Fix terrain streaming and surface spawning

internal/world/spawn_surface_test.go

@@ -0,0 +1,41 @@
+package world
+
+import "testing"
+
+func TestSafeSpawnYUsesGeneratedSurface(t *testing.T) {
+	cache := NewCache(-1, GenerateFlat)
+	y, ok := cache.SafeSpawnY(8, 8)
+	if !ok || y != FlatSurfaceY+1 {
+		t.Fatalf("SafeSpawnY = %d, %v; want %d, true", y, ok, FlatSurfaceY+1)
+	}
+}
+
+func TestSafeSpawnYRejectsUnderwaterColumn(t *testing.T) {
+	cache := NewCache(-1, func(cx, cz int32) *Chunk {
+		chunk := NewChunk(cx, cz, BiomePlains)
+		chunk.setBlockRaw(8, 60, 8, StateStone)
+		for y := 61; y <= SeaLevel; y++ {
+			chunk.setBlockRaw(8, y, 8, StateWater)
+		}
+		return chunk
+	})
+	if y, ok := cache.SafeSpawnY(8, 8); ok {
+		t.Fatalf("SafeSpawnY = %d, true; want underwater column rejected", y)
+	}
+}
+
+func TestSafeSpawnYAcceptsNonOpaqueSolidFloor(t *testing.T) {
+	stairs := nameToStateID("minecraft:oak_stairs", nil)
+	if stairs == StateAir {
+		t.Fatal("oak stairs state is unavailable")
+	}
+	cache := NewCache(-1, func(cx, cz int32) *Chunk {
+		chunk := NewChunk(cx, cz, BiomePlains)
+		chunk.setBlockRaw(8, 70, 8, stairs)
+		return chunk
+	})
+	y, ok := cache.SafeSpawnY(8, 8)
+	if !ok || y != 71 {
+		t.Fatalf("SafeSpawnY = %d, %v; want 71, true for stairs", y, ok)
+	}
+}