From 4fa0a5e55490f33dd4ff00e453c59a18dc72957e Mon Sep 17 00:00:00 2001
From: Daniar Mannanov <daniar@dev.tatar>
Date: Mon, 17 Aug 2026 10:56:05 +0300
Subject: [PATCH] Match very biased height sampling

---
 internal/world/store.go            |  2 +-
 internal/worldgen/features.go      |  5 ++---
 internal/worldgen/features_test.go | 20 ++++++++++++++++++++
 3 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/internal/world/store.go b/internal/world/store.go
index 7011bf6..26b0fd9 100644
--- a/internal/world/store.go
+++ b/internal/world/store.go
@@ -33,7 +33,7 @@ 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 = 19
+const generatorVersion = 20
 
 // generatorVersionTag is the NBT key holding generatorVersion. It is namespaced
 // because it is ours, not part of the vanilla chunk format.
diff --git a/internal/worldgen/features.go b/internal/worldgen/features.go
index 774f966..82a18c5 100644
--- a/internal/worldgen/features.go
+++ b/internal/worldgen/features.go
@@ -210,9 +210,8 @@ func (p PlacementPlan) SampleY(r RandomSource, minY, height int) int {
 		if span-inner <= 0 {
 			return lo
 		}
-		first := lo + inner + int(r.NextIntN(int32(span-inner)))
-		second := lo + int(r.NextIntN(int32(first-lo)))
-		return lo + int(r.NextIntN(int32(second-lo+inner)))
+		first := int(r.NextIntN(int32(span - inner)))
+		return lo + int(r.NextIntN(int32(first+inner)))
 	}
 	return lo + int(r.NextIntN(int32(span)))
 }
diff --git a/internal/worldgen/features_test.go b/internal/worldgen/features_test.go
index 23eb42f..a0aef29 100644
--- a/internal/worldgen/features_test.go
+++ b/internal/worldgen/features_test.go
@@ -137,6 +137,26 @@ func TestPlacementHeightDistributionsStayWithinInclusiveBounds(t *testing.T) {
 	}
 }
 
+func TestVeryBiasedToBottomConsumesVanillaDraws(t *testing.T) {
+	plan := PlacementPlan{
+		HeightDistribution: "minecraft:very_biased_to_bottom",
+		MinY:               HeightProvider{Absolute: intPtr(-64)},
+		MaxY:               HeightProvider{Absolute: intPtr(320)},
+	}
+	gotRandom := NewLegacy(12345)
+	wantRandom := NewLegacy(12345)
+	for sample := 0; sample < 16; sample++ {
+		first := int(wantRandom.NextIntN(377))
+		want := -64 + int(wantRandom.NextIntN(int32(first+8)))
+		if got := plan.SampleY(gotRandom, -64, 384); got != want {
+			t.Fatalf("sample %d = %d, want %d", sample, got, want)
+		}
+	}
+	if got, want := gotRandom.NextLong(), wantRandom.NextLong(); got != want {
+		t.Fatalf("random state after samples = %d, want %d", got, want)
+	}
+}
+
 func TestPlacementPositionsPreservesModifierOrder(t *testing.T) {
 	modifier := func(raw string) PlacementModifier {
 		var value PlacementModifier