Ore veins from the noise router

The router has carried vein_toggle, vein_ridged and vein_gap since the whole of
it was parsed, and nothing read them. So the copper and iron mega-veins -- the
long branching sheets that run through the deepslate and the copper band, not
the small scattered ore blobs -- did not exist.

OreVeinifier is not a decoration feature. It is the second entry of the same
MaterialRuleList the aquifer heads: the aquifer answers first, and only where it
says the position is solid rock does the veinifier get a turn at what would
otherwise be plain stone. That is why a vein never opens into a cave, and it is
why this lands in the density pass rather than in decorate.

Three random draws per position, in a fixed order, from a factory hashed off
"minecraft:ore": solidness, then richness, then the rare raw-ore block.
Reordering them or hoisting the vein_gap compute above the second draw would
change every vein in the world, so the code follows the bytecode's order rather
than the one that reads better.

The Y windows are the sharp edge worth knowing about: a vein's type comes from
the sign of the veininess noise, but its window comes from the type, so
veininess <= 0 anywhere outside -60..-8 is simply not a vein. Over 49 sampled
chunks copper lands in 0..49 and iron in -57..-8, and raw ore comes out at 1.9%
of ore blocks against vanilla's 2%.

internal/worldgen/loader.go

@@ -50,8 +50,10 @@ type OverworldDensity struct {
 	AquifersEnabled bool
 	OreVeinsEnabled bool
 
-	// AquiferRandom places the aquifer cell centres.
+	// AquiferRandom places the aquifer cell centres; OreRandom rolls the
+	// per-position ore-vein draws.
 	AquiferRandom PositionalRandomFactory
+	OreRandom     PositionalRandomFactory
 
 	// Surface samples the noises SurfaceSystem reads per column, before the
 	// rule tree runs.
@@ -103,6 +105,7 @@ func LoadOverworldFinalDensity(seed int64) (*OverworldDensity, error) {
 		AquifersEnabled: settings.AquifersEnabled,
 		OreVeinsEnabled: settings.OreVeinsEnabled,
 		AquiferRandom:   l.rs.AquiferRandom(),
+		OreRandom:       l.rs.OreRandom(),
 		prelim:          newLevelCache(),
 	}
 

internal/worldgen/orevein.go

@@ -0,0 +1,103 @@
+package worldgen
+
+import "math"
+
+// orevein.go ports net.minecraft.world.level.levelgen.OreVeinifier.
+//
+// Ore veins are not a decoration feature: they run during the density pass, as
+// the second entry of the same MaterialRuleList the aquifer heads. Where the
+// aquifer says "this position is solid rock", the veinifier gets a chance to
+// replace what would have been plain stone with copper or iron and its
+// surrounding filler. That is why veins are long branching sheets rather than
+// blobs, and why they never open into a cave: a position the aquifer hollowed
+// out never reaches this code.
+
+// Vein constants, as widened from OreVeinifier's float fields. Written out
+// rather than computed so the double each float comparison widens to is
+// unambiguous.
+const (
+	veininessThreshold       = 0.4000000059604645  // (double)(float)0.4f
+	edgeRoundoffBegin        = 20.0                // an int field, used as a double
+	maxEdgeRoundoff          = -0.2                // a real double literal, not a widened float
+	minRichness              = 0.10000000149011612 // (double)(float)0.1f
+	maxRichness              = 0.30000001192092896 // (double)(float)0.3f
+	maxRichnessThreshold     = 0.6000000238418579  // (double)(float)0.6f
+	skipOreIfGapNoiseIsBelow = -0.3000000119209290 // (double)(float)-0.3f
+
+	veinSolidness       float32 = 0.7  // compared as a float
+	chanceOfRawOreBlock float32 = 0.02 // compared as a float
+)
+
+// veinType is OreVeinifier.VeinType. Note the asymmetry: copper's ore is the
+// plain stone variant while iron's is the deepslate one, and neither switches
+// with depth — the block is fixed per type.
+type veinType struct {
+	ore, rawOreBlock, filler uint16
+	minY, maxY               int
+}
+
+var (
+	veinCopper = veinType{ore: 25313, rawOreBlock: 29578, filler: 2, minY: 0, maxY: 50}
+	veinIron   = veinType{ore: 132, rawOreBlock: 29577, filler: 23452, minY: -60, maxY: -8}
+)
+
+// OreVeinifier decides whether a solid position becomes part of an ore vein.
+type OreVeinifier struct {
+	toggle, ridged, gap DensityFunction
+	random              PositionalRandomFactory
+}
+
+// NewOreVeinifier returns nil when the settings or the router leave veins off,
+// which the caller reads as "always place the default block".
+func NewOreVeinifier(od *OverworldDensity) *OreVeinifier {
+	if !od.OreVeinsEnabled || od.VeinToggle == nil || od.VeinRidged == nil || od.VeinGap == nil || od.OreRandom == nil {
+		return nil
+	}
+	return &OreVeinifier{toggle: od.VeinToggle, ridged: od.VeinRidged, gap: od.VeinGap, random: od.OreRandom}
+}
+
+// Calculate returns the block a vein places at this position, or ok=false to
+// leave the default block. ctx must carry the cell-interpolated values: both
+// vein_toggle and vein_ridged are minecraft:interpolated in the datapack, while
+// vein_gap is a plain per-block noise.
+//
+// The three random draws happen in a fixed order and are the whole shape of the
+// output; reordering them, or hoisting the vein_gap compute above the second
+// draw, changes every vein in the world.
+func (o *OreVeinifier) Calculate(ctx FunctionContext, x, y, z int) (uint16, bool) {
+	veininess := o.toggle.Compute(ctx)
+	vein := &veinIron
+	if veininess > 0 {
+		vein = &veinCopper
+	}
+	// The type comes from the sign of the noise but the Y window comes from the
+	// type, so a position outside its type's band is simply not a vein — which
+	// is what keeps copper above 0 and iron below -8.
+	distanceToTop := vein.maxY - y
+	distanceToBottom := y - vein.minY
+	if distanceToBottom < 0 || distanceToTop < 0 {
+		return 0, false
+	}
+	edgeDistance := min(distanceToTop, distanceToBottom)
+	edgeRoundoff := clampedMap(float64(edgeDistance), 0.0, edgeRoundoffBegin, maxEdgeRoundoff, 0.0)
+	absVeininess := math.Abs(veininess)
+	if absVeininess+edgeRoundoff < veininessThreshold {
+		return 0, false
+	}
+
+	random := o.random.At(x, y, z)
+	if random.NextFloat() > veinSolidness {
+		return 0, false
+	}
+	if o.ridged.Compute(ctx) >= 0 {
+		return 0, false
+	}
+	richness := clampedMap(absVeininess, veininessThreshold, maxRichnessThreshold, minRichness, maxRichness)
+	if float64(random.NextFloat()) < richness && o.gap.Compute(ctx) > skipOreIfGapNoiseIsBelow {
+		if random.NextFloat() < chanceOfRawOreBlock {
+			return vein.rawOreBlock, true
+		}
+		return vein.ore, true
+	}
+	return vein.filler, true
+}