Merge consecutive segments for downloading.

This speeds up downloads where segments have the same URL with different
byte ranges. We limit the merged segments to 20 seconds to ensure the download
progress of demuxed streams is roughly in line with the playable media duration.

Issue:#5978
PiperOrigin-RevId: 280410761

RELEASENOTES.md

@@ -137,6 +137,8 @@
     [Cast demo app](https://github.com/google/ExoPlayer/tree/dev-v2/demos/cast).
 * TestUtils: Publish the `testutils` module to simplify unit testing with
   ExoPlayer ([#6267](https://github.com/google/ExoPlayer/issues/6267)).
+* Downloads: Merge downloads in `SegmentDownloader` to improve overall download
+  speed ([#5978](https://github.com/google/ExoPlayer/issues/5978)).
 
 ### 2.10.7 (2019-11-12) ###
 

library/core/src/main/java/com/google/android/exoplayer2/offline/SegmentDownloader.java

@@ -25,11 +25,13 @@
 import com.google.android.exoplayer2.upstream.cache.CacheDataSource;
 import com.google.android.exoplayer2.upstream.cache.CacheKeyFactory;
 import com.google.android.exoplayer2.upstream.cache.CacheUtil;
+import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.PriorityTaskManager;
 import com.google.android.exoplayer2.util.Util;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicBoolean;
 
@@ -62,6 +64,7 @@ public int compareTo(Segment other) {
   }
 
   private static final int BUFFER_SIZE_BYTES = 128 * 1024;
+  private static final long MAX_MERGED_SEGMENT_START_TIME_DIFF_US = 20 * C.MICROS_PER_SECOND;
 
   private final DataSpec manifestDataSpec;
   private final Cache cache;
@@ -108,6 +111,8 @@ public final void download(@Nullable ProgressListener progressListener)
         manifest = manifest.copy(streamKeys);
       }
       List<Segment> segments = getSegments(dataSource, manifest, /* allowIncompleteList= */ false);
+      Collections.sort(segments);
+      mergeSegments(segments, cacheKeyFactory);
 
       // Scan the segments, removing any that are fully downloaded.
       int totalSegments = segments.size();
@@ -134,7 +139,6 @@ public final void download(@Nullable ProgressListener progressListener)
           contentLength = C.LENGTH_UNSET;
         }
       }
-      Collections.sort(segments);
 
       // Download the segments.
       @Nullable ProgressNotifier progressNotifier = null;
@@ -232,6 +236,44 @@ protected static DataSpec getCompressibleDataSpec(Uri uri) {
         /* flags= */ DataSpec.FLAG_ALLOW_GZIP);
   }
 
+  private static void mergeSegments(List<Segment> segments, CacheKeyFactory keyFactory) {
+    HashMap<String, Integer> lastIndexByCacheKey = new HashMap<>();
+    int nextOutIndex = 0;
+    for (int i = 0; i < segments.size(); i++) {
+      Segment segment = segments.get(i);
+      String cacheKey = keyFactory.buildCacheKey(segment.dataSpec);
+      @Nullable Integer lastIndex = lastIndexByCacheKey.get(cacheKey);
+      @Nullable Segment lastSegment = lastIndex == null ? null : segments.get(lastIndex);
+      if (lastSegment == null
+          || segment.startTimeUs > lastSegment.startTimeUs + MAX_MERGED_SEGMENT_START_TIME_DIFF_US
+          || !canMergeSegments(lastSegment.dataSpec, segment.dataSpec)) {
+        lastIndexByCacheKey.put(cacheKey, nextOutIndex);
+        segments.set(nextOutIndex, segment);
+        nextOutIndex++;
+      } else {
+        long mergedLength =
+            segment.dataSpec.length == C.LENGTH_UNSET
+                ? C.LENGTH_UNSET
+                : lastSegment.dataSpec.length + segment.dataSpec.length;
+        DataSpec mergedDataSpec = lastSegment.dataSpec.subrange(/* offset= */ 0, mergedLength);
+        segments.set(
+            Assertions.checkNotNull(lastIndex),
+            new Segment(lastSegment.startTimeUs, mergedDataSpec));
+      }
+    }
+    Util.removeRange(segments, /* fromIndex= */ nextOutIndex, /* toIndex= */ segments.size());
+  }
+
+  private static boolean canMergeSegments(DataSpec dataSpec1, DataSpec dataSpec2) {
+    return dataSpec1.uri.equals(dataSpec2.uri)
+        && dataSpec1.length != C.LENGTH_UNSET
+        && (dataSpec1.absoluteStreamPosition + dataSpec1.length == dataSpec2.absoluteStreamPosition)
+        && Util.areEqual(dataSpec1.key, dataSpec2.key)
+        && dataSpec1.flags == dataSpec2.flags
+        && dataSpec1.httpMethod == dataSpec2.httpMethod
+        && dataSpec1.httpRequestHeaders.equals(dataSpec2.httpRequestHeaders);
+  }
+
   private static final class ProgressNotifier implements CacheUtil.ProgressListener {
 
     private final ProgressListener progressListener;