Preserve trapezoid height plateaus

internal/worldgen/features.go

@@ -122,6 +122,7 @@ type PlacementPlan struct {
 	Count              CountProvider
 	RarityChance       int
 	HeightDistribution string
+	HeightPlateau      int
 	MinY               HeightProvider
 	MaxY               HeightProvider
 }
@@ -195,13 +196,15 @@ func (p PlacementPlan) SampleY(r RandomSource, minY, height int) int {
 	}
 	span := hi - lo + 1
 	if p.HeightDistribution == "minecraft:trapezoid" {
-		// Vanilla's TrapezoidHeight works with the inclusive range distance,
-		// then performs two inclusive random draws around the midpoint.
 		rangeSize := span - 1
 		if rangeSize <= 0 {
 			return lo
 		}
-		left := rangeSize / 2
+		plateau := p.HeightPlateau
+		if plateau < 0 || plateau > rangeSize {
+			return lo
+		}
+		left := (rangeSize - plateau) / 2
 		right := rangeSize - left
 		return lo + int(r.NextIntN(int32(right+1))) + int(r.NextIntN(int32(left+1)))
 	}
@@ -500,6 +503,7 @@ func (s *FeatureSet) Placement(name string) (PlacementPlan, error) {
 			var value struct {
 				Height struct {
 					Type    string          `json:"type"`
+					Plateau int             `json:"plateau"`
 					Min     json.RawMessage `json:"min_inclusive"`
 					Max     json.RawMessage `json:"max_inclusive"`
 				} `json:"height"`
@@ -519,7 +523,7 @@ func (s *FeatureSet) Placement(name string) (PlacementPlan, error) {
 				value.Height.Type != "minecraft:very_biased_to_bottom" {
 				return PlacementPlan{}, fmt.Errorf("worldgen: %s unsupported height distribution %q", name, value.Height.Type)
 			}
-			plan.HeightDistribution, plan.MinY, plan.MaxY = value.Height.Type, min, max
+			plan.HeightDistribution, plan.HeightPlateau, plan.MinY, plan.MaxY = value.Height.Type, value.Height.Plateau, min, max
 		case "minecraft:in_square", "minecraft:biome", "minecraft:surface_water_depth_filter",
 			"minecraft:heightmap", "minecraft:block_predicate_filter", "minecraft:noise_threshold_count",
 			"minecraft:random_offset":
@@ -746,6 +750,7 @@ func placementHeightPlan(raw json.RawMessage) (PlacementPlan, error) {
 	var value struct {
 		Height struct {
 			Type    string          `json:"type"`
+			Plateau int             `json:"plateau"`
 			Min     json.RawMessage `json:"min_inclusive"`
 			Max     json.RawMessage `json:"max_inclusive"`
 		} `json:"height"`
@@ -765,7 +770,7 @@ func placementHeightPlan(raw json.RawMessage) (PlacementPlan, error) {
 		value.Height.Type != "minecraft:very_biased_to_bottom" {
 		return PlacementPlan{}, fmt.Errorf("worldgen: unsupported height distribution %q", value.Height.Type)
 	}
-	return PlacementPlan{HeightDistribution: value.Height.Type, MinY: min, MaxY: max}, nil
+	return PlacementPlan{HeightDistribution: value.Height.Type, HeightPlateau: value.Height.Plateau, MinY: min, MaxY: max}, nil
 }
 
 func parseIntProvider(raw json.RawMessage) (CountProvider, error) {

internal/worldgen/features_test.go

@@ -2,10 +2,35 @@ package worldgen
 
 import (
 	"encoding/json"
+	"os"
 	"reflect"
 	"testing"
 )
 
+func TestTrapezoidPlateauDiagnostic(t *testing.T) {
+	if os.Getenv("REGIONIO_TRAPEZOID_DIAGNOSTIC") != "1" {
+		t.Skip("set REGIONIO_TRAPEZOID_DIAGNOSTIC=1 to list non-zero height plateaus")
+	}
+	set, err := LoadFeatureSet()
+	if err != nil {
+		t.Fatal(err)
+	}
+	for name, placed := range set.Placed {
+		for _, modifier := range placed.Placement {
+			if modifier.Type != "minecraft:height_range" {
+				continue
+			}
+			plan, err := placementHeightPlan(modifier.Raw)
+			if err != nil {
+				t.Fatalf("%s: %v", name, err)
+			}
+			if plan.HeightDistribution == "minecraft:trapezoid" && plan.HeightPlateau != 0 {
+				t.Logf("%s plateau=%d", name, plan.HeightPlateau)
+			}
+		}
+	}
+}
+
 func TestFeatureDatapackLoadsAndLinks(t *testing.T) {
 	set, err := LoadFeatureSet()
 	if err != nil {
@@ -157,6 +182,27 @@ func TestVeryBiasedToBottomConsumesVanillaDraws(t *testing.T) {
 	}
 }
 
+func TestTrapezoidHeightHonorsPlateau(t *testing.T) {
+	plan := PlacementPlan{
+		HeightDistribution: "minecraft:trapezoid",
+		HeightPlateau:      4,
+		MinY:               HeightProvider{Absolute: intPtr(0)},
+		MaxY:               HeightProvider{Absolute: intPtr(20)},
+	}
+	gotRandom := NewLegacy(12345)
+	wantRandom := NewLegacy(12345)
+	for sample := 0; sample < 16; sample++ {
+		left := (20 - 4) / 2
+		want := int(wantRandom.NextIntN(int32(20-left+1))) + int(wantRandom.NextIntN(int32(left+1)))
+		if got := plan.SampleY(gotRandom, 0, 21); 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