allow user to press up and down to select different options in options

fcore/options/options-update.sml

@@ -1,5 +1,43 @@
 structure OptionsUpdate =
 struct
-  fun update (options, input, userKeys) =
+  open OptionsType
+
+  fun default (options, userKeys) =
     {mode = GameType.OPTIONS options, userKeys = userKeys}
+
+  fun withFocus (newFocus, userKeys) =
+    {mode = GameType.OPTIONS {focus = newFocus}, userKeys = userKeys}
+
+  fun update (options, input: FrameInputType.t, userKeys) =
+    case #focus options of
+      LEFT_KEY =>
+        if #downHeld input then withFocus (RIGHT_KEY, userKeys)
+        else default (options, userKeys)
+    | RIGHT_KEY =>
+        if #upHeld input then withFocus (LEFT_KEY, userKeys)
+        else if #downHeld input then withFocus (UP_KEY, userKeys)
+        else default (options, userKeys)
+    | UP_KEY =>
+        if #upHeld input then withFocus (RIGHT_KEY, userKeys)
+        else if #downHeld input then withFocus (DOWN_KEY, userKeys)
+        else default (options, userKeys)
+    | DOWN_KEY =>
+        if #upHeld input then withFocus (UP_KEY, userKeys)
+        else if #downHeld input then withFocus (JUMP_KEY, userKeys)
+        else default (options, userKeys)
+    | JUMP_KEY =>
+        if #upHeld input then withFocus (DOWN_KEY, userKeys)
+        else if #downHeld input then withFocus (ATTACK_KEY, userKeys)
+        else default (options, userKeys)
+    | ATTACK_KEY =>
+        if #upHeld input then withFocus (JUMP_KEY, userKeys)
+        else if #downHeld input then withFocus (SAVE_BUTTON, userKeys)
+        else default (options, userKeys)
+    | SAVE_BUTTON =>
+        if #upHeld input then withFocus (ATTACK_KEY, userKeys)
+        else if #downHeld input then withFocus (CANCEL_BUTTON, userKeys)
+        else default (options, userKeys)
+    | CANCEL_BUTTON =>
+        if #upHeld input then withFocus (SAVE_BUTTON, userKeys)
+        else default (options, userKeys)
 end