Schedule source chunk features globally

internal/world/feature_scheduler.go

@@ -0,0 +1,21 @@
+package world
+
+// decorationSource is a source chunk whose feature pass may inspect or write a
+// target chunk. Vanilla FEATURES has a one-chunk block-state write radius.
+type decorationSource struct {
+	X, Z int32
+}
+
+// decorationSources returns source chunks in deterministic X-major/Z-minor
+// order. Replaying all nine against one mutable 3x3 terrain region makes target
+// output independent of cache request order while preserving each source's own
+// decoration seed and placement origin.
+func decorationSources(targetX, targetZ int32) []decorationSource {
+	sources := make([]decorationSource, 0, 9)
+	for sourceX := targetX - 1; sourceX <= targetX+1; sourceX++ {
+		for sourceZ := targetZ - 1; sourceZ <= targetZ+1; sourceZ++ {
+			sources = append(sources, decorationSource{X: sourceX, Z: sourceZ})
+		}
+	}
+	return sources
+}

internal/world/feature_scheduler_test.go

@@ -0,0 +1,27 @@
+package world
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestDecorationSourcesCoverVanillaFeatureWriteRadius(t *testing.T) {
+	got := decorationSources(4, -7)
+	want := []decorationSource{
+		{3, -8}, {3, -7}, {3, -6},
+		{4, -8}, {4, -7}, {4, -6},
+		{5, -8}, {5, -7}, {5, -6},
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("sources = %v, want %v", got, want)
+	}
+}
+
+func TestDecorationSourcesAreRequestOrderIndependent(t *testing.T) {
+	first := decorationSources(-2, 9)
+	_ = decorationSources(100, -100)
+	second := decorationSources(-2, 9)
+	if !reflect.DeepEqual(first, second) {
+		t.Fatalf("sources changed after unrelated request: %v != %v", first, second)
+	}
+}