the ConfigManager's state from a memento object.
     *
     * This method takes a memento and restores the configuration to the state
     * that was saved in that memento. This is useful for implementing undo
     * functionality or rolling back failed configuration changes.
     *
     * @param ConfigSnapshot $snapshot The memento to restore from
     */
    public function restore(ConfigSnapshot $snapshot): void
    {
        $this->config = $snapshot->getState();
        echo "ConfigManager: Configuration restored from snapshot.\n";
    }

    /**
     * Additional business method: Reset to defaults
     *
     * Resets the configuration to default values. This is another operation
     * that might benefit from creating a backup before execution.
     */
    public function resetToDefaults(): void
    {
        echo "ConfigManager: Resetting configuration to defaults...\n";
        $this->config = [
            'maintenance_mode' => false,
            'theme' => 'light',
            'debug' => false
        ];
        echo "ConfigManager: Configuration reset to defaults.\n";
    }

    /**
     * Additional business method: Enable maintenance mode
     *
     * Quickly enables maintenance mode across the application.
     */
    public function enableMaintenanceMode(): void
    {
        echo "ConfigManager: Enabling maintenance mode...\n";
        $this->config['maintenance_mode'] = true;
        $this->config['maintenance_message'] = 'System under maintenance. Please try again later.';
        echo "ConfigManager: Maintenance mode enabled.\n";
    }
}

/**
 * The Memento interface provides a way to retrieve the memento's metadata, such
 * as creation date or name. However, it doesn't expose the Originator's state.
 *
 * This interface ensures that external classes can work with mementos without
 * having direct access to the stored state. Only the originator should be able
 * to extract the actual state data.
 */
interface ConfigMemento
{
    /**
     * Returns a user-friendly name for this memento.
     *
     * @return string A descriptive name for this snapshot
     */
    public function getName(): string;

    /**
     * Returns the date when this memento was created.
     *
     * @return string The creation timestamp
     */
    public function getDate(): string;
}

/**
 * The Concrete Memento contains the infrastructure for storing the Originator's
 * state.
 *
 * This class stores a snapshot of the configuration state along with metadata
 * about when the snapshot was created. The actual state is stored privately and
 * can only be accessed by the originator through the getState() method.
 */
class ConfigSnapshot implements ConfigMemento
{
    /**
     * @var array The configuration state at the time this snapshot was created
     */
    private $state;

    /**
     * @var string The timestamp when this snapshot was created
     */
    private $date;

    /**
     * Constructor stores the provided state and records the current timestamp.
     *
     * @param array $state The configuration state to store
     */
    public function __construct(array $state)
    {
        $this->state = $state;
        $this->date = date('Y-m-d H:i:s');
        echo "ConfigSnapshot: Created snapshot with " . count($state) . " config items.\n";
    }

    /**
     * The Originator uses this method when restoring its state.
     *
     * This method provides access to the stored state data. It should only be
     * called by the ConfigManager when restoring configuration.
     *
     * @return array The stored configuration state
     */
    public function getState(): array
    {
        return $this->state;
    }

    /**
     * The rest of the methods are used by the Caretaker to display metadata.
     *
     * Returns a descriptive name that includes the timestamp and a preview of
     * the configuration content.
     *
     * @return string A user-friendly name for this snapshot
     */
    public function getName(): string
    {
        $configCount = count($this->state);
        $maintenanceStatus = $this->state['maintenance_mode'] ?? 'unknown';
        return $this->date . " / ({$configCount} items, maintenance: {$maintenanceStatus})";
    }

    /**
     * Returns the creation date of this snapshot.
     *
     * @return string The timestamp when this snapshot was created
     */
    public function getDate(): string
    {
        return $this->date;
    }
}

/**
 * The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
 * doesn't have access to the originator's state, stored inside the memento. It
 * works with all mementos via the base Memento interface.
 *
 * The ConfigHistory class manages a collection of configuration snapshots and
 * provides undo functionality. It demonstrates how the caretaker can manage
 * mementos without knowing their internal structure.
 */
class ConfigHistory
{
    /**
     * @var ConfigSnapshot[] Array of stored configuration snapshots
     */
    private $snapshots = [];

    /**
     * @var ConfigManager Reference to the configuration manager
     */
    private $configManager;

    /**
     * Constructor establishes the relationship with the originator.
     *
     * @param ConfigManager $configManager The configuration manager to work
     * with
     */
    public function __construct(ConfigManager $configManager)
    {
        $this->configManager = $configManager;
        echo "ConfigHistory: History manager initialized.\n";
    }

    /**
     * Creates a backup of the current configuration state.
     *
     * This method asks the originator to create a memento and stores it in the
     * history. This should be called before making changes that might need to
     * be undone.
     */
    public function backup(): void
    {
        echo "\nConfigHistory: Creating backup of current configuration...\n";
        $this->snapshots[] = $this->configManager->save();
        echo "ConfigHistory: Backup created. Total backups: " . count($this->snapshots) . "\n";
    }

    /**
     * Restores the configuration to the most recent backup.
     *
     * This method retrieves the most recent memento from the history and asks
     * the originator to restore its state from that memento.
     */
    public function undo(): void
    {
        if (!count($this->snapshots)) {
            echo "ConfigHistory: No backups available for undo.\n";
            return;
        }

        $memento = array_pop($this->snapshots);

        echo "ConfigHistory: Restoring configuration to: " . $memento->getName() . "\n";
        try {
            $this->configManager->restore($memento);
            echo "ConfigHistory: Undo completed successfully.\n";
        } catch (\Exception $e) {
            echo "ConfigHistory: Undo failed, trying previous backup...\n";
            $this->undo();
        }
    }

    /**
     * Displays the history of all saved configuration snapshots.
     *
     * This method shows all available backups using only the information
     * available through the memento interface, without accessing the actual
     * configuration data.
     */
    public function showHistory(): void
    {
        echo "\nConfigHistory: Available configuration backups:\n";
        if (empty($this->snapshots)) {
            echo "No backups available.\n";
        } else {
            foreach ($this->snapshots as $index => $memento) {
                echo "[{$index}] " . $memento->getName() . "\n";
            }
        }
        echo "\n";
    }

    /**
     * Clears all stored backups.
     *
     * This method removes all mementos from the history, which might be useful
     * when starting fresh or to free up memory.
     */
    public function clearHistory(): void
    {
        $count = count($this->snapshots);
        $this->snapshots = [];
        echo "ConfigHistory: Cleared {$count} backups from history.\n";
    }

    /**
     * Gets the number of available backups.
     *
     * @return int The number of stored backups
     */
    public function getBackupCount(): int
    {
        return count($this->snapshots);
    }
}



/**
 * ============================================================================
 * CLIENT CODE - DEMONSTRATION AND USAGE EXAMPLES
 * ============================================================================
 */

echo "=== Configuration Manager with Memento Pattern Demo ===\n\n";

/**
 * Example 1: Basic configuration management with backup/restore
 */
echo "--- Example 1: Basic Configuration Management ---\n";

// Initialize configuration manager with default settings
$config = new ConfigManager([
    'maintenance_mode' => false,
    'theme' => 'light',
    'seo' => ['title' => 'My Website', 'description' => 'Welcome to my site!'],
    'debug' => false,
    'max_users' => 1000
]);

// Create history manager
$history = new ConfigHistory($config);

echo "\nInitial configuration:\n";
print_r($config->getConfig());

/**
 * Example 2: Making changes with backups
 */
echo "\n--- Example 2: Making Changes with Backups ---\n";

// Create backup before making changes
$history->backup();

// Update theme settings
$config->updateConfig([
    'theme' => 'dark',
    'theme_options' => ['sidebar' => 'collapsed', 'font_size' => 'large']
]);

echo "\nAfter theme update:\n";
print_r($config->getConfig());

// Create another backup
$history->backup();

// Enable maintenance mode
$config->enableMaintenanceMode();

echo "\nAfter enabling maintenance mode:\n";
print_r($config->getConfig());

/**
 * Example 3: Demonstrating undo functionality
 */
echo "\n--- Example 3: Undo Functionality ---\n";

// Show current history
$history->showHistory();

// Undo last change (maintenance mode)
echo "Undoing maintenance mode activation...\n";
$history->undo();

echo "\nAfter first undo:\n";
print_r($config->getConfig());

// Undo theme changes
echo "\nUndoing theme changes...\n";
$history->undo();

echo "\nAfter second undo (back to original):\n";
print_r($config->getConfig());

/**
 * Example 4: Multiple configuration scenarios
 */
echo "\n--- Example 4: Multiple Configuration Scenarios ---\n";

// Scenario A: SEO Configuration
$history->backup();
echo "\nScenario A: Updating SEO settings...\n";
$config->updateConfig([
    'seo' => [
        'title' => 'Best Products Online',
        'description' => 'Find the best products at great prices!',
        'keywords' => 'products, online, shopping, deals'
    ],
    'analytics' => ['google_id' => 'GA-123456', 'facebook_pixel' => 'FB-789012']
]);

echo "SEO configuration updated:\n";
print_r($config->getConfig());

// Scenario B: Performance Settings
$history->backup();
echo "\nScenario B: Updating performance settings...\n";
$config->updateConfig([
    'cache_enabled' => true,
    'cache_duration' => 3600,
    'compression' => 'gzip',
    'max_users' => 2000
]);

echo "Performance settings updated:\n";
print_r($config->getConfig());

// Scenario C: Emergency rollback
echo "\nScenario C: Emergency rollback to SEO-only changes...\n";
$history->undo(); // Remove performance changes
echo "Rolled back performance changes:\n";
print_r($config->getConfig());

/**
 * Example 5: Reset and recovery
 */
echo "\n--- Example 5: Reset and Recovery ---\n";

// Save current state before reset
$history->backup();

// Reset configuration
$config->resetToDefaults();

echo "\nAfter reset to defaults:\n";
print_r($config->getConfig());

// Restore previous configuration
echo "\nRestoring previous configuration...\n";
$history->undo();

echo "\nAfter restoration:\n";
print_r($config->getConfig());

/**
 * Example 6: History management
 */
echo "\n--- Example 6: History Management ---\n";

// Show complete history
$history->showHistory();

echo "Total backups available: " . $history->getBackupCount() . "\n";

// Clear history
echo "\nClearing history...\n";
$history->clearHistory();

// Show history after clearing
$history->showHistory();

/**
 * Example 7: Real-world workflow simulation
 */
echo "\n--- Example 7: Real-World Workflow Simulation ---\n";

// Simulate a typical configuration update workflow
echo "Simulating typical admin workflow...\n\n";

// Step 1: Admin wants to update site for promotion
$history->backup();
echo "Step 1: Preparing for Black Friday promotion...\n";
$config->updateConfig([
    'promotion_banner' => 'Black Friday Sale - 50% Off!',
    'theme' => 'dark',
    'special_offers' => ['discount' => 50, 'code' => 'BLACKFRIDAY50']
]);

// Step 2: Update SEO for promotion
$history->backup();
echo "\nStep 2: Updating SEO for promotion visibility...\n";
$config->updateConfig([
    'seo' => [
        'title' => 'Black Friday Sale - 50% Off Everything!',
        'description' => 'Huge Black Friday discounts on all products. Limited time offer!',
        'keywords' => 'black friday, sale, discount, deals, promotion'
    ]
]);

// Step 3: Something goes wrong, need to rollback SEO changes only
echo "\nStep 3: SEO changes caused issues, rolling back SEO only...\n";
$history->undo();

echo "Final configuration after workflow:\n";
print_r($config->getConfig());

echo "\nFinal history state:\n";
$history->showHistory();

=== Configuration Manager with Memento Pattern Demo ===

--- Example 1: Basic Configuration Management ---
ConfigManager: Initialized with 5 config items.
ConfigHistory: History manager initialized.

Initial configuration:
Array
(
    [maintenance_mode] => 
    [theme] => light
    [seo] => Array
        (
            [title] => My Website
            [description] => Welcome to my site!
        )

    [debug] => 
    [max_users] => 1000
)

--- Example 2: Making Changes with Backups ---

ConfigHistory: Creating backup of current configuration...
ConfigManager: Saving current configuration state...
ConfigSnapshot: Created snapshot with 5 config items.
ConfigHistory: Backup created. Total backups: 1
ConfigManager: Updating configuration with new values...
ConfigManager: Configuration updated. Current config has 6 items.

After theme update:
Array
(
    [maintenance_mode] => 
    [theme] => dark
    [seo] => Array
        (
            [title] => My Website
            [description] => Welcome to my site!
        )

    [debug] => 
    [max_users] => 1000
    [theme_options] => Array
        (
            [sidebar] => collapsed
            [font_size] => large
        )

)

ConfigHistory: Creating backup of current configuration...
ConfigManager: Saving current configuration state...
ConfigSnapshot: Created snapshot with 6 config items.
ConfigHistory: Backup created. Total backups: 2
ConfigManager: Enabling maintenance mode...
ConfigManager: Maintenance mode enabled.

After enabling maintenance mode:
Array
(
    [maintenance_mode] => 1
    [theme] => dark
    [seo] => Array
        (
            [title] => My Website
            [description] => Welcome to my site!
        )

    [debug] => 
    [max_users] => 1000
    [theme_options] => Array
        (
            [sidebar] => collapsed
            [font_size] => large
        )

    [maintenance_message] => System under maintenance. Please try again later.
)

--- Example 3: Undo Functionality ---

ConfigHistory: Available configuration backups:
[0] 2025-07-28 15:04:16 / (5 items, maintenance: )
[1] 2025-07-28 15:04:16 / (6 items, maintenance: )

Undoing maintenance mode activation...
ConfigHistory: Restoring configuration to: 2025-07-28 15:04:16 / (6 items, maintenance: )
ConfigManager: Configuration restored from snapshot.
ConfigHistory: Undo completed successfully.

After first undo:
Array
(
    [maintenance_mode] => 
    [theme] => dark
    [seo] => Array
        (
            [title] => My Website
            [description] => Welcome to my site!
        )

    [debug] => 
    [max_users] => 1000
    [theme_options] => Array
        (
            [sidebar] => collapsed
            [font_size] => large
        )

)

Undoing theme changes...
ConfigHistory: Restoring configuration to: 2025-07-28 15:04:16 / (5 items, maintenance: )
ConfigManager: Configuration restored from snapshot.
ConfigHistory: Undo completed successfully.

After second undo (back to original):
Array
(
    [maintenance_mode] => 
    [theme] => light
    [seo] => Array
        (
            [title] => My Website
            [description] => Welcome to my site!
        )

    [debug] => 
    [max_users] => 1000
)

--- Example 4: Multiple Configuration Scenarios ---

ConfigHistory: Creating backup of current configuration...
ConfigManager: Saving current configuration state...
ConfigSnapshot: Created snapshot with 5 config items.
ConfigHistory: Backup created. Total backups: 1

Scenario A: Updating SEO settings...
ConfigManager: Updating configuration with new values...
ConfigManager: Configuration updated. Current config has 6 items.
SEO configuration updated:
Array
(
    [maintenance_mode] => 
    [theme] => light
    [seo] => Array
        (
            [title] => Best Products Online
            [description] => Find the best products at great prices!
            [keywords] => products, online, shopping, deals
        )

    [debug] => 
    [max_users] => 1000
    [analytics] => Array
        (
            [google_id] => GA-123456
            [facebook_pixel] => FB-789012
        )

)

ConfigHistory: Creating backup of current configuration...
ConfigManager: Saving current configuration state...
ConfigSnapshot: Created snapshot with 6 config items.
ConfigHistory: Backup created. Total backups: 2

Scenario B: Updating performance settings...
ConfigManager: Updating configuration with new values...
ConfigManager: Configuration updated. Current config has 9 items.
Performance settings updated:
Array
(
    [maintenance_mode] => 
    [theme] => light
    [seo] => Array
        (
            [title] => Best Products Online
            [description] => Find the best products at great prices!
            [keywords] => products, online, shopping, deals
        )

    [debug] => 
    [max_users] => 2000
    [analytics] => Array
        (
            [google_id] => GA-123456
            [facebook_pixel] => FB-789012
        )

    [cache_enabled] => 1
    [cache_duration] => 3600
    [compression] => gzip
)

Scenario C: Emergency rollback to SEO-only changes...
ConfigHistory: Restoring configuration to: 2025-07-28 15:04:16 / (6 items, maintenance: )
ConfigManager: Configuration restored from snapshot.
ConfigHistory: Undo completed successfully.
Rolled back performance changes:
Array
(
    [maintenance_mode] => 
    [theme] => light
    [seo] => Array
        (
            [title] => Best Products Online
            [description] => Find the best products at great prices!
            [keywords] => products, online, shopping, deals
        )

    [debug] => 
    [max_users] => 1000
    [analytics] => Array
        (
            [google_id] => GA-123456
            [facebook_pixel] => FB-789012
        )

)

--- Example 5: Reset and Recovery ---

ConfigHistory: Creating backup of current configuration...
ConfigManager: Saving current configuration state...
ConfigSnapshot: Created snapshot with 6 config items.
ConfigHistory: Backup created. Total backups: 2
ConfigManager: Resetting configuration to defaults...
ConfigManager: Configuration reset to defaults.

After reset to defaults:
Array
(
    [maintenance_mode] => 
    [theme] => light
    [debug] => 
)

Restoring previous configuration...
ConfigHistory: Restoring configuration to: 2025-07-28 15:04:16 / (6 items, maintenance: )
ConfigManager: Configuration restored from snapshot.
ConfigHistory: Undo completed successfully.

After restoration:
Array
(
    [maintenance_mode] => 
    [theme] => light
    [seo] => Array
        (
            [title] => Best Products Online
            [description] => Find the best products at great prices!
            [keywords] => products, online, shopping, deals
        )

    [debug] => 
    [max_users] => 1000
    [analytics] => Array
        (
            [google_id] => GA-123456
            [facebook_pixel] => FB-789012
        )

)

--- Example 6: History Management ---

ConfigHistory: Available configuration backups:
[0] 2025-07-28 15:04:16 / (5 items, maintenance: )

Total backups available: 1

Clearing history...
ConfigHistory: Cleared 1 backups from history.

ConfigHistory: Available configuration backups:
No backups available.


--- Example 7: Real-World Workflow Simulation ---
Simulating typical admin workflow...


ConfigHistory: Creating backup of current configuration...
ConfigManager: Saving current configuration state...
ConfigSnapshot: Created snapshot with 6 config items.
ConfigHistory: Backup created. Total backups: 1
Step 1: Preparing for Black Friday promotion...
ConfigManager: Updating configuration with new values...
ConfigManager: Configuration updated. Current config has 8 items.

ConfigHistory: Creating backup of current configuration...
ConfigManager: Saving current configuration state...
ConfigSnapshot: Created snapshot with 8 config items.
ConfigHistory: Backup created. Total backups: 2

Step 2: Updating SEO for promotion visibility...
ConfigManager: Updating configuration with new values...
ConfigManager: Configuration updated. Current config has 8 items.

Step 3: SEO changes caused issues, rolling back SEO only...
ConfigHistory: Restoring configuration to: 2025-07-28 15:04:16 / (8 items, maintenance: )
ConfigManager: Configuration restored from snapshot.
ConfigHistory: Undo completed successfully.
Final configuration after workflow:
Array
(
    [maintenance_mode] => 
    [theme] => dark
    [seo] => Array
        (
            [title] => Best Products Online
            [description] => Find the best products at great prices!
            [keywords] => products, online, shopping, deals
        )

    [debug] => 
    [max_users] => 1000
    [analytics] => Array
        (
            [google_id] => GA-123456
            [facebook_pixel] => FB-789012
        )

    [promotion_banner] => Black Friday Sale - 50% Off!
    [special_offers] => Array
        (
            [discount] => 50
            [code] => BLACKFRIDAY50
        )

)

Final history state:

ConfigHistory: Available configuration backups:
[0] 2025-07-28 15:04:16 / (6 items, maintenance: )
```

### python

```python
from __future__ import annotations
from abc import ABC, abstractmethod
from datetime import datetime
from random import sample
from string import ascii_letters


class Originator:
    """
    The Originator holds some important state that may change over time. It also
    defines a method for saving the state inside a memento and another method
    for restoring the state from it.
    """

    _state = None
    """
    For the sake of simplicity, the originator's state is stored inside a single
    variable.
    """

    def __init__(self, state: str) -> None:
        self._state = state
        print(f"Originator: My initial state is: {self._state}")

    def do_something(self) -> None:
        """
        The Originator's business logic may affect its internal state.
        Therefore, the client should backup the state before launching methods
        of the business logic via the save() method.
        """

        print("Originator: I'm doing something important.")
        self._state = self._generate_random_string(30)
        print(f"Originator: and my state has changed to: {self._state}")

    @staticmethod
    def _generate_random_string(length: int = 10) -> str:
        return "".join(sample(ascii_letters, length))

    def save(self) -> Memento:
        """
        Saves the current state inside a memento.
        """

        return ConcreteMemento(self._state)

    def restore(self, memento: Memento) -> None:
        """
        Restores the Originator's state from a memento object.
        """

        self._state = memento.get_state()
        print(f"Originator: My state has changed to: {self._state}")


class Memento(ABC):
    """
    The Memento interface provides a way to retrieve the memento's metadata,
    such as creation date or name. However, it doesn't expose the Originator's
    state.
    """

    @abstractmethod
    def get_name(self) -> str:
        pass

    @abstractmethod
    def get_date(self) -> str:
        pass


class ConcreteMemento(Memento):
    def __init__(self, state: str) -> None:
        self._state = state
        self._date = str(datetime.now())[:19]

    def get_state(self) -> str:
        """
        The Originator uses this method when restoring its state.
        """
        return self._state

    def get_name(self) -> str:
        """
        The rest of the methods are used by the Caretaker to display metadata.
        """

        return f"{self._date} / ({self._state[0:9]}...)"

    def get_date(self) -> str:
        return self._date


class Caretaker:
    """
    The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
    doesn't have access to the originator's state, stored inside the memento. It
    works with all mementos via the base Memento interface.
    """

    def __init__(self, originator: Originator) -> None:
        self._mementos = []
        self._originator = originator

    def backup(self) -> None:
        print("\nCaretaker: Saving Originator's state...")
        self._mementos.append(self._originator.save())

    def undo(self) -> None:
        if not len(self._mementos):
            return

        memento = self._mementos.pop()
        print(f"Caretaker: Restoring state to: {memento.get_name()}")
        try:
            self._originator.restore(memento)
        except Exception:
            self.undo()

    def show_history(self) -> None:
        print("Caretaker: Here's the list of mementos:")
        for memento in self._mementos:
            print(memento.get_name())


if __name__ == "__main__":
    originator = Originator("Super-duper-super-puper-super.")
    caretaker = Caretaker(originator)

    caretaker.backup()
    originator.do_something()

    caretaker.backup()
    originator.do_something()

    caretaker.backup()
    originator.do_something()

    print()
    caretaker.show_history()

    print("\nClient: Now, let's rollback!\n")
    caretaker.undo()

    print("\nClient: Once more!\n")
    caretaker.undo()

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: wQAehHYOqVSlpEXjyIcgobrxsZUnat

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: lHxNORKcsgMWYnJqoXjVCbQLEIeiSp

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: cvIYsRilNOtwynaKdEZpDCQkFAXVMf

Caretaker: Here's the list of mementos:
2019-01-26 21:11:24 / (Super-dup...)
2019-01-26 21:11:24 / (wQAehHYOq...)
2019-01-26 21:11:24 / (lHxNORKcs...)

Client: Now, let's rollback!

Caretaker: Restoring state to: 2019-01-26 21:11:24 / (lHxNORKcs...)
Originator: My state has changed to: lHxNORKcsgMWYnJqoXjVCbQLEIeiSp

Client: Once more!

Caretaker: Restoring state to: 2019-01-26 21:11:24 / (wQAehHYOq...)
Originator: My state has changed to: wQAehHYOqVSlpEXjyIcgobrxsZUnat
```

### ruby

```ruby
# The Originator holds some important state that may change over time. It also
# defines a method for saving the state inside a memento and another method for
# restoring the state from it.
class Originator
  # For the sake of simplicity, the originator's state is stored inside a single
  # variable.
  attr_accessor :state
  private :state

  # @param [String] state
  def initialize(state)
    @state = state
    puts "Originator: My initial state is: #{@state}"
  end

  # The Originator's business logic may affect its internal state. Therefore,
  # the client should backup the state before launching methods of the business
  # logic via the save() method.
  def do_something
    puts 'Originator: I\'m doing something important.'
    @state = generate_random_string(30)
    puts "Originator: and my state has changed to: #{@state}"
  end

  private def generate_random_string(length = 10)
    ascii_letters = [*'a'..'z', *'A'..'Z']
    (0...length).map { ascii_letters.sample }.join
  end

  # Saves the current state inside a memento.
  def save
    ConcreteMemento.new(@state)
  end

  # Restores the Originator's state from a memento object.
  def restore(memento)
    @state = memento.state
    puts "Originator: My state has changed to: #{@state}"
  end
end

# The Memento interface provides a way to retrieve the memento's metadata, such
# as creation date or name. However, it doesn't expose the Originator's state.
class Memento
  # @abstract
  #
  # @return [String]
  def name
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  # @abstract
  #
  # @return [String]
  def date
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

class ConcreteMemento < Memento
  # @param [String] state
  def initialize(state)
    @state = state
    @date = Time.now.strftime('%F %T')
  end

  # The Originator uses this method when restoring its state.
  attr_reader :state

  # The rest of the methods are used by the Caretaker to display metadata.
  def name
    "#{@date} / (#{@state[0, 9]}...)"
  end

  # @return [String]
  attr_reader :date
end

# The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
# doesn't have access to the originator's state, stored inside the memento. It
# works with all mementos via the base Memento interface.
class Caretaker
  # @param [Originator] originator
  def initialize(originator)
    @mementos = []
    @originator = originator
  end

  def backup
    puts "\nCaretaker: Saving Originator's state..."
    @mementos << @originator.save
  end

  def undo
    return if @mementos.empty?

    memento = @mementos.pop
    puts "Caretaker: Restoring state to: #{memento.name}"

    begin
      @originator.restore(memento)
    rescue StandardError
      undo
    end
  end

  def show_history
    puts 'Caretaker: Here\'s the list of mementos:'

    @mementos.each { |memento| puts memento.name }
  end
end

originator = Originator.new('Super-duper-super-puper-super.')
caretaker = Caretaker.new(originator)

caretaker.backup
originator.do_something

caretaker.backup
originator.do_something

caretaker.backup
originator.do_something

puts "\n"
caretaker.show_history

puts "\nClient: Now, let's rollback!\n"
caretaker.undo

puts "\nClient: Once more!\n"
caretaker.undo

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: CHYzYSIWbqvWkCzIHOqTyEJWfQlFMn

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: vbkhwCeAEQBpLwQLlhmpcvUnwzxVnT

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: SBWlQnAEPLsitiOQAZbGlXHZAeWBoW

Caretaker: Here's the list of mementos:
2023-08-11 15:02:35 / (Super-dup...)
2023-08-11 15:02:35 / (CHYzYSIWb...)
2023-08-11 15:02:35 / (vbkhwCeAE...)

Client: Now, let's rollback!
Caretaker: Restoring state to: 2023-08-11 15:02:35 / (vbkhwCeAE...)
Originator: My state has changed to: vbkhwCeAEQBpLwQLlhmpcvUnwzxVnT

Client: Once more!
Caretaker: Restoring state to: 2023-08-11 15:02:35 / (CHYzYSIWb...)
Originator: My state has changed to: CHYzYSIWbqvWkCzIHOqTyEJWfQlFMn
```

### rust

```rust
trait Memento<T> {
    fn restore(self) -> T;
    fn print(&self);
}

struct Originator {
    state: u32,
}

impl Originator {
    pub fn save(&self) -> OriginatorBackup {
        OriginatorBackup {
            state: self.state.to_string(),
        }
    }
}

struct OriginatorBackup {
    state: String,
}

impl Memento<Originator> for OriginatorBackup {
    fn restore(self) -> Originator {
        Originator {
            state: self.state.parse().unwrap(),
        }
    }

    fn print(&self) {
        println!("Originator backup: '{}'", self.state);
    }
}

fn main() {
    let mut history = Vec::<OriginatorBackup>::new();

    let mut originator = Originator { state: 0 };

    originator.state = 1;
    history.push(originator.save());

    originator.state = 2;
    history.push(originator.save());

    for moment in history.iter() {
        moment.print();
    }

    let originator = history.pop().unwrap().restore();
    println!("Restored to state: {}", originator.state);

    let originator = history.pop().unwrap().restore();
    println!("Restored to state: {}", originator.state);
}

Originator backup: '1'
Originator backup: '2'
Restored to state: 2
Restored to state: 1

use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
struct Originator {
    state: u32,
}

use serde::{Deserialize, Serialize};

/// An object to be stored. It derives a default
/// `Serialize` and `Deserialize` trait implementation, which
/// allows to convert it into many different formats (e.g. JSON).
#[derive(Serialize, Deserialize)]
struct Originator {
    state: u32,
}

impl Originator {
    /// Serializes an originator into a string of JSON format.
    pub fn save(&self) -> String {
        serde_json::to_string(self).unwrap()
    }

    /// Deserializes an originator into a string of JSON format.
    pub fn restore(json: &str) -> Self {
        serde_json::from_str(json).unwrap()
    }
}

fn main() {
    // A stack of mementos.
    let mut history = Vec::<String>::new();

    let mut originator = Originator { state: 0 };

    originator.state = 1;
    history.push(originator.save());

    originator.state = 2;
    history.push(originator.save());

    for moment in history.iter() {
        println!("{}", moment);
    }

    let originator = Originator::restore(&history.pop().unwrap());
    println!("Restored to state: {}", originator.state);

    let originator = Originator::restore(&history.pop().unwrap());
    println!("Restored to state: {}", originator.state);
}

{"state":1}
{"state":2}
Restored to state: 2
Restored to state: 1
```

### swift

```swift
import XCTest

/// The Originator holds some important state that may change over time. It also
/// defines a method for saving the state inside a memento and another method
/// for restoring the state from it.
class Originator {

    /// For the sake of simplicity, the originator's state is stored inside a
    /// single variable.
    private var state: String

    init(state: String) {
        self.state = state
        print("Originator: My initial state is: \(state)")
    }

    /// The Originator's business logic may affect its internal state.
    /// Therefore, the client should backup the state before launching methods
    /// of the business logic via the save() method.
    func doSomething() {
        print("Originator: I'm doing something important.")
        state = generateRandomString()
        print("Originator: and my state has changed to: \(state)")
    }

    private func generateRandomString() -> String {
        return String(UUID().uuidString.suffix(4))
    }

    /// Saves the current state inside a memento.
    func save() -> Memento {
        return ConcreteMemento(state: state)
    }

    /// Restores the Originator's state from a memento object.
    func restore(memento: Memento) {
        guard let memento = memento as? ConcreteMemento else { return }
        self.state = memento.state
        print("Originator: My state has changed to: \(state)")
    }
}

/// The Memento interface provides a way to retrieve the memento's metadata,
/// such as creation date or name. However, it doesn't expose the Originator's
/// state.
protocol Memento {

    var name: String { get }
    var date: Date { get }
}

/// The Concrete Memento contains the infrastructure for storing the
/// Originator's state.
class ConcreteMemento: Memento {

    /// The Originator uses this method when restoring its state.
    private(set) var state: String
    private(set) var date: Date

    init(state: String) {
        self.state = state
        self.date = Date()
    }

    /// The rest of the methods are used by the Caretaker to display metadata.
    var name: String { return state + " " + date.description.suffix(14).prefix(8) }
}

/// The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
/// doesn't have access to the originator's state, stored inside the memento. It
/// works with all mementos via the base Memento interface.
class Caretaker {

    private lazy var mementos = [Memento]()
    private var originator: Originator

    init(originator: Originator) {
        self.originator = originator
    }

    func backup() {
        print("\nCaretaker: Saving Originator's state...\n")
        mementos.append(originator.save())
    }

    func undo() {

        guard !mementos.isEmpty else { return }
        let removedMemento = mementos.removeLast()

        print("Caretaker: Restoring state to: " + removedMemento.name)
        originator.restore(memento: removedMemento)
    }

    func showHistory() {
        print("Caretaker: Here's the list of mementos:\n")
        mementos.forEach({ print($0.name) })
    }
}

/// Let's see how it all works together.
class MementoConceptual: XCTestCase {

    func testMementoConceptual() {

        let originator = Originator(state: "Super-duper-super-puper-super.")
        let caretaker = Caretaker(originator: originator)

        caretaker.backup()
        originator.doSomething()

        caretaker.backup()
        originator.doSomething()

        caretaker.backup()
        originator.doSomething()

        print("\n")
        caretaker.showHistory()

        print("\nClient: Now, let's rollback!\n\n")
        caretaker.undo()

        print("\nClient: Once more!\n\n")
        caretaker.undo()
    }
}

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...

Originator: I'm doing something important.
Originator: and my state has changed to: 1923

Caretaker: Saving Originator's state...

Originator: I'm doing something important.
Originator: and my state has changed to: 74FB

Caretaker: Saving Originator's state...

Originator: I'm doing something important.
Originator: and my state has changed to: 3681


Caretaker: Here's the list of mementos:

Super-duper-super-puper-super. 11:45:44
1923 11:45:44
74FB 11:45:44

Client: Now, let's rollback!


Caretaker: Restoring state to: 74FB 11:45:44
Originator: My state has changed to: 74FB

Client: Once more!


Caretaker: Restoring state to: 1923 11:45:44
Originator: My state has changed to: 1923

import XCTest

class MementoRealWorld: XCTestCase {

    /// State and Command are often used together when the previous state of the
    /// object should be restored in case of failure of some operation.
    ///
    /// Note: UndoManager can be used as an alternative.

    func test() {

        let textView = UITextView()
        let undoStack = UndoStack(textView)

        textView.text = "First Change"
        undoStack.save()

        textView.text = "Second Change"
        undoStack.save()

        textView.text = (textView.text ?? "") + " & Third Change"
        textView.textColor = .red
        undoStack.save()

        print(undoStack)

        print("Client: Perform Undo operation 2 times\n")
        undoStack.undo()
        undoStack.undo()

        print(undoStack)
    }
}

class UndoStack: CustomStringConvertible {

    private lazy var mementos = [Memento]()
    private let textView: UITextView

    init(_ textView: UITextView) {
        self.textView = textView
    }

    func save() {
        mementos.append(textView.memento)
    }

    func undo() {
        guard !mementos.isEmpty else { return }
        textView.restore(with: mementos.removeLast())
    }

    var description: String {
        return mementos.reduce("", { $0 + $1.description })
    }
}

protocol Memento: CustomStringConvertible {

    var text: String { get }
    var date: Date { get }
}

extension UITextView {

    var memento: Memento {
        return TextViewMemento(text: text,
                               textColor: textColor,
                               selectedRange: selectedRange)
    }

    func restore(with memento: Memento) {
        guard let textViewMemento = memento as? TextViewMemento else { return }

        text = textViewMemento.text
        textColor = textViewMemento.textColor
        selectedRange = textViewMemento.selectedRange
    }

    struct TextViewMemento: Memento {

        let text: String
        let date = Date()

        let textColor: UIColor?
        let selectedRange: NSRange

        var description: String {
            let time = Calendar.current.dateComponents([.hour, .minute, .second, .nanosecond],
                                                       from: date)
            let color = String(describing: textColor)
            return "Text: \(text)\n" + "Date: \(time.description)\n"
                + "Color: \(color)\n" + "Range: \(selectedRange)\n\n"
        }
    }
}

Text: First Change
Date: hour: 12 minute: 21 second: 50 nanosecond: 821737051 isLeapMonth: false
Color: nil
Range: {12, 0}

Text: Second Change
Date: hour: 12 minute: 21 second: 50 nanosecond: 826483011 isLeapMonth: false
Color: nil
Range: {13, 0}

Text: Second Change & Third Change
Date: hour: 12 minute: 21 second: 50 nanosecond: 829187035 isLeapMonth: false
Color: Optional(UIExtendedSRGBColorSpace 1 0 0 1)
Range: {28, 0}


Client: Perform Undo operation 2 times

Text: First Change
Date: hour: 12 minute: 21 second: 50 nanosecond: 821737051 isLeapMonth: false
Color: nil
Range: {12, 0}
```

### typescript

```typescript
/**
 * The Originator holds some important state that may change over time. It also
 * defines a method for saving the state inside a memento and another method for
 * restoring the state from it.
 */
class Originator {
    /**
     * For the sake of simplicity, the originator's state is stored inside a
     * single variable.
     */
    private state: string;

    constructor(state: string) {
        this.state = state;
        console.log(`Originator: My initial state is: ${state}`);
    }

    /**
     * The Originator's business logic may affect its internal state. Therefore,
     * the client should backup the state before launching methods of the
     * business logic via the save() method.
     */
    public doSomething(): void {
        console.log('Originator: I\'m doing something important.');
        this.state = this.generateRandomString(30);
        console.log(`Originator: and my state has changed to: ${this.state}`);
    }

    private generateRandomString(length: number = 10): string {
        const charSet = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

        return Array
            .apply(null, { length })
            .map(() => charSet.charAt(Math.floor(Math.random() * charSet.length)))
            .join('');
    }

    /**
     * Saves the current state inside a memento.
     */
    public save(): Memento {
        return new ConcreteMemento(this.state);
    }

    /**
     * Restores the Originator's state from a memento object.
     */
    public restore(memento: Memento): void {
        this.state = memento.getState();
        console.log(`Originator: My state has changed to: ${this.state}`);
    }
}

/**
 * The Memento interface provides a way to retrieve the memento's metadata, such
 * as creation date or name. However, it doesn't expose the Originator's state.
 */
interface Memento {
    getState(): string;

    getName(): string;

    getDate(): string;
}

/**
 * The Concrete Memento contains the infrastructure for storing the Originator's
 * state.
 */
class ConcreteMemento implements Memento {
    private state: string;

    private date: string;

    constructor(state: string) {
        this.state = state;
        this.date = new Date().toISOString().slice(0, 19).replace('T', ' ');
    }

    /**
     * The Originator uses this method when restoring its state.
     */
    public getState(): string {
        return this.state;
    }

    /**
     * The rest of the methods are used by the Caretaker to display metadata.
     */
    public getName(): string {
        return `${this.date} / (${this.state.substr(0, 9)}...)`;
    }

    public getDate(): string {
        return this.date;
    }
}

/**
 * The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
 * doesn't have access to the originator's state, stored inside the memento. It
 * works with all mementos via the base Memento interface.
 */
class Caretaker {
    private mementos: Memento[] = [];

    private originator: Originator;

    constructor(originator: Originator) {
        this.originator = originator;
    }

    public backup(): void {
        console.log('\nCaretaker: Saving Originator\'s state...');
        this.mementos.push(this.originator.save());
    }

    public undo(): void {
        if (!this.mementos.length) {
            return;
        }
        const memento = this.mementos.pop();

        console.log(`Caretaker: Restoring state to: ${memento.getName()}`);
        this.originator.restore(memento);
    }

    public showHistory(): void {
        console.log('Caretaker: Here\'s the list of mementos:');
        for (const memento of this.mementos) {
            console.log(memento.getName());
        }
    }
}

/**
 * Client code.
 */
const originator = new Originator('Super-duper-super-puper-super.');
const caretaker = new Caretaker(originator);

caretaker.backup();
originator.doSomething();

caretaker.backup();
originator.doSomething();

caretaker.backup();
originator.doSomething();

console.log('');
caretaker.showHistory();

console.log('\nClient: Now, let\'s rollback!\n');
caretaker.undo();

console.log('\nClient: Once more!\n');
caretaker.undo();

Originator: My initial state is: Super-duper-super-puper-super.

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: qXqxgTcLSCeLYdcgElOghOFhPGfMxo

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: iaVCJVryJwWwbipieensfodeMSWvUY

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: oSUxsOCiZEnohBMQEjwnPWJLGnwGmy

Caretaker: Here's the list of mementos:
2019-02-17 15:14:05 / (Super-dup...)
2019-02-17 15:14:05 / (qXqxgTcLS...)
2019-02-17 15:14:05 / (iaVCJVryJ...)

Client: Now, let's rollback!

Caretaker: Restoring state to: 2019-02-17 15:14:05 / (iaVCJVryJ...)
Originator: My state has changed to: iaVCJVryJwWwbipieensfodeMSWvUY

Client: Once more!

Caretaker: Restoring state to: 2019-02-17 15:14:05 / (qXqxgTcLS...)
Originator: My state has changed to: qXqxgTcLSCeLYdcgElOghOFhPGfMxo
```