Render blocks through a ring buffer which is a binary heap ordered by descending y coords. This establisches a stream of partial y ordered blocks to reduce the the over all overdraw. Speeds up rendering about 25%.

common/coords_test.go

@@ -5,6 +5,7 @@
 package common
 
 import (
+	"math"
 	"math/rand"
 	"testing"
 )
@@ -180,6 +181,15 @@ func outsiders(zmin, zmax int64, fn func(int64)) {
 	}
 }
 
+func TestInvert16(t *testing.T) {
+	if invert16(math.MaxInt16) != math.MinInt16 {
+		t.Errorf("invert16(max) != min\n")
+	}
+	if invert16(math.MinInt16) != math.MaxInt16 {
+		t.Errorf("invert16(max) != min\n")
+	}
+}
+
 func TestBigMin(t *testing.T) {
 	const tries = 20
 	for i := 0; i < tries; i++ {