add functions to add and remove pixels from layer-tree

fcore/layer-tree.sml

@@ -1,10 +1,8 @@
 structure LayerTree =
 struct
-  datatype tree =
-    NODE of {key: int, value: Grid.t, left: tree, right: tree}
-  | LEAF
+  datatype t = NODE of {key: int, value: Grid.t, left: t, right: t} | LEAF
 
-  val emptyTree = LEAF
+  val empty = LEAF
 
   fun insert (newKey, newValue, tree) =
     case tree of
@@ -65,15 +63,26 @@ struct
               else
                 valuePixel) valueYAxis) value
 
-  fun makeEmptyGrid maxSide =
-    Vector.tabulate (maxSide, fn _ =>
-      Vector.tabulate (maxSide, fn _ => Grid.emptyPixel))
-
   fun flatten (maxSide, tree) =
-    foldl (helpFlatten, tree, makeEmptyGrid maxSide)
+    foldl (helpFlatten, tree, Grid.makeEmpty maxSide)
 
   fun changeGridSize (maxSide, tree) =
     let val f = Grid.changeGridSize maxSide
     in map (f, tree)
     end
+
+  fun addPixel (key, newX, newY, maxSide, pixel, tree) =
+    let
+      val grid =
+        case get (key, tree) of
+          SOME grid => grid
+        | NONE => Grid.makeEmpty maxSide
+
+      val grid = Grid.updateGrid (grid, newX, newY, pixel)
+    in
+      insert (key, grid, tree)
+    end
+
+  fun removePixel (key, newX, newY, maxSide, tree) =
+    addPixel (key, newX, newY, maxSide, Grid.emptyPixel, tree)
 end