RegionIO

A Minecraft Java Edition server core written in Go, targeting version 26.1.2 (protocol 775). RegionIO implements the connection lifecycle (status → login → configuration → play), multiplayer chunk streaming, shared block editing, persistent worlds, and an overworld generator built on the real noise_router final_density tree.

Status

• Network: full handshake/status/login (offline mode)/configuration/play state machine with zlib compression, keep-alive, and chunk streaming.
• Registries: 28 synchronized registries + tags, captured verbatim from the 26.1.2 vanilla server and sent during configuration.
• World: revisioned, concurrency-safe chunk snapshots; memoized level_chunk_with_light frames; bounded LRU cache; Anvil .mca persistence with autosave and seed metadata.
• Generation: vanilla-derived overworld terrain from the embedded datapack (ImprovedNoise/PerlinNoise/BlendedNoise/NormalNoise + the density function interpreter), 3D multi-noise biomes, surface-rule interpretation, deterministic decoration, and basic template structures.
• Gameplay: four-player session registry; player join/leave and movement synchronization; chunk-scoped visibility for players and mobs; shared creative block place/break; broadcast chat; and hotbar item→block mapping.
• Lighting: stored vanilla nibble arrays for sky and block light; horizontal and cross-chunk propagation; incremental updates after edits; persisted SkyLight/BlockLight; and chunk-scoped light_update broadcasts.
• Safety: duplicate chunk generation is coalesced; corrupt stored chunks are not silently regenerated or overwritten; a world cannot reopen with another seed.

Build & run

go build ./...
go run ./cmd/regionio -seed 12345

The world seed defaults to 0; override it with the -seed flag or the REGIONIO_SEED environment variable. The server listens on 0.0.0.0:25565. Changing the seed for an existing world directory is rejected.

Testing

go test ./...
go test -race ./internal/network ./internal/server ./internal/world \
  -run 'Test(Integration|BoundaryEdit|PlayerInfo|PlayerRegistry|Concurrent|Incremental|EncodeLight|Cache|Store|Eviction|Region)'
# or run both gates:
make verify

The integration suite exercises four clients across two visibility regions: join, movement, leaving, mob visibility, and local block/light updates. A two-client scenario separately covers shared block edits and chat. Concurrency tests cover simultaneous frame encoding, editing, autosave, cache misses, and session movement/broadcasts. Lighting tests compare the initial flat chunk and a 31x31x31 glowstone propagation volume against fixtures captured from the official vanilla 26.1.2 server. Optional terrain parity diagnostics compare surface heights against /tmp/vanilla_ground.json when that capture is present.

v0.3 scope

RegionIO v0.3 is a small creative multiplayer server core, not a complete vanilla gameplay implementation. Player and mob visibility is chunk-scoped, but there is no interest prioritization or delta-movement compression yet. Lighting matches vanilla's block-state dampening, emission, and face-occlusion properties and propagates across loaded chunk boundaries. Chunks outside the live cache are recalculated exactly when loaded rather than retained as active light-engine state. Structures, placed features, mob AI, authentication, inventory, and survival mechanics remain intentionally partial. The density router is vanilla-derived, while biome/surface/decoration layers still contain approximations and require stricter parity fixtures.

Project layout

cmd/regionio/      entry point (config, listener, graceful shutdown)
internal/
  protocol/        wire primitives: VarInt, framing, compression, packet IDs
  nbt/             NBT encoder/decoder (with modified UTF-8)
  registry/        embedded synchronized registries + tags
  world/           chunk model, level_chunk encoder, cache, generators, biomes
  worldgen/        noise core + density-function interpreter + climate finder
  network/         per-connection state machine (handler/conn/play/login/...)
  server/          shared core: config, status response, profiles

Notes

The vanilla server.jar and its unpacked libraries//versions/ are not included (obtain them from Mojang). The embedded data under internal/ (registries, biome parameters, the overworld datapack) is derived from vanilla reports and is all that is required to build and run.