Restore the subsurface layers: real above_preliminary_surface and surface depth

Every land column was one block of grass sitting straight on stone. No dirt
under grass, no sandstone under sand, nothing. Two stubs did it together:

above_preliminary_surface compared blockY against the column's actual top block,
so of every position in the column exactly one passed -- and the entire
biome-specific half of the surface rule tree hangs under that condition.
Vanilla compares against a minimum surface level: the preliminary surface level
sampled at the four corners of the 16-block cell, bilinearly interpolated, plus
the surface depth less 8. That is about twenty blocks of reach on ordinary
terrain, which is what the biome subtree is written against.

Surface depth was hardcoded to 0. Vanilla is surfaceNoise*2.75 + 3 with a
per-column jitter, so it comes out around three; it sets how thick the band is
and feeds every add_surface_depth term in the tree. Zero collapsed them all.

Also samples surface_secondary, so stone_depth's secondary_depth_range widens
its band instead of being parsed and dropped.

Grass columns now read grass, two to four dirt, stone -- the histogram over 256
columns is {2: 223, 3: 33}, against vanilla's 2..4. gendump prints it and fails
if the band collapses again; TestGrassColumnsHaveDirt guards it in the suite.

internal/world/banding_verify_test.go

@@ -0,0 +1,67 @@
+package world
+
+import "testing"
+
+// TestGrassColumnsHaveDirt guards the subsurface banding. Before
+// above_preliminary_surface and surfaceDepth were real, every land column read
+// grass-on-stone: the biome subtree was gated to a single block per column and
+// the surface depth that widens the band was hardcoded to zero.
+//
+// Vanilla puts two to four blocks of dirt under the grass cap. The check is
+// deliberately a majority rather than a universal: a column on a steep slope or
+// in a surface "hole" legitimately has none.
+func TestGrassColumnsHaveDirt(t *testing.T) {
+	gen := NewVanillaGenerator(12345)
+	withDirt, total := 0, 0
+	depths := map[int]int{}
+	for cx := int32(-42); cx < -36; cx++ {
+		for cz := int32(-40); cz < -34; cz++ {
+			ch := gen(cx, cz)
+			for lx := 0; lx < 16; lx += 4 {
+				for lz := 0; lz < 16; lz += 4 {
+					topY, ok := grassTop(ch, lx, lz)
+					if !ok {
+						continue
+					}
+					total++
+					depth := 0
+					for y := topY - 1; y >= topY-6; y-- {
+						if ch.GetBlock(lx, y, lz) != StateDirt {
+							break
+						}
+						depth++
+					}
+					depths[depth]++
+					if depth >= 2 {
+						withDirt++
+					}
+				}
+			}
+		}
+	}
+	if total < 50 {
+		t.Fatalf("only %d grass columns found; the scan area has no land", total)
+	}
+	t.Logf("grass columns=%d with a dirt band>=2: %d; depth histogram %v", total, withDirt, depths)
+	if withDirt*4 < total*3 {
+		t.Errorf("only %d of %d grass columns carry a dirt band of 2+; the surface subtree is gated too tightly", withDirt, total)
+	}
+	if depths[6] > total/10 {
+		t.Errorf("%d of %d grass columns have 6+ blocks of dirt; the surface band is running away", depths[6], total)
+	}
+}
+
+// grassTop returns the Y of the column's grass cap, skipping decoration.
+func grassTop(c *Chunk, lx, lz int) (int, bool) {
+	for wy := MinY + WorldHeight - 1; wy >= MinY; wy-- {
+		switch b := c.GetBlock(lx, wy, lz); b {
+		case StateAir, StateWater, StateLava, StateOakLog, StateOakLeaf:
+			continue
+		case StateGrass:
+			return wy, true
+		default:
+			return 0, false
+		}
+	}
+	return 0, false
+}

internal/world/store.go

@@ -32,7 +32,7 @@ const dataVersion26 = 4790
 // first time it ran: chunkAt prefers the store over the generator, so the
 // already-explored area around spawn keeps its old terrain and every later fix
 // looks like it did nothing in exactly the place you are standing.
-const generatorVersion = 3
+const generatorVersion = 4
 
 // generatorVersionTag is the NBT key holding generatorVersion. It is namespaced
 // because it is ours, not part of the vanilla chunk format.

internal/world/vanilla.go

@@ -199,7 +199,7 @@ func fillVanillaColumn(od *worldgen.OverworldDensity, aq *worldgen.Aquifer, flui
 	rng := newColumnRand(wx, wz, int(seed))
 
 	if rule != nil {
-		applySurfaceRule(out, wx, wz, SeaLevel, MinY, biomeName, rule, rng, top)
+		applySurfaceRule(od, out, wx, wz, SeaLevel, MinY, biomeName, rule, rng)
 	} else {
 		fillLegacySurface(out, top, beach, deepWater, rng)
 	}
@@ -241,7 +241,7 @@ func substance(aq *worldgen.Aquifer, fluidPicker worldgen.FluidPicker, x, y, z i
 // One *rand.Rand is created per column (not per block) — bandlands/gradient
 // consume from it sequentially, which is correct because vanilla seeds those
 // per-column too. This avoids ~98k rand.New allocations per chunk.
-func applySurfaceRule(out *[WorldHeight]uint16, wx, wz, seaLevel, minY int, biomeName string, rule worldgen.SurfaceRule, rng chunkRand, topSolid int) {
+func applySurfaceRule(od *worldgen.OverworldDensity, out *[WorldHeight]uint16, wx, wz, seaLevel, minY int, biomeName string, rule worldgen.SurfaceRule, rng chunkRand) {
 	top := -1
 	for i := WorldHeight - 1; i >= 0; i-- {
 		if out[i] != StateAir {
@@ -254,23 +254,23 @@ func applySurfaceRule(out *[WorldHeight]uint16, wx, wz, seaLevel, minY int, biom
 	}
 	// One per-column RNG for all surface rules in this column.
 	colRng := rng.toRand()
-	// Surface noise sample (the "minecraft:surface" noise used by noise_threshold
-	// conditions). Cheap deterministic value derived from the column so the
-	// rule's coarse_dirt/terracotta bands vary per column.
-	surfaceNoise := colRng.Float64()*2 - 1 // [-1, 1]
+	// Column-constant surface quantities, computed once per column exactly as
+	// SurfaceRules.Context.updateXZ does.
+	surfaceDepth := od.Surface.SurfaceDepth(wx, wz)
 	// Reuse one context across the column (mutated per block) to avoid ~98k
 	// heap allocations per chunk; the fields that vary per block are set inside
 	// the loop, the rest are column-constant.
 	sctx := &worldgen.SurfaceContext{
-		X:                  wx,
-		Z:                  wz,
-		SeaLevel:           seaLevel,
-		BiomeName:          biomeName,
-		MinY:               minY,
-		SurfaceNoise:       surfaceNoise,
-		SurfaceDepth:       0,
-		PreliminarySurface: minY + topSolid,
-		Rng:                colRng,
+		X:                wx,
+		Z:                wz,
+		SeaLevel:         seaLevel,
+		BiomeName:        biomeName,
+		MinY:             minY,
+		SurfaceNoise:     od.Surface.Noise(wx, wz),
+		SurfaceSecondary: od.Surface.SurfaceSecondary(wx, wz),
+		SurfaceDepth:     surfaceDepth,
+		MinSurfaceLevel:  od.MinSurfaceLevelAt(wx, wz, surfaceDepth),
+		Rng:              colRng,
 	}
 	stoneDepthAbove := 0
 	waterHeight := worldgen.NoWaterAbove

internal/worldgen/loader.go

@@ -52,6 +52,10 @@ type OverworldDensity struct {
 	// AquiferRandom places the aquifer cell centres.
 	AquiferRandom PositionalRandomFactory
 
+	// Surface samples the noises SurfaceSystem reads per column, before the
+	// rule tree runs.
+	Surface *SurfaceSampler
+
 	prelim *levelCache
 }
 
@@ -148,6 +152,22 @@ func LoadOverworldFinalDensity(seed int64) (*OverworldDensity, error) {
 	// Interpolated nodes are collected as the whole router is parsed, so the
 	// list has to be taken after the loop, not just after final_density.
 	od.Interpolated = l.interpolated
+
+	// SurfaceSystem's own noises. They are not router keys: vanilla pulls them
+	// straight out of the noise registry when it builds the SurfaceSystem.
+	surfaceNoise, err := l.noiseField("minecraft:surface")
+	if err != nil {
+		return nil, fmt.Errorf("surface noise: %w", err)
+	}
+	secondaryNoise, err := l.noiseField("minecraft:surface_secondary")
+	if err != nil {
+		return nil, fmt.Errorf("surface_secondary noise: %w", err)
+	}
+	od.Surface = &SurfaceSampler{
+		surfaceNoise:   surfaceNoise,
+		secondaryNoise: secondaryNoise,
+		positionalRand: l.rs.Positional(),
+	}
 	return od, nil
 }
 

internal/worldgen/randomstate.go

@@ -33,6 +33,11 @@ func (rs *RandomState) AquiferRandom() PositionalRandomFactory { return rs.aquif
 // (RandomState.oreRandom).
 func (rs *RandomState) OreRandom() PositionalRandomFactory { return rs.ore }
 
+// Positional returns the root positional factory (RandomState.random). It is
+// what SurfaceSystem jitters the surface depth with and what the surface rules'
+// vertical_gradient and clay bands derive their own factories from.
+func (rs *RandomState) Positional() PositionalRandomFactory { return rs.factory }
+
 // Noise returns the NormalNoise for the named noise parameters, seeded as
 // NormalNoise.create(factory.fromHashOf(name), params) and cached.
 func (rs *RandomState) Noise(name string, firstOctave int, amplitudes []float64) *NormalNoise {

internal/worldgen/surface.go

@@ -51,12 +51,18 @@ type SurfaceContext struct {
 	// Steep is true when the local slope exceeds the vanilla steep threshold
 	// (~1.0 surface-depth delta between neighbours).
 	Steep bool
-	// SurfaceDepth is the vanilla surface-depth value at this column (a small
-	// noise-driven integer 0..N) added to stone depth comparisons.
+	// SurfaceDepth is how thick the biome's surface layers are at this column
+	// (SurfaceSystem.getSurfaceDepth): usually 3, sometimes 0 or less, which is
+	// what "hole" tests for. It widens the stone_depth bands.
 	SurfaceDepth int
-	// PreliminarySurface is the top solid Y in this column; the
-	// above_preliminary_surface condition passes for blocks above it.
-	PreliminarySurface int
+	// SurfaceSecondary is the "minecraft:surface_secondary" noise at this
+	// column, which widens a stone_depth band further when the rule sets
+	// secondary_depth_range.
+	SurfaceSecondary float64
+	// MinSurfaceLevel is the lowest Y the biome surface subtree may reach:
+	// the interpolated preliminary surface level plus SurfaceDepth less 8.
+	// above_preliminary_surface tests Y against it.
+	MinSurfaceLevel int
 	// Rng is a per-column deterministic source for vertical_gradient and
 	// bandlands. It is seeded by the column so results are stable across runs.
 	Rng *rand.Rand
@@ -253,11 +259,11 @@ func (t stoneDepthTest) Test(ctx *SurfaceContext) bool {
 	if t.addSurfaceDepth {
 		surfaceDepth = ctx.SurfaceDepth
 	}
-	// Vanilla widens the band by map(surface_secondary noise, -1..1, 0..range).
-	// That noise is not sampled yet, so the secondary term stays 0; the two
-	// rules that use it also set add_surface_depth, and both currently reduce to
-	// the same single-block band either way.
-	return depth <= 1+t.offset+surfaceDepth
+	secondary := 0
+	if t.secondaryRange != 0 {
+		secondary = int(mapRange(ctx.SurfaceSecondary, -1.0, 1.0, 0.0, float64(t.secondaryRange)))
+	}
+	return depth <= 1+t.offset+surfaceDepth+secondary
 }
 
 // noiseThresholdTest passes when the named surface noise is within [min,max].
@@ -309,13 +315,12 @@ func (t verticalGradientTest) Test(ctx *SurfaceContext) bool {
 	return ctx.Rng.Float64() > float64(pos)/float64(band)
 }
 
-// abovePreliminarySurfaceTest passes for blocks at or above the column's
-// preliminary surface (the top solid Y). Vanilla gates the biome dispatch on
-// this so submerged blocks far below the surface keep stone.
+// abovePreliminarySurfaceTest gates the whole biome surface subtree: below the
+// column's minimum surface level nothing is dressed and the stone stays stone.
 type abovePreliminarySurfaceTest struct{}
 
 func (abovePreliminarySurfaceTest) Test(ctx *SurfaceContext) bool {
-	return ctx.Y >= ctx.PreliminarySurface
+	return ctx.Y >= ctx.MinSurfaceLevel
 }
 
 // ---- Parser ------------------------------------------------------------

internal/worldgen/surface_system.go

@@ -0,0 +1,61 @@
+package worldgen
+
+import "math"
+
+// surface_system.go holds the per-column quantities SurfaceSystem computes
+// before the rule tree runs: the surface depth (how thick the biome's surface
+// layers are here) and the minimum surface level (how far down the biome
+// subtree is allowed to reach at all).
+//
+// Both were stubbed out — surface depth at a constant 0, minimum surface level
+// at the actual top block — and between them they collapsed every land column
+// to a single block of grass sitting straight on stone.
+
+// SurfaceSampler is the noise half of SurfaceSystem.
+type SurfaceSampler struct {
+	surfaceNoise   *NormalNoise
+	secondaryNoise *NormalNoise
+	positionalRand PositionalRandomFactory
+}
+
+// SurfaceDepth is SurfaceSystem.getSurfaceDepth: roughly three blocks, varied
+// by the surface noise and jittered by a per-column draw. It can come out zero
+// or negative, which is exactly what the "hole" condition looks for.
+func (s *SurfaceSampler) SurfaceDepth(blockX, blockZ int) int {
+	noiseValue := s.surfaceNoise.GetValue(float64(blockX), 0, float64(blockZ))
+	jitter := s.positionalRand.At(blockX, 0, blockZ).NextDouble() * 0.25
+	return int(noiseValue*2.75 + 3.0 + jitter)
+}
+
+// SurfaceSecondary is SurfaceSystem.getSurfaceSecondary, the noise that widens
+// a stone_depth band when the rule sets secondary_depth_range.
+func (s *SurfaceSampler) SurfaceSecondary(blockX, blockZ int) float64 {
+	return s.secondaryNoise.GetValue(float64(blockX), 0, float64(blockZ))
+}
+
+// Noise returns the primary surface noise value at a column, which the
+// noise_threshold conditions on "minecraft:surface" range over.
+func (s *SurfaceSampler) Noise(blockX, blockZ int) float64 {
+	return s.surfaceNoise.GetValue(float64(blockX), 0, float64(blockZ))
+}
+
+// MinSurfaceLevelAt is SurfaceRules.Context.getMinSurfaceLevel: the preliminary
+// surface level sampled at the four corners of the 16-block cell containing the
+// column, bilinearly interpolated, then offset by the surface depth less 8.
+//
+// Every biome-specific surface rule hangs under above_preliminary_surface,
+// which tests blockY against this. Comparing against the column's actual top
+// block instead — what we did before — let exactly one block per column through.
+func (od *OverworldDensity) MinSurfaceLevelAt(blockX, blockZ, surfaceDepth int) int {
+	cellX := blockX >> 4
+	cellZ := blockZ >> 4
+	c00 := float64(od.PreliminarySurfaceLevelAt(cellX<<4, cellZ<<4))
+	c10 := float64(od.PreliminarySurfaceLevelAt((cellX+1)<<4, cellZ<<4))
+	c01 := float64(od.PreliminarySurfaceLevelAt(cellX<<4, (cellZ+1)<<4))
+	c11 := float64(od.PreliminarySurfaceLevelAt((cellX+1)<<4, (cellZ+1)<<4))
+	// Vanilla forms the fractions in float before widening; both are exact here
+	// because the divisor is a power of two.
+	fx := float64(float32(blockX&15) / 16)
+	fz := float64(float32(blockZ&15) / 16)
+	return int(math.Floor(lerp2(fx, fz, c00, c10, c01, c11))) + surfaceDepth - 8
+}

internal/worldgen/surface_test.go

@@ -37,7 +37,7 @@ func TestSurfaceRuleNoPanic(t *testing.T) {
 				X: 100, Y: y, Z: 100,
 				StoneDepthAbove: 100 - y, StoneDepthBelow: y + 1,
 				SeaLevel: 63, BiomeName: b, MinY: -64,
-				PreliminarySurface: 100, WaterHeight: NoWaterAbove,
+				MinSurfaceLevel: 80, WaterHeight: NoWaterAbove,
 				Rng: rand.New(rand.NewSource(1)),
 			}
 			rule.Apply(ctx) // must not panic
@@ -55,7 +55,7 @@ func TestSurfaceBedrockFloor(t *testing.T) {
 	ctx := &SurfaceContext{
 		X: 0, Y: -64, Z: 0, StoneDepthAbove: 1, StoneDepthBelow: 1,
 		SeaLevel: 63, BiomeName: "minecraft:plains", MinY: -64,
-		PreliminarySurface: 70, WaterHeight: NoWaterAbove,
+		MinSurfaceLevel: 62, WaterHeight: NoWaterAbove,
 		Rng: rand.New(rand.NewSource(1)),
 	}
 	state, ok := rule.Apply(ctx)