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lr2arena_protocol.md

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LR2Arena protocol

LR2Arena relies on UDP to communicate with both the "implant" (the DLL being injected into LR2) and the remote client. Ports are distributed as follows:

  • 2222: used for communication from the implant to LR2Arena
  • 2223: used for communication from LR2Arena to the implant
  • 2224: used for communication between clients (LR2Arena to LR2Arena)

Since even the communication between the implant and LR2Arena relies on UDP, it is possible to develop a new implant to work with LR2Arena, for example if you want to adapt LR2Arena to a different BMS client.

The UDP packets have a very simple structure:

[ id (1 byte) | data (n bytes) ]

id defines the operation carried by the UDP packet. If you wish to develop a new implant for LR2Arena, you need to implement the following operations:

Send (implant -> LR2Arena)

  • 1: returns the full path to the selected BMS in data, and a generated random. The path must be encoded in Shift-JIS. This should be sent right after selecting the chart. Packet format is as follows:
[ 1 | random (28 bytes) | path (n bytes) ]

random can be anything you want as long as both players can play using the same random with it; this is the implant's duty to implement it properly. In LR2mind's implementation, random does NOT correspond to the lanes' order. random always has to be generated, even if the player did not choose the Random option.

  • 2: returns the player's current score data. It should be sent on each note. Packet format is as follows (note that the score field refers to the old-style score (out of 200000) and not the exscore):
[ 2 | poor (4 bytes) | bad (4 bytes) | good (4 bytes) | great (4 bytes) | pgreat (4 bytes) | max_combo (4 bytes) | score (4 bytes) ]
  • 3: should be sent when the player cancels chart loading (before the chart has started). data is not checked, so it can be empty.
  • 4: should be sent when the chart is fully loaded and ready. data is not checked, so it can be empty.

Receive (LR2Arena -> implant)

  • 1: receives the remote player's exscore. It can be used to update the pacemaker in-game. data is a 4-byte long unsigned int (little-endian).
  • 2: notifies that the remote player is ready and has selected the correct chart. Can be used for syncing between clients. data is not checked, so it can be empty.
  • 3: receives a random to use for the next chart. This is always sent, even if the player did not choose the Random option. Structure is as follows:
[ 3 | random (28 bytes) ]
  • 4: tells whether if random flip should be activated or not. Random flip means that randoms should be mirrored. Structure is as follows:
[ 4 | random_flip_enabled (1 byte) ]