Formalise a bit in each cell as an "IsActive()" bit

internal/maps/maps.go

@@ -69,8 +69,8 @@ func (h Header) MapSetFilename() string {
 }
 
 type ObjRef struct {
-	AreaByte            byte
-	FrameAndUnknownByte byte
+	AreaByte          byte
+	SpriteAndFlagByte byte
 }
 
 // The index into a set palette to retrieve the object
@@ -78,8 +78,14 @@ func (o ObjRef) Index() int {
 	return int(o.AreaByte)
 }
 
-func (o ObjRef) Frame() int {
-	return int(o.FrameAndUnknownByte - 0x80)
+func (o ObjRef) Sprite() int {
+	// The top bit seems to be a flag of some kind
+	return int(o.SpriteAndFlagByte & 0x7f)
+}
+
+// The top bit seems to say whether we should draw or not.
+func (o ObjRef) IsActive() bool {
+	return (o.SpriteAndFlagByte & 0x80) == 0x80
 }
 
 type Cell struct {
@@ -108,19 +114,19 @@ func (c *Cell) At(n int) byte {
 	case 3:
 		return c.Surface.AreaByte
 	case 4:
-		return c.Surface.FrameAndUnknownByte
+		return c.Surface.SpriteAndFlagByte
 	case 5:
 		return c.Left.AreaByte
 	case 6:
-		return c.Left.FrameAndUnknownByte
+		return c.Left.SpriteAndFlagByte
 	case 7:
 		return c.Right.AreaByte
 	case 8:
-		return c.Right.FrameAndUnknownByte
+		return c.Right.SpriteAndFlagByte
 	case 9:
 		return c.Center.AreaByte
 	case 10:
-		return c.Center.FrameAndUnknownByte
+		return c.Center.SpriteAndFlagByte
 	case 11:
 		return c.Unknown11
 	case 12:
@@ -139,7 +145,6 @@ func (c *Cell) At(n int) byte {
 // Cells is always a fixed size; use At to get a cell according to x,y,z
 type Cells []Cell
 
-// FIXME: Ordering may be incorrect? I assume z,y,x for now...
 func (c Cells) At(x, y, z int) Cell {
 	return c[(z*MaxLength*MaxWidth)+(y*MaxWidth)+x]
 }