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; ticket-aware bounded LRU cache; shared frame admission limit; 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, configured carvers, aquifers, noise-router ore veins, deterministic decoration, and basic template structures. The complete vanilla feature and biome datapack graph is embedded; feature ordering, placement modifiers, vertical anchors/providers, biome filters, and mutable region writes are implemented. Production region replay now covers cross-chunk ores, disks, underwater magma, stage-2 amethyst geodes, and lush-cave moss ground patches directly from their datapack configurations.
• 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; load-time border reconciliation; and chunk-scoped light_update broadcasts.
• Chunk lifecycle: per-client view and prefetch tickets, strict near-first ring streaming, stale-recenter cutoff, explicit client unload packets, and eviction only after the final owner releases a chunk.
• 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 -port 25565 -viewdistance 2

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. The server caps the client-requested chunk radius at 2 by default because cold density-based generation is expensive; raise it with -viewdistance 3 after the surrounding world has been generated and cached.

Testing

go test ./...
go test -race -timeout 20m ./...
# or run build, vet, ordinary tests, and race tests:
make verify

# strict block/biome comparison; requires a fixture generated with Java 25:
go run ./cmd/vanillacapture -server server.jar
make parity

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. A 16-client lifecycle test exercises overlapping ticket ownership, bounded global frame work, packet output, and cleanup after disconnect. Lighting tests compare the initial flat chunk and a 31x31x31 glowstone propagation volume against fixtures captured from the official vanilla 26.1.2 server. The committed overworld fixture exhaustively compares 393,216 block states, 6,144 biome cells, and three heightmaps across four fixed chunks. The canonical single-chunk generator currently matches 95.378% of fixture blocks, while the production region replay path matches 98.028%; both match all fixture biomes and heightmaps. CI guards the 91% regression floor while make parity requires exact equality. GitHub Actions runs build/vet/tests, fixture regression checks, and the full race suite on every push and pull request. Env-gated diagnostics in internal/world break the remaining gap down per subsystem (ore paths per state, base-terrain defects with sample coordinates).

v0.4 scope

RegionIO v0.4 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 reconciles persisted borders when chunks re-enter the live cache. Streaming prioritizes Chebyshev rings and abandons unstarted stale work; an already admitted frame calculation completes atomically rather than being interrupted halfway. Unowned clean chunks remain as an LRU warm cache until capacity pressure evicts them. Structures, placed features, mob AI, authentication, inventory, and survival mechanics remain intentionally partial. The density router, configured carvers, and noise-router ore veins are vanilla-derived. Surface and biome selection are ported but still need broader runtime captures. The production cache now uses atomic batch publication and the datapack-driven region replay path, with cross-chunk writes isolated per target. Underground decoration (ores with deepslate targets, underwater magma, disks) runs from the vanilla stage-6 schedule; the largest remaining fidelity gap is surface decoration — trees, flora, springs, and lakes are still hand-written — plus a few hundred underground cells where our carver or aquifer verdict differs from vanilla. The biome parameter finder uses an exact spatial index and overlapping region requests share a bounded immutable terrain cache, keeping cold 3x3 generation near one second on the reference Ryzen 5 5600X development machine.

The production cache uses atomic batch publication: a miss builds and publishes a complete decorated 3x3 neighborhood, while persisted neighbors still take precedence. This closes the chunk-lifecycle integration boundary without exposing undecorated chunks.

The remaining block-parity gap is entirely inside feature replay: a vanilla capture with every feature and structure set stripped out matches our undecorated pipeline on all 393,216 cells, so density, surface rules, carvers, aquifers, and noise-router veins are already bit-exact. Monster rooms now replay from their datapack configuration between the geodes and the ores, so the air pockets ore ellipsoids roll discards against exist where the schedule puts them; the committed fixture happens to contain no dungeons, leaving the measured parity at 98.028%. What still does not replay is the stage-1 lake schedule and the structure sets — ruined portal and mineshaft pieces are visible in the fixture — and those are the next worldgen milestones on the way to exact equality while keeping cold batch generation within an acceptable latency budget.

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/density core, climate finder, feature datapack runtime
  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.