# Protocol Document
This document describes the protocol for client-server communication in our application. It defines the structure of requests and responses, the supported commands along with their request parameters, response formats, and possible errors.


# Table of Contents
- [Protocol Document](#protocol-document)
- [Table of Contents](#table-of-contents)
- [General structure of requests](#general-structure-of-requests)
- [Preconditions](#preconditions)
  - [Parsing](#parsing)
    - [Error Response](#error-response)
  - [Command dispatching](#command-dispatching)
    - [Error Response](#error-response-1)
  - [Command parsing](#command-parsing)
    - [Error Response](#error-response-2)
- [Pre-execution checks](#pre-execution-checks)
  - [UserLoggedInCheck](#userloggedincheck)
    - [Error Response](#error-response-3)
- [Commands](#commands)
  - [PING command](#ping-command)
    - [Required pre-execution checks](#required-pre-execution-checks)
    - [Request Parameters](#request-parameters)
    - [Success Response](#success-response)
    - [Example Request](#example-request)
    - [Example Response](#example-response)
  - [CHECK\_USERNAME command](#check_username-command)
    - [Required pre-execution checks](#required-pre-execution-checks-1)
    - [Request Parameters](#request-parameters-1)
    - [Success Response](#success-response-1)
    - [Example Request](#example-request-1)
    - [Example Response](#example-response-1)
  - [LOGIN command](#login-command)
    - [Required pre-execution checks](#required-pre-execution-checks-2)
    - [Request Parameters](#request-parameters-2)
    - [Success Response](#success-response-2)
    - [Error Response](#error-response-4)
    - [Example Request](#example-request-2)
    - [Example Response](#example-response-2)
  - [LOGOUT command](#logout-command)
    - [Required pre-execution checks](#required-pre-execution-checks-3)
    - [Request Parameters](#request-parameters-3)
    - [Success Response](#success-response-3)
    - [Error Response](#error-response-5)
    - [Example Request](#example-request-3)
    - [Example Response](#example-response-3)
  - [LIST\_USERS command](#list_users-command)
    - [Required pre-execution checks](#required-pre-execution-checks-4)
    - [Request Parameters](#request-parameters-4)
    - [Success Response](#success-response-4)
    - [Error Response](#error-response-6)
    - [Example Request](#example-request-4)
    - [Example Response](#example-response-4)
  - [GET_LOBBY_LIST command](#get_lobby_list-command)
    - [Required pre-execution checks](#required-pre-execution-checks)
    - [Request Parameters](#request-parameters)
    - [Success Response](#success-response)
    - [Example Request](#example-request)
    - [Example Response](#example-response)

<!-- Please see the comments for copy ‚ n' paste ready examples -->

# General structure of requests
As mentioned before, our protocol is based on POP3. 
Each command is represented as a single line of text, starting with the command name followed by parameters. The server responds with a status line indicating success or failure, followed by the body.

Requests can have parameters that provide additional information for the command. Parameters are key-value pairs separated by an equal sign (`=`).

Responses are collections of key-value pairs, containing either a value or another collection, allowing for nested structures. 
Each collection is ended with the `END` keyword.



# Preconditions
The serverside pipeline to process incoming requests consists of multiple stages.
Each of these stages can yield an error response if the request does not meet the requirements of that stage.

## Parsing
One of these stages is the parsing. It is responsible for parsing the raw request into a structured format that can be easily processed by the command handlers. It validates the syntax of the request as well.

### Error Response
| Code            | Description                                                                                     |
| :-------------- | :---------------------------------------------------------------------------------------------- |
| `PARSING_ERROR` | The body of the request contains syntax errors (see message field of response for more details) |


## Command dispatching
After the request has been successfully parsed, the next stage is to dispatch the `PrimitiveRequest` to the appropriate `CommandParser`. This is done by the `CommandDispatcherDispatcher`, which uses the command name to determine which parser to use.

### Error Response
| Code              | Description                                                       |
| :---------------- | :---------------------------------------------------------------- |
| `UNKNOWN_COMMAND` | The command is unknown to this server. No parser has been defined |


## Command parsing
Once the `PrimitiveRequest` has been dispatched to the appropriate `CommandParser`, the parser is responsible for parsing the parameters of the request and creating a `Request` that can be executed by the responsible `CommandHandler`.

### Error Response
| Code                | Description                                                                                       |
| :------------------ | :------------------------------------------------------------------------------------------------ |
| `MISSING_PARAMETER` | A required parameter is missing from the request (see message field of response for more details) |



# Pre-execution checks
Pre-execution checks are reusable validation steps that can be registered on command handlers. 
They are implemented as `HandlerCheck` instances and are executed by the `CommandHandlerExecutor` before the handler's main logic is invoked.

<!--
## Name of the check
Description of the check, what it does and when it should be used.

### Response if not met
| Code         | Description              |
| :----------- | :----------------------- |
| `ERROR_CODE` | Description of the error |
-->

## UserLoggedInCheck
The `UserLoggedInCheck` is a common pre-execution check that verifies whether the user is logged in (i.e. has a user associated with his session).

### Error Response
| Code                 | Description               |
| :------------------- | :------------------------ |
| `USER_NOT_LOGGED_IN` | The user is not logged in |



# Commands
Commands are the core of our protocol, representing the various actions that clients can request from the server. Each command has a unique name and may require specific parameters in addition to pre-execution checks.
The server processes these commands and responds accordingly.

<!--
## Name of the command
Description of the command, what it does, and when it should be used.

### Required pre-execution checks
- [`Check1`](#check1)

### Request Parameters
| Parameter Name | Type   | Optional                            | Description           |
| :------------- | :----- | :---------------------------------- | :-------------------- |
| `param1`       | `type` | If the parameter is optional or not | Description of param1 |

### Success Response
| Field    | Type                | Description           |
| :------- | :------------------ | :-------------------- |
| `field1` | `type`              | Description of field1 |
| `field2` | `Collection<type2>` |                       |
| `field3` | `Enum<type3>`       | Description of field3 |

| Fields of `type2` | Type   | Description           |
| :---------------- | :----- | :-------------------- |
| `field1`          | `type` | Description of field1 |

| Members of `type` | Description            |
| :---------------- | :--------------------- |
| `MEMBER1`         | Description of member1 |

### Error Response
| Code         | Description              |
| :----------- | :----------------------- |
| `ERROR_CODE` | Description of the error |

### Example Request
```
COMMAND_NAME PARAM1='value1' PARAM2='value2'
```

### Example Response
```
+OK
  KEY1=VALUE1
  FIELDS
    FIELD
      NESTED_KEY=NESTED_VALUE
  END
  KEY2=VALUE2
END
```
-->

## PING command
The `PING` command is a simple command that can be used to check if the server is responsive.

### Required pre-execution checks
None.

### Request Parameters
No parameters.

### Success Response
No response fields.

### Example Request
```
PING
``` 

### Example Response
```
+OK
END
```


## CHECK_USERNAME command
The `CHECK_USERNAME` command is used to check if a username is already taken by another user. Additional users can still log in with the same username, but their name will be substituted with a suffix.

### Required pre-execution checks
None.

### Request Parameters
| Parameter Name | Type     | Optional | Description                            |
| :------------- | :------- | :------- | :------------------------------------- |
| `USERNAME`     | `String` | no       | The username to check for availability |

### Success Response
| Field    | Type                         | Description                                                             |
| :------- | :--------------------------- | :---------------------------------------------------------------------- |
| `STATUS` | `Enum<UsernameAvailability>` | Member of enum indicating if the username is available or already taken |

| Members of `UsernameAvailability` | Description                |
| :-------------------------------- | :------------------------- |
| `FREE`                            | Username is available      |
| `TAKEN`                           | Username is already in use |

### Example Request
```
CHECK_USERNAME USERNAME='Lars'
```

### Example Response
```
+OK 
  STATUS=FREE
END
```

## LOGIN command
The `LOGIN` command is used to log in a user with a specified username. If the username is already taken by another user, the server will append a suffix to the username to make it unique.

### Required pre-execution checks
None.

### Request Parameters
| Parameter Name | Type     | Optional | Description                          |
| :------------- | :------- | :------- | :----------------------------------- |
| `USERNAME`     | `String` | no       | The username to create the user with |

### Success Response
| Field      | Type                                                                    | Description                                                           |
| :--------- | :---------------------------------------------------------------------- | :-------------------------------------------------------------------- |
| `USERNAME` | `String`                                                                | Username of the newly created user, can differ from the requested one |
| `ID`       | [`UUID`](https://docs.oracle.com/javase/8/docs/api/java/util/UUID.html) | The ID of the created user                                            |

### Error Response
| Code                | Description                                                                    |
| :------------------ | :----------------------------------------------------------------------------- |
| `ALREADY_LOGGED_IN` | The session is already associated with a user, logging in again is prohibited. |

### Example Request
```
LOGIN USERNAME='Lars'
```

### Example Response
```
+OK
  USERNAME='Lars_1234'
  ID=e47a671e-2b2a-42df-bb82-953fe2ebd307
END
```


## LOGOUT command
Description of the command, what it does, and when it should be used.

### Required pre-execution checks
None.

### Request Parameters
No parameters.

### Success Response
No response fields.

### Error Response
| Code                 | Description                                                      |
| :------------------- | :--------------------------------------------------------------- |
| `NO_USER_ASSOCIATED` | The session has no user associated, logging out is not possible. |

### Example Request
```
LOGOUT
```

### Example Response
```
+OK
END
```

## LIST_USERS command
The `LIST_USERS` command is used to retrieve a list of all currently logged-in users.

### Required pre-execution checks
None.

### Request Parameters
No parameters.

### Success Response
| Field   | Type               | Description                              |
| :------ | :----------------- | :--------------------------------------- |
| `USERS` | `Collection<User>` | Collection of all users currently online |

| Fields of `User` | Type                                                                    | Description                                                           |
| :--------------- | :---------------------------------------------------------------------- | :-------------------------------------------------------------------- |
| `USERNAME`       | `String`                                                                | Username of the newly created user, can differ from the requested one |
| `ID`             | [`UUID`](https://docs.oracle.com/javase/8/docs/api/java/util/UUID.html) | The ID of the created user                                            |


### Error Response
None.

### Example Request
```
LIST_USERS
```

### Example Response
```
+OK
  USERS
    USER
      USERNAME=Lars_001
      ID=56765d0f-8cd3-4eec-91b2-7e36265c1a5d
    END
    USER
      USERNAME=Lars_002
      ID=b7bbd9b3-0d49-4c92-8306-b1c8506d2ff0
    END
    USER
      USERNAME=Lars
      ID=982bc78e-547f-495a-a821-433a3603f92c
    END
  END
END
```

## SEND_MESSAGE command
The `SEND_MESSAGE` command is used to transfer the chat message sent by a user to the server.
### Required pre-execution checks
None.

### Request Parameters

| Field    | Type            | Description                                                        |
|:---------|:----------------|:-------------------------------------------------------------------|
| `TYPE`   | `Enum<ChatType` | Member of Enum, indicating with chat type is used                  |
| `GAME`   | `int`           | ID for identifying the Lobby Chat                                  |
| `USER`   | `String`        | Username of the player that sent the message                       |
| `TARGET` | `String`        | Username of to which the message is sent to (only in Whisper Chat) |
| `TIME`   | `String`        | Timestamp, when the message was sent                               |
| `TEXT`   | `String`        | Content of the message                                             |

| Members of `ChatType` | Description                     |
|:----------------------|:--------------------------------|
| `GLOBAL`              | Message is for the global chat  |
| `LOBBY`               | Message is for the lobby chat   |
| `WHISPER`             | Message is for the whisper chat |

### Success Response
No response fields.

### Error Response
None.

### Example Request
```
SEND_MESSAGE TYPE=GLOBAL GAME=1 USER=player1 TARGET=null TIME='10:30' TEXT='Hello World'
```

### Example Response
```
+OK
END
```

## GET_MESSAGE_COUNT command
The `GET_MESSAGE_COUNT` is used to get the current number of messages that are stored in the queue for a client.
### Required pre-execution checks
None.

### Request Parameters
No parameters.

### Success Response

| Field   | Type  | Description                    |
|:--------|:------|:-------------------------------|
| `COUNT` | `int` | The current number of messages |

### Error Response
| Code                 | Description                                                                      |
| :------------------- |:---------------------------------------------------------------------------------|
| `NO_USER_ASSOCIATED` | The session has no user associated, there is no queue of messages for the client |


### Example Request
```
GET_MESSAGE_COUNT
```

### Example Response
```
+OK
COUNT=10
END
```

## GET_NEXT_MESSAGE command
The `GET_NEXT_MESSAGE` command is used to get the next message stored in a queue for the client.
### Required pre-execution checks
None.

### Request Parameters
No parameters.

### Success Response
| Field    | Type            | Description                                                        |
|:---------|:----------------|:-------------------------------------------------------------------|
| `TYPE`   | `Enum<ChatType` | Member of Enum, indicating with chat type is used                  |
| `GAME`   | `int`           | ID for identifying the Lobby Chat                                  |
| `USER`   | `String`        | Username of the player that sent the message                       |
| `TARGET` | `String`        | Username of to which the message is sent to (only in Whisper Chat) |
| `TIME`   | `String`        | Timestamp, when the message was sent                               |
| `TEXT`   | `String`        | Content of the message                                             |

| Members of `ChatType` | Description                     |
|:----------------------|:--------------------------------|
| `GLOBAL`              | Message is for the global chat  |
| `LOBBY`               | Message is for the lobby chat   |
| `WHISPER`             | Message is for the whisper chat |

### Error Response
| Code                 | Description                                                                      |
| :------------------- |:---------------------------------------------------------------------------------|
| `NO_USER_ASSOCIATED` | The session has no user associated, there is no queue of messages for the client |

### Example Request
```
GET_NEXT_MESSAGE
```

### Example Response
```
+OK
TYPE=GLOBAL GAME=-1 USER=player1 TARGET=null TIME=9:30 TEXT="Guten Tag"
END
```

### Example Response (success)

```
+OK
END
```

## JOIN_LOBBY command

The `JOIN_LOBBY` command requests the server to add the currently logged-in user to the lobby with the given identifier. The server resolves the username from the session; clients must only provide the `ID` parameter.

### Required pre-execution checks

- [`UserLoggedInCheck`](#userloggedincheck)

### Request Parameters

| Parameter Name | Type | Optional | Description |
| :------------- | :--- | :------: | :---------- |
| `ID` | `int` | no | Numeric id of the target lobby |

### Implementation notes

- Parser: `JoinLobbyParser` — reads the `ID` parameter and builds `JoinLobbyRequest`.
- Handler: `JoinLobbyHandler` — resolves the username from `UserRegistry` using the session id, attempts to add the user to the lobby via `LobbyManager.addPlayerToLobby(...)` and returns appropriate responses.

### Success Response

No additional response fields. The server replies with a simple `+OK` on success.

### Error Response

| Code | Description |
| :--- | :---------- |
| `USER_NOT_LOGGED_IN` | No user associated with the session (pre-execution check failed) |
| `LOBBY_NOT_FOUND` | The specified lobby id does not exist |
| `LOBBY_FULL_OR_ALREADY_IN` | Lobby is full or the user is already in the lobby |

### Example Request

```
JOIN_LOBBY ID=1
```

### Example Response (success)

```
+OK
END
```

### Example Response (error)

```
-ERR
  CODE=LOBBY_NOT_FOUND
  MESSAGE=Lobby not found
END
```

## GET_LOBBY_LIST command

The `GET_LOBBY_LIST` command requests the server to return a list of currently available lobbies with basic metadata.

### Required pre-execution checks
None.

### Request Parameters
No parameters.

### Implementation notes

- Parser: `GetLobbyListParser` — creates `GetLobbyListRequest` (no parameters).
- Handler: `GetLobbyListHandler` — queries `LobbyManager#getAllLobbies()` and returns `GetLobbyListResponse`.
- Response: `GetLobbyListResponse` — builds a `LOBBIES` collection with repeated `LOBBY` blocks containing `ID`, `NAME`, `PLAYER_COUNT`.

### Success Response

The response contains a `LOBBIES` collection with nested `LOBBY` entries.

Example response structure:

```
+OK
  LOBBIES
    LOBBY
      ID=1
      NAME=Room 1
      PLAYER_COUNT=2
    END
    LOBBY
      ID=2
      NAME=Room 2
      PLAYER_COUNT=3
    END
  END
END
```

### Example Request

```
GET_LOBBY_LIST
```

### Example Response (error)

```
-ERR
  CODE=NO_LOBBIES_AVAILABLE
  MESSAGE=No lobbies available
END
```

## GET_LOBBY_STATUS command

The `GET_LOBBY_STATUS` command requests the server to return the current state of a lobby, including the list of players and their ready state.

### Required pre-execution checks
None.

### Request Parameters
| Parameter Name | Type | Optional | Description |
| :------------- | :--- | :------: | :---------- |
| `ID` | `int` | no | Numeric id of the target lobby |

### Implementation notes

- Parser: `GetLobbyStatusParser` — reads the `ID` parameter and builds `GetLobbyStatusRequest`.
- Handler: `GetLobbyStatusHandler` — queries `LobbyManager` for the lobby and builds a `GetLobbyStatusResponse` containing player entries.

### Success Response

The response contains a `LOBBY` collection with nested `PLAYERS` and one or more `PLAYER` entries. Each `PLAYER` entry contains `USERNAME` and `READY` fields.

Example response structure:

```
+OK
  LOBBY
    ID=1
    PLAYERS
      PLAYER
        USERNAME=Lars_001
        READY=false
      END
      PLAYER
        USERNAME=Anna
        READY=true
      END
    END
  END
END
```

### Error Response
| Code | Description |
| :--- | :---------- |
| `LOBBY_NOT_FOUND` | The specified lobby id does not exist |

### Example Request

```
GET_LOBBY_STATUS ID=1
```

### Example Response (error)

```
-ERR
  CODE=LOBBY_NOT_FOUND
  MESSAGE=Lobby not found
END
```