Move MediaPeriodHolder and MediaPeriodHolderQueue to top level

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183065262

library/core/src/main/java/com/google/android/exoplayer2/MediaPeriodHolderQueue.java

+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2;
+
+import com.google.android.exoplayer2.util.Assertions;
+
+/**
+ * Holds a queue of {@link MediaPeriodHolder}s from the currently playing period holder at the front
+ * to the loading period holder at the end of the queue. Also has a reference to the reading period
+ * holder.
+ */
+/* package */ final class MediaPeriodHolderQueue {
+
+  private MediaPeriodHolder playing;
+  private MediaPeriodHolder reading;
+  private MediaPeriodHolder loading;
+  private int length;
+
+  /**
+   * Returns the loading period holder which is at the end of the queue, or null if the queue is
+   * empty.
+   */
+  public MediaPeriodHolder getLoadingPeriod() {
+    return loading;
+  }
+
+  /**
+   * Returns the playing period holder which is at the front of the queue, or null if the queue is
+   * empty or hasn't started playing.
+   */
+  public MediaPeriodHolder getPlayingPeriod() {
+    return playing;
+  }
+
+  /**
+   * Returns the reading period holder, or null if the queue is empty or the player hasn't started
+   * reading.
+   */
+  public MediaPeriodHolder getReadingPeriod() {
+    return reading;
+  }
+
+  /**
+   * Returns the period holder in the front of the queue which is the playing period holder when
+   * playing, or null if the queue is empty.
+   */
+  public MediaPeriodHolder getFrontPeriod() {
+    return hasPlayingPeriod() ? playing : loading;
+  }
+
+  /** Returns the current length of the queue. */
+  public int getLength() {
+    return length;
+  }
+
+  /** Returns whether the reading and playing period holders are set. */
+  public boolean hasPlayingPeriod() {
+    return playing != null;
+  }
+
+  /**
+   * Continues reading from the next period holder in the queue.
+   *
+   * @return The updated reading period holder.
+   */
+  public MediaPeriodHolder advanceReadingPeriod() {
+    Assertions.checkState(reading != null && reading.next != null);
+    reading = reading.next;
+    return reading;
+  }
+
+  /** Enqueues a new period holder at the end, which becomes the new loading period holder. */
+  public void enqueueLoadingPeriod(MediaPeriodHolder mediaPeriodHolder) {
+    Assertions.checkState(mediaPeriodHolder != null);
+    if (loading != null) {
+      Assertions.checkState(hasPlayingPeriod());
+      loading.next = mediaPeriodHolder;
+    }
+    loading = mediaPeriodHolder;
+    length++;
+  }
+
+  /**
+   * Dequeues the playing period holder from the front of the queue and advances the playing period
+   * holder to be the next item in the queue. If the playing period holder is unset, set it to the
+   * item in the front of the queue.
+   *
+   * @return The updated playing period holder, or null if the queue is or becomes empty.
+   */
+  public MediaPeriodHolder advancePlayingPeriod() {
+    if (playing != null) {
+      if (playing == reading) {
+        reading = playing.next;
+      }
+      playing.release();
+      playing = playing.next;
+      length--;
+      if (length == 0) {
+        loading = null;
+      }
+    } else {
+      playing = loading;
+      reading = loading;
+    }
+    return playing;
+  }
+
+  /**
+   * Removes all period holders after the given period holder. This process may also remove the
+   * currently reading period holder. If that is the case, the reading period holder is set to be
+   * the same as the playing period holder at the front of the queue.
+   *
+   * @param mediaPeriodHolder The media period holder that shall be the new end of the queue.
+   * @return Whether the reading period has been removed.
+   */
+  public boolean removeAfter(MediaPeriodHolder mediaPeriodHolder) {
+    Assertions.checkState(mediaPeriodHolder != null);
+    boolean removedReading = false;
+    loading = mediaPeriodHolder;
+    while (mediaPeriodHolder.next != null) {
+      mediaPeriodHolder = mediaPeriodHolder.next;
+      if (mediaPeriodHolder == reading) {
+        reading = playing;
+        removedReading = true;
+      }
+      mediaPeriodHolder.release();
+      length--;
+    }
+    loading.next = null;
+    return removedReading;
+  }
+
+  /** Clears the queue. */
+  public void clear() {
+    MediaPeriodHolder front = getFrontPeriod();
+    if (front != null) {
+      front.release();
+      removeAfter(front);
+    }
+    playing = null;
+    loading = null;
+    reading = null;
+    length = 0;
+  }
+}