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5918: Constify snap more (inspired by upstream) r=Robyt3 a=ChillerDragon Incorporate const added in those upstream commits:d86d576217e6b8518b49## Checklist - [x] Tested the change ingame - [ ] Provided screenshots if it is a visual change - [ ] Tested in combination with possibly related configuration options - [ ] Written a unit test (especially base/) or added coverage to integration test - [ ] Considered possible null pointers and out of bounds array indexing - [x] Changed no physics that affect existing maps - [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional) Co-authored-by: ChillerDragon <ChillerDragon@gmail.com>
58 lines
3 KiB
Plaintext
58 lines
3 KiB
Plaintext
Title: Snapshots
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Section: Overview
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Topic: Definitions
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- *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.
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- *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.
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Topic: Structure
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A snapshot contains a series of items. Each item have a type, id and data.
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- *Type*. Type of item. Could be projectile or character for example.
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- *Id*. A unique id so the client can identify the item between two snapshots.
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- *Data*. A series of 32-bit integers that contains the per item type specific data.
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Section: Server Side
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Topic: Creating
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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.
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Section: Client Side
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Topic: Inspection
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<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.
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Topic: Rendering
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DOCTODO
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Section: In depth
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Topic: Compression
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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.
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- *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.
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- *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.
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> ESDDDDDD EDDDDDDD EDDDDDDD EDDDDDDD
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> E = extend
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> S = sign
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> D = data bit
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- *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.
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Topic: Interval
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The interval for how often a client receives a snapshot changes during the course of the connection. There are three different snapshot rates.
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- *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.
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- *Full*. Snapshot for every tick or every even tick depending on server configuration. This is used during normal gameplay and everything is running smooth.
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- *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.
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