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Bounce Out

bounce out gif

A multiplayer, synchronous turn-based game with a Javascript client and an Elixir server. The code that simulates the physics of the sliding balls is done on the client. The server takes care of validating player inputs.

The goal of this project is to create a minimalistic web game that uses only the Matterjs library on the frontend, with an Elixir backend.

The client and server communicate with websockets.

Client and Server Interaction

A good way to see how the client and server interact is by going through a couple rounds of a game:

  1. Server starts the game with multiple clients and information on the starting location of their balls.
  2. Clients choose launch vectors for each of their balls.
  3. Each client caps the launch vectors to a pre-determined maximum magnitude in case a client tried to cheat by value that was too big.
  4. Clients send their vectors and ball locations to the server.
  5. Server sends out all vectors of the client's opponent to each client ,with the current location of the balls that the server has.
  6. Clients use the launch vectors and ball locations from the server to animate the game state updating. This way, if a bad actor tried changing the local location of the balls before the simulation occurs, it will have no effect.
  7. Immediately after the animation is finished, clients sejknd the updated location of their balls to the server.
  8. The server updates the position of the balls in its state.
  9. If all of the balls of a client have fallen off, that client loses. A game can result in a draw. If no client has lost, return to step 2.
  10. The server stores the game info in the database temporarily so that the game can be analyzed in the event that a player reports cheating.

With this system, none of the simulation code has to be run on the server. This helps us in the following ways:

  • We do not have to worry about finding a way to have both the client and server run a consistent simulation every round.
  • The server is more performant and the cost of running the server is cheaper since it does not have to do as much work.

Implementing the launch mechanism

Two different algorithms were considered when implementing the launch mechanism. In the app, the algorithm outlined in plan 2 was used because it had the least amount of things to change and keep track of each round of gameplay.

Plan 1:

  1. Pieces are assigned collisionFilters that correspond to the player that controls while still allowing all pieces to collide with each other.
  2. Each player's mouse is assigned their unique collisionFilter to correspond to the pieces they can set the launch vector for.
  3. At the beginning of the round, all pieces start off as being set to static = true.
  4. Players drag their launch vectors into place.
  5. Once time is up:
    1. The collisionFilter of the mice are changed so that they are no longer matching the pieces.
    2. The static property of each piece is set to false
    3. The pieces are launched
  6. Once pieces have stopped moving, return to step 3

Plan 2:

  1. Pieces are assigned a collisionFilter such that they can collide with each other.
  2. Each player's mouse is assigned a collisionFilter such that it cannot collide with the pieces.
  3. At the beginning of the round, a sensor is created on top of each piece controlled by the player with a collisionFilter that matches the player's mouse.
  4. Players drag their launch vectors into place.
  5. Once time is up:
    1. The sensors are destroyed
    2. The pieces are launched
  6. Once pieces have stopped moving, return to step 3

Assigning Pieces Owned by each Client at Start of Game

The first plan to assign which pieces each client owns was the following:

  1. A game lobby is created with a set maximum number of players.
  2. The server sends a player integer, starting at 0 to the person that creates the lobby.
  3. Every time a person joins the lobby, player is incremented and sent to the client that joined until the maximum number of players for that lobby is reached.
  4. Clients take the array [1, 2, 3], then add 3 * player to each index to get an array of the ids of the pieces that they own.

At the start of the game, the server creates a map, pieces, that has keys of piece_id to values of a tuple created with player_id and piece_position

Initializing Client's Game State Upon Reconnecting

To initialize the state of the game when a client reconnects after temporarily disconnecting, the server needs to send this information to the client that reconnects:

  • The playerId of the client (sent as an integer playerId)
  • The server's pieces map (keys of piece_id to values of a tuple: player_id, piece_position)

With this information from the server, the client reconstructs the state of the game by iterating through the pieces map and creating the corresponding pieces while keeping track of the newly created pieceIds.

After all pieces are created, each client sends the server what each local pieceId maps to the server's piece_id. This is needed because the game may have less pieces in its current state than what it started with, therefore, pieceIds that server has would not match up with those that were newly created.

The piece colors that correspond to each player are stored in the client.

Remembering the player id of a client after disconnecting

When a client reconnects to the server after temporarily disconnecting, the server needs to send the correct playerId to that client. This is how the server does that.