Added a previous level button

src/superposition/game/LevelPlaylist.scala

@@ -15,6 +15,9 @@ final class LevelPlaylist(engine: Engine) {
   /** The list of level factories. */
   private var factories: Seq[LevelFactory] = Nil
 
+  /** The index of the current level. */
+  private var currentIndex: Int = 0
+
   /** The current level. */
   private var _current: Option[Level] = None
 
@@ -31,17 +34,24 @@ final class LevelPlaylist(engine: Engine) {
 
   /** Advances to the next level in the playlist. */
   def next(): Unit = {
-    factories = factories match {
-      case Nil => Nil
-      case _ :: next => next
+    if (currentIndex < factories.size - 1) {
+      currentIndex += 1
+      play()
+    }
+  }
+
+  /** Retreats to the previous level in the playlist. */
+  def prev(): Unit = {
+    if (currentIndex > 0) {
+      currentIndex -= 1
+      play()
     }
-    play()
   }
 
   /** Plays the current level or resets the current level if it is already playing. */
   def play(): Unit = {
     current.foreach(removeLevel(engine))
-    _current = factories.headOption map (_ ())
+    _current = Some(factories(currentIndex)())
     current.foreach(addLevel(engine))
   }
 }

src/superposition/system/LevelSystem.scala

@@ -2,7 +2,7 @@ package superposition.system
 
 import com.badlogic.ashley.core.{Engine, Entity, EntitySystem, Family}
 import com.badlogic.gdx.Gdx.input
-import com.badlogic.gdx.Input.Keys.{N, R}
+import com.badlogic.gdx.Input.Keys._
 import superposition.component._
 import superposition.game.LevelPlaylist
 
@@ -25,15 +25,21 @@ final class LevelSystem(levels: LevelPlaylist) extends EntitySystem {
   }
 
   override def update(deltaTime: Float): Unit = {
+    if (input.isKeyJustPressed(P)) {
+      levels.prev()
+      return
+    }
     if (input.isKeyJustPressed(R)) {
       levels.play()
+      return
     }
     val multiverse = levels.current.get.multiverse
     val allExitSquares = exits.flatMap(ClassicalPosition.mapper.get(_).cells).toSet
     val playersAtExits = players.map(QuantumPosition.mapper.get(_).cell)
       .forall(entityCell => multiverse.universes.map(_.state(entityCell)).forall(allExitSquares.contains))
     if (input.isKeyJustPressed(N) || playersAtExits) {
       levels.next()
+      return
     }
   }
 }