The locked lab 2: puzzles and the way out

Problem

All the doors are open

In the first lesson the lab is locked only in words: the player goes wherever they want. The key and the note just sit there and are no use. The game has no end.

An escape game needs:

• a locked door that opens with an item;
• a code lock, whose code you put together from clues in different rooms;
• an end: the player is outside, with a move count to brag about;
• a save code: the game is long, and the player wants to stop and go on later.

The save code is a short piece of text. The game prints it, and the player types it back. If the text is broken, the game has to say so, not crash.

The idea

A locked door

The room gets a lock. The secret is the name of the item that opens it, or the code:

Java

private String secret = null;
private boolean codeLock = false;
private boolean locked = false;

public void lockWithKey(String keyName) {
    secret = keyName;
    codeLock = false;
    locked = true;
}

public void lockWithCode(String code) {
    secret = code;
    codeLock = true;
    locked = true;
}

public boolean isLocked() {
    return locked;
}

public boolean isCodeLock() {
    return codeLock;
}

public boolean opensWith(String attempt) {
    return locked && secret.equals(attempt);
}

public void unlock() {
    locked = false;
}

C#

private string secret = null;

public bool IsLocked { get; private set; } = false;
public bool IsCodeLock { get; private set; } = false;

public void LockWithKey(string keyName)
{
    secret = keyName;
    IsCodeLock = false;
    IsLocked = true;
}

public void LockWithCode(string code)
{
    secret = code;
    IsCodeLock = true;
    IsLocked = true;
}

public bool OpensWith(string attempt)
{
    return IsLocked && secret == attempt;
}

public void Unlock()
{
    IsLocked = false;
}

Python

def __init__(self, name, description):
    self._name = name
    self._description = description
    self._exits = {}
    self._items = []
    self._secret = None
    self._code_lock = False
    self._locked = False

def lock_with_key(self, key_name):
    self._secret = key_name
    self._code_lock = False
    self._locked = True

def lock_with_code(self, code):
    self._secret = code
    self._code_lock = True
    self._locked = True

@property
def is_locked(self):
    return self._locked

@property
def is_code_lock(self):
    return self._code_lock

def opens_with(self, attempt):
    return self._locked and self._secret == attempt

def unlock(self):
    self._locked = False

JavaScript

#secret = null;
#codeLock = false;
#locked = false;

lockWithKey(keyName) {
  this.#secret = keyName;
  this.#codeLock = false;
  this.#locked = true;
}

lockWithCode(code) {
  this.#secret = code;
  this.#codeLock = true;
  this.#locked = true;
}

get isLocked() {
  return this.#locked;
}

get isCodeLock() {
  return this.#codeLock;
}

opensWith(attempt) {
  return this.#locked && this.#secret === attempt;
}

unlock() {
  this.#locked = false;
}

"go" now stops in front of a locked door:

Java

class GoCommand implements Command {
    @Override
    public String execute(Game game, String argument) {
        Room next = game.getCurrent().getExit(argument);
        if (next == null) {
            return "There is no exit that way.";
        }
        if (next.isLocked()) {
            return "The door to the " + next.getName() + " is locked.";
        }
        game.setCurrent(next);
        return next.describe();
    }
}

C#

class GoCommand : ICommand
{
    public string Execute(Game game, string argument)
    {
        Room next = game.Current.GetExit(argument);
        if (next == null)
        {
            return "There is no exit that way.";
        }
        if (next.IsLocked)
        {
            return "The door to the " + next.Name + " is locked.";
        }
        game.Current = next;
        return next.Describe();
    }
}

Python

class GoCommand(Command):
    def execute(self, game, argument):
        next_room = game.current.get_exit(argument)
        if next_room is None:
            return "There is no exit that way."
        if next_room.is_locked:
            return f"The door to the {next_room.name} is locked."
        game.current = next_room
        return next_room.describe()

JavaScript

class GoCommand {
  execute(game, argument) {
    const next = game.current.getExit(argument);
    if (next === undefined) {
      return "There is no exit that way.";
    }
    if (next.isLocked) {
      return `The door to the ${next.name} is locked.`;
    }
    game.current = next;
    return next.describe();
  }
}

In the world, the office is locked with the key from the storeroom, and the new lobby with a code:

Java

Room foyer = new Room("lobby", "The glass door to the street is open.");
corridor.addExit("north", foyer);
foyer.addExit("south", corridor);
office.lockWithKey("key");
foyer.lockWithCode("4719");

C#

Room foyer = new Room("lobby", "The glass door to the street is open.");
corridor.AddExit("north", foyer);
foyer.AddExit("south", corridor);
office.LockWithKey("key");
foyer.LockWithCode("4719");

Python

foyer = Room("lobby", "The glass door to the street is open.")
corridor.add_exit("north", foyer)
foyer.add_exit("south", corridor)
office.lock_with_key("key")
foyer.lock_with_code("4719")

JavaScript

const foyer = new Room("lobby", "The glass door to the street is open.");
corridor.addExit("north", foyer);
foyer.addExit("south", corridor);
office.lockWithKey("key");
foyer.lockWithCode("4719");

The idea

The key unlocks

"use key" checks the backpack, then looks for a locked neighboring door that opens with this item:

Java

class UseCommand implements Command {
    @Override
    public String execute(Game game, String argument) {
        if (!game.has(argument)) {
            return "You have no " + argument + ".";
        }
        for (Room room : game.getCurrent().getNeighbours()) {
            if (!room.isCodeLock() && room.opensWith(argument)) {
                game.unlock(room);
                return "You unlock the door to the " + room.getName() + ".";
            }
        }
        return "The " + argument + " does not help here.";
    }
}

C#

class UseCommand : ICommand
{
    public string Execute(Game game, string argument)
    {
        if (!game.Has(argument))
        {
            return "You have no " + argument + ".";
        }
        foreach (Room room in game.Current.GetNeighbours())
        {
            if (!room.IsCodeLock && room.OpensWith(argument))
            {
                game.Unlock(room);
                return "You unlock the door to the " + room.Name + ".";
            }
        }
        return "The " + argument + " does not help here.";
    }
}

Python

class UseCommand(Command):
    def execute(self, game, argument):
        if not game.has(argument):
            return f"You have no {argument}."
        for room in game.current.get_neighbours():
            if not room.is_code_lock and room.opens_with(argument):
                game.unlock(room)
                return f"You unlock the door to the {room.name}."
        return f"The {argument} does not help here."

JavaScript

class UseCommand {
  execute(game, argument) {
    if (!game.has(argument)) {
      return `You have no ${argument}.`;
    }
    for (const room of game.current.getNeighbours()) {
      if (!room.isCodeLock && room.opensWith(argument)) {
        game.unlock(room);
        return `You unlock the door to the ${room.name}.`;
      }
    }
    return `The ${argument} does not help here.`;
  }
}

Unlocking goes through the game, not straight through the room. That way the game remembers which doors are open. You will need that for the save code.

Try it

Which key, which door?

The player types these commands from the start of the game. What does the game answer to the three "use key" commands, in order?

go north
use key
go east
take key
use key
go west
use key

Answer

You have no key.
You have no key.
You unlock the door to the office.

The key does not help here.
The key does not help here.
You unlock the door to the office.

You have no key.
The key does not help here.
You unlock the door to the office.

You have no key.
You unlock the door to the office.
The key does not help here.

Check

The idea

A code lock from clues

The lobby is locked with four digits. The game does not say them outright:

• the note in the office: "Lobby code: 47 and the stocktaking day.";
• the calendar in the storeroom: "Stocktaking on October 19".

The code is 4719. The command "enter" checks it against the code lock of a neighboring door:

Java

class CodeCommand implements Command {
    @Override
    public String execute(Game game, String argument) {
        for (Room room : game.getCurrent().getNeighbours()) {
            if (room.isCodeLock() && room.isLocked()) {
                if (room.opensWith(argument)) {
                    game.unlock(room);
                    return "The lock clicks. The door to the " + room.getName() + " is open.";
                }
                return "Wrong code. The lock flashes red.";
            }
        }
        return "There is no code lock here.";
    }
}

C#

class CodeCommand : ICommand
{
    public string Execute(Game game, string argument)
    {
        foreach (Room room in game.Current.GetNeighbours())
        {
            if (room.IsCodeLock && room.IsLocked)
            {
                if (room.OpensWith(argument))
                {
                    game.Unlock(room);
                    return "The lock clicks. The door to the " + room.Name + " is open.";
                }
                return "Wrong code. The lock flashes red.";
            }
        }
        return "There is no code lock here.";
    }
}

Python

class CodeCommand(Command):
    def execute(self, game, argument):
        for room in game.current.get_neighbours():
            if room.is_code_lock and room.is_locked:
                if room.opens_with(argument):
                    game.unlock(room)
                    return f"The lock clicks. The door to the {room.name} is open."
                return "Wrong code. The lock flashes red."
        return "There is no code lock here."

JavaScript

class CodeCommand {
  execute(game, argument) {
    for (const room of game.current.getNeighbours()) {
      if (room.isCodeLock && room.isLocked) {
        if (room.opensWith(argument)) {
          game.unlock(room);
          return `The lock clicks. The door to the ${room.name} is open.`;
        }
        return "Wrong code. The lock flashes red.";
      }
    }
    return "There is no code lock here.";
  }
}

The puzzle is in the data, not in the code: a different code and different clues make a different game with the same classes.

See it

Outside

The game is won when the player reaches the lobby:

Java

public boolean isWon() {
    return current.getName().equals("lobby");
}

C#

public bool IsWon()
{
    return Current.Name == "lobby";
}

Python

def is_won(self):
    return self._current.name == "lobby"

JavaScript

isWon() {
  return this.#current.name === "lobby";
}

Handle counts every command it understands as a move. The loop in Main stops when you win:

Java

Game game = new Game();
System.out.println(game.getCurrent().describe());
Scanner in = new Scanner(System.in);
while (!game.isWon() && in.hasNextLine()) {
    String line = in.nextLine();
    System.out.println("> " + line);
    System.out.println(game.handle(line));
}
if (game.isWon()) {
    System.out.println("You got out in " + game.getMoves() + " moves.");
}

Output

[lab] The tables are full of flasks. The door behind you locked by itself.
You see: flashlight
Exits: north
> go north
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go west
The door to the office is locked.
> go east
[storeroom] Shelves of boxes up to the ceiling. A calendar on the wall says: Stocktaking on October 19.
You see: key
Exits: west
> take key
You take the key. A small brass key. Its tag says office.
> go west
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> use key
You unlock the door to the office.
> go west
[office] A desk, a chair and a cabinet full of folders.
You see: note
Exits: east
> take note
You take the note. Lobby code: 47 and the stocktaking day.
> go east
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go north
The door to the lobby is locked.
> enter 1234
Wrong code. The lock flashes red.
> enter 4719
The lock clicks. The door to the lobby is open.
> go north
[lobby] The glass door to the street is open.
Exits: south
You got out in 13 moves.

C#

Game game = new Game();
Console.WriteLine(game.Current.Describe());
string line = Console.ReadLine();
while (!game.IsWon() && line != null)
{
    Console.WriteLine("> " + line);
    Console.WriteLine(game.Handle(line));
    line = Console.ReadLine();
}
if (game.IsWon())
{
    Console.WriteLine("You got out in " + game.Moves + " moves.");
}

Output

[lab] The tables are full of flasks. The door behind you locked by itself.
You see: flashlight
Exits: north
> go north
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go west
The door to the office is locked.
> go east
[storeroom] Shelves of boxes up to the ceiling. A calendar on the wall says: Stocktaking on October 19.
You see: key
Exits: west
> take key
You take the key. A small brass key. Its tag says office.
> go west
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> use key
You unlock the door to the office.
> go west
[office] A desk, a chair and a cabinet full of folders.
You see: note
Exits: east
> take note
You take the note. Lobby code: 47 and the stocktaking day.
> go east
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go north
The door to the lobby is locked.
> enter 1234
Wrong code. The lock flashes red.
> enter 4719
The lock clicks. The door to the lobby is open.
> go north
[lobby] The glass door to the street is open.
Exits: south
You got out in 13 moves.

Python

game = Game()
print(game.current.describe())
for line in sys.stdin:
    line = line.rstrip("\n")
    print(f"> {line}")
    print(game.handle(line))
    if game.is_won():
        break
if game.is_won():
    print(f"You got out in {game.moves} moves.")

Output

[lab] The tables are full of flasks. The door behind you locked by itself.
You see: flashlight
Exits: north
> go north
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go west
The door to the office is locked.
> go east
[storeroom] Shelves of boxes up to the ceiling. A calendar on the wall says: Stocktaking on October 19.
You see: key
Exits: west
> take key
You take the key. A small brass key. Its tag says office.
> go west
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> use key
You unlock the door to the office.
> go west
[office] A desk, a chair and a cabinet full of folders.
You see: note
Exits: east
> take note
You take the note. Lobby code: 47 and the stocktaking day.
> go east
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go north
The door to the lobby is locked.
> enter 1234
Wrong code. The lock flashes red.
> enter 4719
The lock clicks. The door to the lobby is open.
> go north
[lobby] The glass door to the street is open.
Exits: south
You got out in 13 moves.

JavaScript

const game = new Game();
console.log(game.current.describe());
let line = await ask("");
while (!game.isWon() && line !== undefined) {
  console.log(`> ${line}`);
  console.log(game.handle(line));
  line = await ask("");
}
rl.close();
if (game.isWon()) {
  console.log(`You got out in ${game.moves} moves.`);
}

Output

[lab] The tables are full of flasks. The door behind you locked by itself.
You see: flashlight
Exits: north
> go north
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go west
The door to the office is locked.
> go east
[storeroom] Shelves of boxes up to the ceiling. A calendar on the wall says: Stocktaking on October 19.
You see: key
Exits: west
> take key
You take the key. A small brass key. Its tag says office.
> go west
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> use key
You unlock the door to the office.
> go west
[office] A desk, a chair and a cabinet full of folders.
You see: note
Exits: east
> take note
You take the note. Lobby code: 47 and the stocktaking day.
> go east
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go north
The door to the lobby is locked.
> enter 1234
Wrong code. The lock flashes red.
> enter 4719
The lock clicks. The door to the lobby is open.
> go north
[lobby] The glass door to the street is open.
Exits: south
You got out in 13 moves.

With this input:

Input

go north
go west
go east
take key
go west
use key
go west
take note
go east
go north
enter 1234
enter 4719
go north

Game game = new Game();
Console.WriteLine(game.Current.Describe());
string line = Console.ReadLine();
while (!game.IsWon() && line != null)
{
    Console.WriteLine("> " + line);
    Console.WriteLine(game.Handle(line));
    line = Console.ReadLine();
}
if (game.IsWon())
{
    Console.WriteLine("You got out in " + game.Moves + " moves.");
}

Step by step, in words

1. You start in the lab. The door to the office opens with the key, and the door to the lobby with a four-digit code.
2. Move 1: "go north". You enter the corridor.
3. Move 2: "go west". The door to the office is locked: "go" stops before it moves you.
4. Move 3: "go east". You enter the storeroom. On the wall: "Stocktaking on October 19".
5. Move 4: "take key". You take the key. It unlocks the office.
6. Move 5: "go west". You enter the corridor.
7. Move 6: "use key". The key is in the backpack and opens the locked neighboring door to the office.
8. Move 7: "go west". You enter the office.
9. Move 8: "take note". You take the note: "Lobby code: 47 and the stocktaking day."
10. Move 9: "go east". You enter the corridor.
11. Move 10: "go north". The door to the lobby is locked: "go" stops before it moves you.
12. Move 11: "enter 1234". 1234 is not the code. The lock flashes red.
13. Move 12: "enter 4719". 47 from the note and 19 from the calendar: 4719. The lobby lock clicks.
14. Move 13: "go north". You enter the lobby.
15. You reach the lobby: IsWon() is true, and the loop stops. You get out in 13 moves.

The idea

A save code

The whole state of the game is four things: where the player is, what they carry, which doors they opened and how many moves they played. They go into a short piece of text, separated by ;:

Java

public String saveCode() {
    List<String> names = new ArrayList<>();
    for (Item item : inventory) {
        names.add(item.getName());
    }
    String items = names.isEmpty() ? "-" : String.join(",", names);
    String doors = unlocked.isEmpty() ? "-" : String.join(",", unlocked);
    return current.getName() + ";" + items + ";" + doors + ";" + moves;
}

C#

public string SaveCode()
{
    List<string> names = new List<string>();
    foreach (Item item in Inventory)
    {
        names.Add(item.Name);
    }
    string items = names.Count == 0 ? "-" : string.Join(",", names);
    string doors = unlocked.Count == 0 ? "-" : string.Join(",", unlocked);
    return Current.Name + ";" + items + ";" + doors + ";" + Moves;
}

Python

def save_code(self):
    names = []
    for item in self._inventory:
        names.append(item.name)
    items = ",".join(names) if names else "-"
    doors = ",".join(self._unlocked) if self._unlocked else "-"
    return f"{self._current.name};{items};{doors};{self._moves}"

JavaScript

saveCode() {
  const names = [];
  for (const item of this.#inventory) {
    names.push(item.name);
  }
  const items = names.length === 0 ? "-" : names.join(",");
  const doors = this.#unlocked.length === 0 ? "-" : this.#unlocked.join(",");
  return `${this.#current.name};${items};${doors};${this.#moves}`;
}

For example office;key,note;office;9. The lists use commas, and an empty list is -, so there is never an empty spot between two ; signs.

The game prints the code, and the player types it back with "load". You do not need a file: the code can be copied onto paper or sent to a friend.

More precisely, if you are curious

The C# course has no lesson on files, so the game keeps the state only as text. Writing it to a file is one line with File.WriteAllText, when you get there.

The idea

A broken code

The player may get a letter wrong or try to cheat: appear in the locked office. Then loading throws its own exception:

Java

class InvalidSaveCodeException extends Exception {
    public InvalidSaveCodeException(String message) {
        super(message);
    }
}

C#

class InvalidSaveCodeException : Exception
{
    public InvalidSaveCodeException(string message) : base(message)
    {
    }
}

Python

class InvalidSaveCodeError(Exception):
    pass

JavaScript

class InvalidSaveCodeError extends Error {
  constructor(message) {
    super(message);
    this.name = "InvalidSaveCodeError";
  }
}

Load builds a new world and checks the code part by part on it. Only when everything has passed does the new world become the current one:

Java

public void load(String code) throws InvalidSaveCodeException {
    String[] parts = code.split(";");
    if (parts.length != 4) {
        throw new InvalidSaveCodeException("it needs 4 parts separated by ;");
    }
    Map<String, Room> world = buildWorld();
    Room room = world.get(parts[0]);
    if (room == null) {
        throw new InvalidSaveCodeException("there is no room " + parts[0]);
    }
    List<Item> items = new ArrayList<>();
    if (!parts[1].equals("-")) {
        for (String name : parts[1].split(",")) {
            Item found = takeFrom(world, name);
            if (found == null) {
                throw new InvalidSaveCodeException("there is no item " + name);
            }
            items.add(found);
        }
    }
    List<String> doors = new ArrayList<>();
    if (!parts[2].equals("-")) {
        for (String name : parts[2].split(",")) {
            Room door = world.get(name);
            if (door == null || !door.isLocked()) {
                throw new InvalidSaveCodeException("there is no locked door to " + name);
            }
            door.unlock();
            doors.add(name);
        }
    }
    if (room.isLocked()) {
        throw new InvalidSaveCodeException("the room " + room.getName() + " is locked");
    }
    int savedMoves;
    try {
        savedMoves = Integer.parseInt(parts[3]);
    } catch (NumberFormatException e) {
        throw new InvalidSaveCodeException("the moves are not a number: " + parts[3]);
    }
    rooms = world;
    current = room;
    inventory = items;
    unlocked = doors;
    moves = savedMoves;
}

private Item takeFrom(Map<String, Room> world, String name) {
    for (Room room : world.values()) {
        Item found = room.takeItem(name);
        if (found != null) {
            return found;
        }
    }
    return null;
}

C#

public void Load(string code)
{
    string[] parts = code.Split(';');
    if (parts.Length != 4)
    {
        throw new InvalidSaveCodeException("it needs 4 parts separated by ;");
    }
    Dictionary<string, Room> world = BuildWorld();
    if (!world.ContainsKey(parts[0]))
    {
        throw new InvalidSaveCodeException("there is no room " + parts[0]);
    }
    Room room = world[parts[0]];
    List<Item> items = new List<Item>();
    if (parts[1] != "-")
    {
        foreach (string name in parts[1].Split(','))
        {
            Item found = TakeFrom(world, name);
            if (found == null)
            {
                throw new InvalidSaveCodeException("there is no item " + name);
            }
            items.Add(found);
        }
    }
    List<string> doors = new List<string>();
    if (parts[2] != "-")
    {
        foreach (string name in parts[2].Split(','))
        {
            if (!world.ContainsKey(name) || !world[name].IsLocked)
            {
                throw new InvalidSaveCodeException("there is no locked door to " + name);
            }
            world[name].Unlock();
            doors.Add(name);
        }
    }
    if (room.IsLocked)
    {
        throw new InvalidSaveCodeException("the room " + room.Name + " is locked");
    }
    int savedMoves;
    try
    {
        savedMoves = int.Parse(parts[3]);
    }
    catch (FormatException)
    {
        throw new InvalidSaveCodeException("the moves are not a number: " + parts[3]);
    }
    rooms = world;
    Current = room;
    Inventory = items;
    unlocked = doors;
    Moves = savedMoves;
}

private Item TakeFrom(Dictionary<string, Room> world, string name)
{
    foreach (Room room in world.Values)
    {
        Item found = room.TakeItem(name);
        if (found != null)
        {
            return found;
        }
    }
    return null;
}

Python

def load(self, code):
    parts = code.split(";")
    if len(parts) != 4:
        raise InvalidSaveCodeError("it needs 4 parts separated by ;")
    world = self._build_world()
    room = world.get(parts[0])
    if room is None:
        raise InvalidSaveCodeError(f"there is no room {parts[0]}")
    items = []
    if parts[1] != "-":
        for name in parts[1].split(","):
            found = self._take_from(world, name)
            if found is None:
                raise InvalidSaveCodeError(f"there is no item {name}")
            items.append(found)
    doors = []
    if parts[2] != "-":
        for name in parts[2].split(","):
            door = world.get(name)
            if door is None or not door.is_locked:
                raise InvalidSaveCodeError(f"there is no locked door to {name}")
            door.unlock()
            doors.append(name)
    if room.is_locked:
        raise InvalidSaveCodeError(f"the room {room.name} is locked")
    try:
        saved_moves = int(parts[3])
    except ValueError:
        raise InvalidSaveCodeError(f"the moves are not a number: {parts[3]}")
    self._rooms = world
    self._current = room
    self._inventory = items
    self._unlocked = doors
    self._moves = saved_moves

def _take_from(self, world, name):
    for room in world.values():
        found = room.take_item(name)
        if found is not None:
            return found
    return None

JavaScript

load(code) {
  const parts = code.split(";");
  if (parts.length !== 4) {
    throw new InvalidSaveCodeError("it needs 4 parts separated by ;");
  }
  const world = this.#buildWorld();
  const room = world.get(parts[0]);
  if (room === undefined) {
    throw new InvalidSaveCodeError(`there is no room ${parts[0]}`);
  }
  const items = [];
  if (parts[1] !== "-") {
    for (const name of parts[1].split(",")) {
      const found = this.#takeFrom(world, name);
      if (found === null) {
        throw new InvalidSaveCodeError(`there is no item ${name}`);
      }
      items.push(found);
    }
  }
  const doors = [];
  if (parts[2] !== "-") {
    for (const name of parts[2].split(",")) {
      const door = world.get(name);
      if (door === undefined || !door.isLocked) {
        throw new InvalidSaveCodeError(`there is no locked door to ${name}`);
      }
      door.unlock();
      doors.push(name);
    }
  }
  if (room.isLocked) {
    throw new InvalidSaveCodeError(`the room ${room.name} is locked`);
  }
  if (!/^\d+$/.test(parts[3])) {
    throw new InvalidSaveCodeError(`the moves are not a number: ${parts[3]}`);
  }
  this.#rooms = world;
  this.#current = room;
  this.#inventory = items;
  this.#unlocked = doors;
  this.#moves = Number(parts[3]);
}

#takeFrom(world, name) {
  for (const room of world.values()) {
    const found = room.takeItem(name);
    if (found !== null) {
      return found;
    }
  }
  return null;
}

The "load" command catches the exception and tells the player what is wrong:

Java

class LoadCommand implements Command {
    @Override
    public String execute(Game game, String argument) {
        try {
            game.load(argument);
            return "You pick up where you left off.\n" + game.getCurrent().describe();
        } catch (InvalidSaveCodeException e) {
            return "The code does not work: " + e.getMessage() + ".";
        }
    }
}

C#

class LoadCommand : ICommand
{
    public string Execute(Game game, string argument)
    {
        try
        {
            game.Load(argument);
            return "You pick up where you left off.\n" + game.Current.Describe();
        }
        catch (InvalidSaveCodeException e)
        {
            return "The code does not work: " + e.Message + ".";
        }
    }
}

Python

class LoadCommand(Command):
    def execute(self, game, argument):
        try:
            game.load(argument)
            return "You pick up where you left off.\n" + game.current.describe()
        except InvalidSaveCodeError as e:
            return f"The code does not work: {e}."

JavaScript

class LoadCommand {
  execute(game, argument) {
    try {
      game.load(argument);
      return `You pick up where you left off.\n${game.current.describe()}`;
    } catch (e) {
      if (!(e instanceof InvalidSaveCodeError)) {
        throw e;
      }
      return `The code does not work: ${e.message}.`;
    }
  }
}

More precisely, if you are curious

• int.Parse throws FormatException, and Load turns it into its own exception. That way "load" catches only one exception, and the message is in the language of the game.

• The code is no protection against cheating: anyone who figures out the format can write "lobby". The check that the room is not locked guards only against the obvious. Real games add a checksum: a number computed from the whole text, which does not match if someone has touched it.

Try it

Find the bug

The player types load office;hammer;-;3. The game says "The code does not work: there is no item hammer.", but after "look" the player is in the office, which is locked. On which line is the bug?

Pick the line with the bug.

1public void Load(string code)2{3 string[] parts = code.Split(';');4 if (parts.Length != 4)5 {6 throw new InvalidSaveCodeException("it needs 4 parts separated by ;");7 }8 Dictionary<string, Room> world = BuildWorld();9 if (!world.ContainsKey(parts[0]))10 {11 throw new InvalidSaveCodeException("there is no room " + parts[0]);12 }13 Room room = world[parts[0]];14 Current = room;15 List<Item> items = new List<Item>();16 if (parts[1] != "-")17 {18 foreach (string name in parts[1].Split(','))19 {20 Item found = TakeFrom(world, name);21 if (found == null)22 {23 throw new InvalidSaveCodeException("there is no item " + name);24 }25 items.Add(found);26 }27 }28 List<string> doors = new List<string>();29 if (parts[2] != "-")30 {31 foreach (string name in parts[2].Split(','))32 {33 if (!world.ContainsKey(name) || !world[name].IsLocked)34 {35 throw new InvalidSaveCodeException("there is no locked door to " + name);36 }37 world[name].Unlock();38 doors.Add(name);39 }40 }41 if (room.IsLocked)42 {43 throw new InvalidSaveCodeException("the room " + room.Name + " is locked");44 }45 int savedMoves;46 try47 {48 savedMoves = int.Parse(parts[3]);49 }50 catch (FormatException)51 {52 throw new InvalidSaveCodeException("the moves are not a number: " + parts[3]);53 }54 rooms = world;55 Inventory = items;56 unlocked = doors;57 Moves = savedMoves;58}5960private Item TakeFrom(Dictionary<string, Room> world, string name)61{62 foreach (Room room in world.Values)63 {64 Item found = room.TakeItem(name);65 if (found != null)66 {67 return found;68 }69 }70 return null;71}

Check

See it

Save and continue

The player takes the key and saves. Then they try three broken codes and finally one that works:

Input

go north
go east
take key
save
load storeroom;hammer;-;4
load corridor;key;office;four
load office;key;-;7
load office;key;office;7
go east

Output

[lab] The tables are full of flasks. The door behind you locked by itself.
You see: flashlight
Exits: north
> go north
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go east
[storeroom] Shelves of boxes up to the ceiling. A calendar on the wall says: Stocktaking on October 19.
You see: key
Exits: west
> take key
You take the key. A small brass key. Its tag says office.
> save
Save code: storeroom;key;-;4
> load storeroom;hammer;-;4
The code does not work: there is no item hammer.
> load corridor;key;office;four
The code does not work: the moves are not a number: four.
> load office;key;-;7
The code does not work: the room office is locked.
> load office;key;office;7
You pick up where you left off.
[office] A desk, a chair and a cabinet full of folders.
You see: note
Exits: east
> go east
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west

The last code was written by hand, but it is valid: the office is unlocked, and the key is in the backpack. The game does not know whether it printed the code itself. It only knows that it is a possible state.

Exercise

More puzzles

Solve the tasks in your editor, with the classes from the lesson.

1. The dark cellar. South of the lab there is a cellar ("A low cellar with pipes along the ceiling."), and in it a spare fuse. You can go in only with a flashlight in the backpack. Hint: a command that extends GoCommand checks for the cellar and calls the parent (Execute in GoCommand has to be virtual). Register it with game.AddCommand in place of "go".

Input

go south
take flashlight
go south
take fuse

Output

[lab] The tables are full of flasks. The door behind you locked by itself.
You see: flashlight
Exits: north, south
> go south
It is too dark. Without light you will not find anything.
> take flashlight
You take the flashlight. It works, and the battery is almost full.
> go south
[cellar] A low cellar with pipes along the ceiling.
You see: fuse
Exits: north
> take fuse
You take the fuse. A spare fuse for the fuse box.

2. A timer. The guards come after 12 moves. If the player is not outside by then, the game is over.

Input

go north
go east
take key
go west
go west
use key
go west
take note
go east
go east
go west
enter 4719
go north

Output

[lab] The tables are full of flasks. The door behind you locked by itself.
You see: flashlight
Exits: north
> go north
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go east
[storeroom] Shelves of boxes up to the ceiling. A calendar on the wall says: Stocktaking on October 19.
You see: key
Exits: west
> take key
You take the key. A small brass key. Its tag says office.
> go west
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go west
The door to the office is locked.
> use key
You unlock the door to the office.
> go west
[office] A desk, a chair and a cabinet full of folders.
You see: note
Exits: east
> take note
You take the note. Lobby code: 47 and the stocktaking day.
> go east
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go east
[storeroom] Shelves of boxes up to the ceiling. A calendar on the wall says: Stocktaking on October 19.
Exits: west
> go west
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> enter 4719
The lock clicks. The door to the lobby is open.
The guards came. They caught you after 12 moves.

3. A hint for a move. A command "hint" says the next step based on the state: the key, the office, the note, the calendar. It costs a move, like every command.

Input

hint
go north
go east
take key
hint
go west
go west
use key
hint

Output

[lab] The tables are full of flasks. The door behind you locked by itself.
You see: flashlight
Exits: north
> hint
Hint: look around the storeroom.
> go north
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go east
[storeroom] Shelves of boxes up to the ceiling. A calendar on the wall says: Stocktaking on October 19.
You see: key
Exits: west
> take key
You take the key. A small brass key. Its tag says office.
> hint
Hint: the key is for the office.
> go west
[corridor] A long corridor. The ceiling light flickers.
Exits: north, south, east, west
> go west
The door to the office is locked.
> use key
You unlock the door to the office.
> hint
Hint: there is something to read in the office.

More tasks

• The fuse. The code lock does not work until the fuse from the cellar has been "used" in the corridor.
• Moves in the code. Make the timer work after "load" too: the code already carries the moves.
• A checksum. Add a fifth part: the sum of the lengths of the first four. A code with a wrong sum is broken.

Test

Check yourself

1. The player is in the storeroom and types "use key". The key is in the backpack. What happens?

"The key does not help here.": the office is not next to the storeroom, and the key only unlocks neighboring doors"You have no key."The office is unlocked

2. The save code is "corridor;key,note;office;9". What does the third part say?

Which doors are unlocked: here only the one to the officeWhere the player isHow many moves have passed

3. Why does Load check everything on a new world and only at the end replace the current game?

If the code turns out to be broken in the middle, the exception stops the method, and the game stays exactly as it wasBecause the old world cannot be changedIt is faster that way

4. Why does the move count go up after Find and not before it?

So that unknown commands are not counted: they do nothing in the gameBecause Find is slowerThere is no difference

Check

Start the lesson over