- *Snapshot*. A is a serialized game state from which a client can render the game from. They are sent from the server at a regular interval and is created specifically for each client in order to reduce bandwidth.
- *Delta Snapshot*. A set of data that can be applied to a snapshot in order to create a new snapshot with the updated game state.
Topic: Structure
A snapshot contains a series of items. Each item have a type, id and data.
- *Type*. Type of item. Could be projectile or character for example.
- *Id*. A unique id so the client can identify the item between two snapshots.
- *Data*. A series of 32-bit integers that contains the per item type specific data.
Section: Server Side
Topic: Creating
Items can be added when <mods_snap> is called using the <snap_new_item> function to insert an item to the snapshot. The server can not inspect the snapshot that is in progress of being created.
<modc_newsnapshot> is called when a new snapshot has arrived for processing. <snap_num_items>, <snap_get_item> and <snap_find_item> can be used to inspect the current and previous snapshot. This can be done anywhere while the client is running and not just in the <modc_newsnapshot> function.
After a snapshot has been created, compression is applied to reduce the bandwidth. There are several steps taken in order to reduce the size of the snapshot.
- *Delta*. The server looks in a clients backlog of snapshots to find a previous acked snapshot. It then compares the two snapshots and creates a delta snapshot containing the changes from the previous acked snapshot to the new one.
- *Variable Integers*. The delta snapshot which is only consisting of 32-bit integers is then encoded into variable integers similar to UTF-8. Each byte has a bit that tells the decoder that it needs one more byte to decode the 32-bit integer. The first byte also contains a bit for telling the sign of the integer.
> ESDDDDDD EDDDDDDD EDDDDDDD EDDDDDDD
> E = extend
> S = sign
> D = data bit
- *Huffman*. The last step is to compress the buffer with a huffman encoder. It uses a static tree that is weighted towards 0 because it's where most of the data will be because of the delta compression.
- *Init*. 5 snapshots per second. Used when a client is connecting and used until the client has acknowledged a snapshot. This mechanism is used because the first snapshot because no delta compression can be done.
- *Full*. Snapshot for every tick or every even tick depending on server configuration. This is used during normal gameplay and everything is running smooth.
- *Recovery*. 1 snapshot each second. A client enters recovery rate when it hasn't acknowledged a snapshot over 1 second. This is to let the client be able to recover if it has a slow connection.
