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Write the section fluid count instead of a hardcoded zero

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LevelChunkSection puts two shorts in front of every section: nonEmptyBlockCount
and fluidCount. We wrote the first and then a literal 0 for the second, under a
comment claiming it was a reserved field that vanilla always leaves at zero.
It is not reserved and vanilla does not.

So every client was told every section is fluid-free, in a world where the
aquifer now fills oceans, lakes and flooded caves. The golden test did not catch
it because its fixture is a superflat chunk whose real fluid count is zero.

The count is per block state, not per block: a waterlogged stair holds a fluid
while a dry one does not, and the flag for that comes from the block-state dump
added with the heightmaps.
This commit is contained in:
Master290 2026-07-27 03:28:49 +03:00
parent 7880531bdb
commit 9e91425c5d
3 changed files with 69 additions and 14 deletions
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31
internal/world/encode.go
View file

@ -391,12 +391,13 @@ func packHeightmap(h [256]uint16) []uint64 {
func (c *Chunk) writeSection(w *protocol.Writer, i int) {
s := c.sections[i]
if s == nil {
w.Uint16(0) // non-air block count
w.Uint16(0) // reserved 2-byte field (always 0 in vanilla)
w.Uint16(0) // nonEmptyBlockCount
w.Uint16(0) // fluidCount
writeSingleValued(w, uint32(StateAir))
} else {
w.Uint16(uint16(nonAirCount(s)))
w.Uint16(0) // reserved 2-byte field
nonEmpty, fluid := sectionCounts(s)
w.Uint16(nonEmpty)
w.Uint16(fluid)
writeBlockPalette(w, s)
}
// Biome container: per-cell palette when present, else the uniform fallback.
@ -407,14 +408,26 @@ func (c *Chunk) writeSection(w *protocol.Writer, i int) {
}
}
func nonAirCount(s *[sectionVol]uint16) int {
n := 0
// sectionCounts is LevelChunkSection.recalcBlockCounts, restricted to the two
// counters that go on the wire: how many blocks are not air, and how many hold
// a fluid.
//
// The second one used to be written as a constant zero, with a comment calling
// it a reserved field. It is not reserved — a client told a section has no
// fluid skips that section when it looks for water to swim in or lava to burn
// on, and the aquifer means almost every section below sea level has some.
// Waterlogged blocks count, which is why the flag is per block state.
func sectionCounts(s *[sectionVol]uint16) (nonEmpty, fluid uint16) {
for _, v := range s {
if v != StateAir {
n++
if v == StateAir {
continue
}
nonEmpty++
if stateFlags(v)&flagFluid != 0 {
fluid++
}
}
return n
return nonEmpty, fluid
}
// writeSingleValued writes a bits-per-entry-0 paletted container (no data).