use binary search to find enemy with ID in 'GameUpdate.checkEnemies' function, because we previously relied on the enemy's index

fcore/enemy.sml

@@ -17,6 +17,24 @@ struct
 
   fun generateTree enemyVec = helpGenerateTree (0, enemyVec, QuadTree.empty)
 
+  fun helpFind (findNum, vec, low, high) =
+    (* should only be called when we know enemy already exists in vec *)
+    let
+      val mid = low + ((high - low) div 2)
+      val enemy = Vector.sub (vec, mid)
+      val {id = curNum, x = _, y = _, health = _} = enemy
+    in
+      if curNum = findNum then
+        enemy
+      else if curNum < findNum then
+        helpFind (findNum, vec, mid + 1, high)
+      else
+        helpFind (findNum, vec, low, mid - 1)
+    end
+
+  fun find (findNum, vec) =
+    helpFind (findNum, vec, 0, Vector.length vec - 1)
+
   fun helpGetDrawVec ({x, y, id = _, health = _}, width, height) =
     let
       val wratio = width / 1920.0

fcore/game-type.sml

@@ -162,7 +162,9 @@ struct
       val platformTree = Platform.generateTree platforms
 
       val enemy1 = {id = 1, x = 300, y = 945, health = 5}
-      val enemies = Vector.fromList [enemy1]
+      val enemy2 = {id = 2, x = 555, y = 945, health = 5}
+      val enemy3 = {id = 3, x = 979, y = 945, health = 5}
+      val enemies = Vector.fromList [enemy1, enemy2, enemy3]
       val enemyTree = Enemy.generateTree enemies
     in
       { player = player

fcore/game-update.sml

@@ -33,7 +33,7 @@ struct
               val pHalfW = Player.size div 2
               val pCentreX = x + pHalfW
 
-              val {x = ex, y = ey, ...} = Vector.sub (enemies, id - 1)
+              val {x = ex, y = ey, ...} = Enemy.find (id, enemies)
               val eFinishX = ex + Enemy.size
               val eHalfW = Enemy.size div 2
               val eCentreX = ex + eHalfW
@@ -47,6 +47,15 @@ struct
         end
     | [] => acc
 
+  fun checkEnemiesWhileAttacking (player, enemies, lst, acc) =
+    let
+      open QuadTree
+    in
+      case lst of
+        enemyID :: tl => (* placeholder *) acc
+      | [] => acc
+    end
+
   (* removes enemies from `enemies` vector when that enemy is in collisions *)
   fun filterEnemyCollisions (pos, collisions, enemies: enemy vector, acc) =
     if pos < 0 then