RegionIO/CLAUDE.md

RegionIO — working notes

A Minecraft Java Edition server core in Go, targeting 26.1.2 / protocol 775. The goal is vanilla fidelity, not a lookalike: where vanilla behaviour is known, match it exactly.

README.md describes what the server does. This file is about how to work on it.

Commands

go build ./... && go vet ./...
make test          # go test ./...
make test-race     # the race-sensitive subset
make verify        # both

go run ./cmd/regionio -seed 12345            # serves on 0.0.0.0:25565
go run ./cmd/regionio -seed 12345 -world ""  # in-memory world, nothing read from or written to disk
go run ./cmd/gendump                         # client-free generator diagnostics

Hard rules

No third-party dependencies. go.mod has none. NBT, zlib framing, MD5 seeding, noise, the density-function interpreter — all in-tree. Keep it that way.

Bump generatorVersion whenever the generator's output changes. It lives in internal/world/store.go and is stamped into every saved chunk; a mismatch makes the chunk regenerate. Without a bump, the chunks already on disk keep their old terrain and your change looks like it did nothing in exactly the area you are standing in — chunkAt prefers the store over the generator. TestGeneratorVersionStampRejectsStaleChunks covers the mechanism. For quick iteration, -world "" sidesteps persistence entirely. (The seed is guarded separately, by the world metadata file, and a seed mismatch is a hard error rather than a regeneration.)

Verify vanilla behaviour against the jar; don't recall it. See below.

Vanilla ground truth

None of these are redistributable, so all are gitignored. Obtain server.jar from Mojang.

Source	What it gives you
.refjava/	Decompiled classes for the parts we port: Climate, SurfaceRules, SurfaceSystem, Aquifer, DensityFunctions, NoiseRouterData, NoiseBasedChunkGenerator, TerrainProvider, PalettedContainer, LevelChunkSection, ClientboundLevelChunkPacketData
versions/26.1.2/server-26.1.2.jar	The real (deobfuscated) server. The outer server.jar is only a bundler
generated/reports/	Datagen output: blocks.json, registries.json, packets.json, biome_parameters/

The inner jar also carries the complete worldgen datapack, which is the source for everything we still approximate: 259 placed_feature, 222 configured_feature, 66 biome (with per-stage features and carvers), 5 configured_carver, 35 structure, 1359 structure NBTs — 648 KB for the feature/biome/carver set.

Three ways in, cheapest first:

unzip -p versions/26.1.2/server-26.1.2.jar data/minecraft/worldgen/biome/plains.json
javap -p -c -classpath versions/26.1.2/server-26.1.2.jar net.minecraft.world.level.chunk.Strategy

javap settles questions the decompiled subset does not cover. It is how the biome palette threshold was pinned down: Strategy$2 switches {0..3} and everything above falls through to the global palette, which .refjava/ alone could not show.

When a constant has to come from vanilla's runtime rather than its source or reports, dump it with a throwaway Java program run against the jar. tools/VanillaLightDump.java is the precedent — it walks the block-state registry and emits opacity, emission and voxel face shapes into internal/world/light_properties.bin. Substring-matching block names is how the light table was wrong before (grass_block matched "grass", bedrock matched "bed"); don't reintroduce that shape of guess anywhere.

Layout

cmd/regionio/     entry point (flags, listener, graceful shutdown)
cmd/gendump/      client-free generator diagnostics — biome spread, surface blocks,
                  subsurface banding, deep-layer composition, bedrock band, fluid census,
                  cross-section
cmd/genblocks/    generates internal/worldgen/generated_blocks.go from the block report
cmd/genlight/     legacy light-table generator, superseded by tools/VanillaLightDump.java
tools/            Java dumpers run against the jar, plus their Go-side fixtures
internal/protocol/  VarInt, framing, compression, packet IDs
internal/nbt/       NBT codec (modified UTF-8)
internal/registry/  28 embedded synced registries + tags, verbatim from vanilla
internal/world/     chunk model, wire encoder, cache + tickets, Anvil store, lighting,
                    the chunk generator itself (vanilla.go), decoration
internal/worldgen/  the library: noise, density-function interpreter, surface rules,
                    aquifer, climate/biome finder, embedded datapack under data/
internal/network/   per-connection state machine
internal/server/    shared core: config, sessions, status, profiles, entity loops

Note the split: internal/worldgen is a library over the datapack; the chunk generator that drives it is internal/world/vanilla.go.

Fidelity status

Bit-exact and parity-tested — treat as settled, change only with a vanilla reference in hand: random.go (Xoroshiro128++, upgradeSeedTo128bit, MD5 seeding), improved_noise.go, perlin_noise.go, normal_noise.go, blended.go, spline.go, density.go, the 4×8×4 cell grid with trilinear interpolation, the climate/biome finder, and the chunk wire encoder (TestGoldenAgainstVanilla compares bytes against a real vanilla chunk).

Ported from the decompiled source and checked by behaviour rather than by bytes — faithful as far as we can tell, but no vanilla capture confirms them: the aquifer (worldgen/aquifer.go, the whole of Aquifer.NoiseBasedAquifer bar shouldScheduleFluidUpdate), the surface-rule interpreter (worldgen/surface.go), and the column pass in world/vanilla.go that mirrors SurfaceSystem.

The whole noise_router is parsed. preliminary_surface_level is reachable through od.PreliminarySurfaceLevelAt, which quart-aligns and memoises across chunks the way NoiseChunk does; the vein_* keys are parsed but nothing reads them yet.

Known gaps, roughly in order of how visible they are:

• above_preliminary_surface is wrong, so there is no subsurface banding: every land column is one grass block directly on stone, no dirt, no sandstone under sand. Vanilla is blockY >= preliminarySurfaceLevel + surfaceDepth - 8 with the level bilinearly interpolated from the four corners of the 16-block cell; we compare against the actual top block, which gates the whole biome surface subtree to a single block per column. gendump prints this. The router value it needs is already available.
• surfaceDepth is always 0. Vanilla is surfaceNoise*2.75 + 3 + rand*0.25, so the dirt band is three-ish blocks deep; ours is one. This is the other half of the missing banding, and it also neuters every add_surface_depth/surface_depth_multiplier term in the rule tree.
• Several surface-rule conditions are stubs: hole is hardcoded false (vanilla is surfaceDepth <= 0), steep is never assigned, the minecraft:surface noise is a per-column random draw rather than the real noise, surface_secondary is not sampled at all (so secondary_depth_range is ignored), and 6 of the 7 noise_threshold noises are unsupported so calcite, ice, packed ice, powder snow, swamp water and gravel patches never appear. bandlands is a 4-colour cycle rather than the 192-band array.
• vertical_gradient ignores absolute anchors, reading only above_bottom. The bedrock floor works because its anchors are above_bottom; the deepslate rule's are absolute 0..8, so both collapse to y=-64 and no deepslate is ever placed — gendump's deep-layer line reads deepslate=0 everywhere.
• No carvers and no ore veins. Caves come only from the density router; configured_carver is not extracted and the OreVeinifier over the parsed vein_* keys is not written.
• Decoration is hand-written heuristics, not the vanilla feature system: oak trees only and without a biome check (so oaks grow in deserts), ores that cannot generate below y≈0 because they only replace stone and never deepslate, and no grass, flowers, lakes or springs.

Testing worldgen

make verify is the gate, but most generator defects are invisible to it — they show up as terrain that looks wrong. cmd/gendump exists for that: biome distribution, top surface blocks, subsurface banding, deep-layer composition, the bedrock band, the underground fluid census, and an ASCII cross-section, with no client involved. Add an assertion to it whenever you fix a class of defect; the bedrock-band check is the model — it prints per-layer counts and fails loudly on any air or water in the floor. The fluid census is the same shape: it prints water as a share of the open volume under inland chunks (3.8% now, 100% before the aquifer) and fails if caves flood again.

Anything gendump can assert on, prefer to also assert in a test — TestCavesAreDry and TestNoFluidUnderBedrock in internal/world are gendump checks that run under make verify.

go test -race needs cgo and a C toolchain; on a Windows box without gcc, make test-race cannot run at all.

internal/world/vanilla_parity_test.go compares surface heights against a capture from the official server and skips when the capture is absent. Note it reads a hardcoded /tmp path, so on Windows it never runs. A capture is produced by running the vanilla server headless at a known seed and reading its region files back with our own regionfile.go + nbt.