add functionality to change grid size for layer tree

fcore/layer-tree.sml

@@ -1,6 +1,8 @@
 structure LayerTree =
 struct
   type square = {r: int, g: int, b: int, a: int}
+  val emptyPixel = {r = 0, g = 0, b = 0, a = 0}
+
   type grid = square vector vector
 
   fun isBlank ({a, ...}: square) = a = 0
@@ -32,6 +34,29 @@ struct
         else if searchKey > key then get (searchKey, right)
         else SOME value
 
+  fun foldl (f, tree, acc) =
+    case tree of
+      LEAF => acc
+    | NODE {value, left, right, ...} =>
+        let
+          val acc = foldl (f, left, acc)
+          val acc = f (value, acc)
+        in
+          foldl (f, right, acc)
+        end
+
+  fun map (f, tree) =
+    case tree of
+      LEAF => LEAF
+    | NODE {key, value, left, right} =>
+        let
+          val left = map (f, left)
+          val right = map (f, right)
+          val newValue = f value
+        in
+          NODE {key = key, value = value, left = left, right = right}
+        end
+
   (* copies non-blank pixels in value vector into acc *)
   fun helpFlatten (value, acc) =
     Vector.mapi
@@ -45,14 +70,28 @@ struct
               else
                 valuePixel) valueYAxis) value
 
-  fun flatten (tree, acc) =
+  fun makeEmptyGrid maxSide =
-    case tree of
+    Vector.tabulate (maxSide, fn _ =>
-      LEAF => acc
+      Vector.tabulate (maxSide, fn _ => emptyPixel))
-    | NODE {value, left, right, ...} =>
+
+  fun flatten (maxSide, tree) =
+    foldl (helpFlatten, tree, makeEmptyGrid maxSide)
+
+  fun helpChangeGridSize maxSide squares =
+    Vector.tabulate (maxSide, fn i =>
+      if i < Vector.length squares then
         let
-          val acc = flatten (left, acc)
+          val yAxis = Vector.sub (squares, i)
-          val acc = helpFlatten (value, acc)
         in
-          flatten (right, acc)
+          Vector.tabulate (maxSide, fn ii =>
+            if ii < Vector.length yAxis then Vector.sub (yAxis, ii)
+            else emptyPixel)
+        end
+      else
+        Vector.tabulate (maxSide, fn _ => emptyPixel))
+
+  fun changeGridSize (maxSide, tree) =
+    let val f = helpChangeGridSize maxSide
+    in map (f, tree)
     end
 end