update state of falling enemies per loop

fcore/enemy/falling-enemies.sml

@@ -0,0 +1,61 @@
+structure FallingEnemies =
+struct
+  open GameType
+
+  fun helpGenerateTree (pos, fallingVec: falling_enemy vector, acc) =
+    if pos = Vector.length fallingVec then
+      acc
+    else
+      let
+        val {x, y, ...} = Vector.sub (fallingVec, pos)
+
+        val size = Constants.enemySize
+
+        val acc = QuadTree.insert (x, y, size, size, pos + 1, acc)
+      in
+        helpGenerateTree (pos + 1, fallingVec, acc)
+      end
+
+  fun generateTree fallingVec =
+    helpGenerateTree
+      ( 0
+      , fallingVec
+      , QuadTree.create (Constants.worldWidth, Constants.worldHeight)
+      )
+
+  fun updateList (pos, vec, acc) =
+    if pos < 0 then
+      acc
+    else
+      let
+        val {x, y, jumped, variant} = Vector.sub (vec, pos)
+
+        val size = Constants.enemySize
+        val ww = Constants.worldWidth
+        val wh = Constants.worldHeight
+      in
+        if Collision.isColliding (x, y, size, size, 0, 0, ww, wh) then
+          (* move falling enemy up or down depending on jumped *)
+          let
+            val updated =
+              if jumped < Constants.jumpLimit then
+                { x = x
+                , y = y - Constants.moveEnemyBy
+                , jumped = jumped + Constants.moveEnemyBy
+                , variant = variant
+                }
+              else
+                { x = x
+                , y = y + Constants.moveEnemyBy
+                , jumped = jumped
+                , variant = variant
+                }
+          in
+            updateList (pos - 1, vec, updated :: acc)
+          end
+        else
+          (* if current is not colliding with world's bounds, then filter out 
+           * as it is off screen *)
+          updateList (pos - 1, vec, acc)
+      end
+end