add functionality to move player by using arrow keys

shell/input-state.sml

@@ -0,0 +1,66 @@
+structure InputState =
+struct
+  (* global state detecting button inputs *)
+  val state =
+    { leftHeld = ref false
+    , rightHeld = ref false
+    , upHeld = ref false
+    , downHeld = ref false
+    }
+
+  fun getSnapshot () =
+    { leftHeld = !(#leftHeld state)
+    , rightHeld = !(#rightHeld state)
+    , upHeld = !(#upHeld state)
+    , downHeld = !(#downHeld state)
+    }
+
+  fun getPlayerXAxis () =
+    let
+      val lh = #leftHeld state
+      val rh = #rightHeld state
+
+      open Player
+    in
+      case (!lh, !rh) of
+        (false, false) => STAY_STILL
+      | (false, true) => MOVE_RIGHT
+      | (true, false) => MOVE_LEFT
+      | (true, true) => STAY_STILL
+    end
+
+  open Input
+
+  fun handleKey (key, action) =
+    if key = ARROW_UP then
+      if action = PRESS then (#upHeld state) := true
+      else if action = RELEASE then (#upHeld state) := false
+      else ()
+    else if key = ARROW_DOWN then
+      if action = PRESS then (#downHeld state) := true
+      else if action = RELEASE then (#downHeld state) := false
+      else ()
+    else if key = ARROW_LEFT then
+      if action = PRESS then (#leftHeld state) := true
+      else if action = RELEASE then (#leftHeld state) := false
+      else ()
+    else if key = ARROW_RIGHT then
+      if action = PRESS then (#rightHeld state) := true
+      else if action = RELEASE then (#rightHeld state) := false
+      else ()
+    else
+      ()
+
+  fun keyCallback (key, scancode, action, mods) =
+    let open Input
+    in if mods = 0 then handleKey (key, action) else ()
+    end
+
+  fun registerCallbacks window =
+    let
+      val () = Input.exportKeyCallback keyCallback
+      val () = Input.setKeyCallback window
+    in
+      ()
+    end
+end