Support looping with LoopingMediaSource

Now you can do cool things (if you really want to!) like
play a video twice, then play a second video, then loop
the whole thing, all seamlessly.

new LoopingMediaSource(
  new LoopingMediaSource(firstVideoSource, 2),
  secondVideoSource));

You can also just loop, which is probably more useful :).

Issue: #490

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132049599

library/src/main/java/com/google/android/exoplayer2/source/LoopingMediaSource.java

@@ -0,0 +1,162 @@
+/*
+ * Copyright (C) 2016 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.source;
+
+import android.util.Log;
+import android.util.Pair;
+import com.google.android.exoplayer2.C;
+import com.google.android.exoplayer2.Timeline;
+import com.google.android.exoplayer2.source.MediaPeriod.Callback;
+import com.google.android.exoplayer2.upstream.Allocator;
+import com.google.android.exoplayer2.util.Assertions;
+import java.io.IOException;
+
+/**
+ * Loops a {@link MediaSource}.
+ */
+public final class LoopingMediaSource implements MediaSource {
+
+  private static final String TAG = "LoopingMediaSource";
+
+  private final MediaSource childSource;
+  private final int loopCount;
+
+  private int childPeriodCount;
+
+  /**
+   * Loops the provided source indefinitely.
+   *
+   * @param childSource The {@link MediaSource} to loop.
+   */
+  public LoopingMediaSource(MediaSource childSource) {
+    this(childSource, Integer.MAX_VALUE);
+  }
+
+  /**
+   * Loops the provided source a specified number of times.
+   *
+   * @param childSource The {@link MediaSource} to loop.
+   * @param loopCount The desired number of loops. Must be strictly positive. The actual number of
+   *     loops will be capped at the maximum value that can achieved without causing the number of
+   *     periods exposed by the source to exceed {@link Integer#MAX_VALUE}.
+   */
+  public LoopingMediaSource(MediaSource childSource, int loopCount) {
+    Assertions.checkArgument(loopCount > 0);
+    this.childSource = childSource;
+    this.loopCount = loopCount;
+  }
+
+  @Override
+  public void prepareSource(final Listener listener) {
+    childSource.prepareSource(new Listener() {
+      @Override
+      public void onSourceInfoRefreshed(Timeline timeline, Object manifest) {
+        childPeriodCount = timeline.getPeriodCount();
+        listener.onSourceInfoRefreshed(new LoopingTimeline(timeline, loopCount), manifest);
+      }
+    });
+  }
+
+  @Override
+  public void maybeThrowSourceInfoRefreshError() throws IOException {
+    childSource.maybeThrowSourceInfoRefreshError();
+  }
+
+  @Override
+  public MediaPeriod createPeriod(int index, Callback callback, Allocator allocator,
+      long positionUs) {
+    return childSource.createPeriod(index % childPeriodCount, callback, allocator, positionUs);
+  }
+
+  @Override
+  public void releasePeriod(MediaPeriod mediaPeriod) {
+    childSource.releasePeriod(mediaPeriod);
+  }
+
+  @Override
+  public void releaseSource() {
+    childSource.releaseSource();
+  }
+
+  private static final class LoopingTimeline extends Timeline {
+
+    private final Timeline childTimeline;
+    private final int childPeriodCount;
+    private final int childWindowCount;
+    private final int loopCount;
+
+    public LoopingTimeline(Timeline childTimeline, int loopCount) {
+      this.childTimeline = childTimeline;
+      childPeriodCount = childTimeline.getPeriodCount();
+      childWindowCount = childTimeline.getWindowCount();
+      // This is the maximum number of loops that can be performed without overflow.
+      int maxLoopCount = Integer.MAX_VALUE / childPeriodCount;
+      if (loopCount > maxLoopCount) {
+        if (loopCount != Integer.MAX_VALUE) {
+          Log.w(TAG, "Capped loops to avoid overflow:" + loopCount + " -> " + maxLoopCount);
+        }
+        this.loopCount = maxLoopCount;
+      } else {
+        this.loopCount = loopCount;
+      }
+    }
+
+    @Override
+    public int getWindowCount() {
+      return childWindowCount * loopCount;
+    }
+
+    @Override
+    public Window getWindow(int windowIndex, Window window, boolean setIds) {
+      childTimeline.getWindow(windowIndex % childWindowCount, window, setIds);
+      int periodIndexOffset = (windowIndex / childWindowCount) * childPeriodCount;
+      window.firstPeriodIndex += periodIndexOffset;
+      window.lastPeriodIndex += periodIndexOffset;
+      return window;
+    }
+
+    @Override
+    public int getPeriodCount() {
+      return childPeriodCount * loopCount;
+    }
+
+    @Override
+    public Period getPeriod(int periodIndex, Period period, boolean setIds) {
+      childTimeline.getPeriod(periodIndex % childPeriodCount, period, setIds);
+      int loopCount = (periodIndex / childPeriodCount);
+      period.windowIndex += loopCount * childWindowCount;
+      if (setIds) {
+        period.uid = Pair.create(loopCount, period.uid);
+      }
+      return period;
+    }
+
+    @Override
+    public int getIndexOfPeriod(Object uid) {
+      if (!(uid instanceof Pair)) {
+        return C.INDEX_UNSET;
+      }
+      Pair<?, ?> loopCountAndChildUid = (Pair<?, ?>) uid;
+      if (!(loopCountAndChildUid.first instanceof Integer)) {
+        return C.INDEX_UNSET;
+      }
+      int loopCount = (Integer) loopCountAndChildUid.first;
+      int periodIndexOffset = loopCount * childPeriodCount;
+      return childTimeline.getIndexOfPeriod(loopCountAndChildUid.second) + periodIndexOffset;
+    }
+  }
+
+}