Horizon searched its own spiral of 48-block steps from 0 0 for a dry,
non-leaf top block, so seed 1891623539983467248 spawned at -48 -48 where
vanilla puts the world spawn at 144 71 144 (the player then lands within
the respawn radius, 152 145 in the report).
Vanilla (MinecraftServer.setInitialSpawn): NoiseBasedChunkGenerator.
getOrigin runs NoiseSpawnFinder over the noise settings' spawn_target
points (SpawnTargetPoint.sampleFitness, a 2048/512 then 512/32 radial
search biased towards 0 0); an 11 by 11 chunk spiral around that chunk
takes the first column PlayerSpawnFinder.getSpawnPosInChunk accepts
(getLevelRespawnPos: MOTION_BLOCKING, WORLD_SURFACE and OCEAN_FLOOR
heights, no liquid, a full-topped collision shape); without one, the
origin chunk's centre at the spawn height. The first player is then
placed by PlayerSpawnFinder.findSpawn: the respawn-radius square in a
random coprime order, a free player box with liquids colliding, else the
spawn with fixupSpawnHeight; adventure mode uses the spawn as is.
horizon-world gains worldgen::spawn_finder and OverworldGenerator::origin;
the client's world_spawn module and game/initial_spawn.rs follow
setInitialSpawn and PlayerSpawnFinder, for single player and the
dedicated server alike. Flat worlds use FlatLevelSource's origin and
spawn height.
tools/test_world_spawn_parity.py compares, for the report's seed, the
chunk harness's seeds and 50 random ones, the origins
(VanillaWorldSpawnOracle against the spawn_origin example) and the
level.dat spawn of worlds created by the vanilla dedicated server and by
Horizon's. Vanilla is not deterministic on chunk corners where trees of
neighbouring chunks overlap (decoration order follows its worker
threads), so a mismatching seed reruns vanilla up to three times.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Ludovic Marie