Detects and corrects for "blockstate mismatch" bugs when connecting to a server.
Blockstates that exist on the server but not the client will be stripped.
Blockstates that exist on the client but not the server will always be set to their default values.
LGPL 3.0 or later.
The client and server independently compute their own Blocks.STATE_IDS table, and assume it always holds the same contents. Clients assume servers have an identical table, servers assume clients have an identical table. This table is consulted for serializing and deserializing chunk packets (among other things), because numbers are a lot cheaper to send over the network than full-blown stringified state IDs.
Usually, the assumption holds and the table happens to be identical: if the list of block IDs is the same, the list of blockstates is also the same. However it's not always the case if there are mods that hack additional blockstates onto vanilla blocks, such as Better End adding new features to chorus plants, and old versions of Trees Do Not Float adding a blockstate to detect player-placed logs.
If a mod that does this is present on only one of the client or the server, you get very weird-looking results ingame where most blocks look OK but many blockstates are replaced with seemingly random other blockstates.
There was gonna be a fancy algorithm to like, rsync the states across, but unascribed was like "why not just gzip and send it in chunks?" and guess what that works perfectly fine lmao. In vanilla I'm already like several orders of magnitude under the chunk threshold hahaha
The server asks the client for its hash of the state_ids table, and if they differ, the server serializes its table, gzips it, and sends it to the client. That's all.