Add support for variable substition in HLS

Issue:#4422

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=209958623

RELEASENOTES.md

@@ -70,6 +70,8 @@
   * Allow configuration of the Loader retry delay
     ([#3370](https://github.com/google/ExoPlayer/issues/3370)).
 * HLS:
+  * Add support for variable substitution
+    ([#4422](https://github.com/google/ExoPlayer/issues/4422)).
   * Add support for PlayReady.
   * Add support for alternative EXT-X-KEY tags.
   * Set the bitrate on primary track sample formats

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/playlist/HlsMasterPlaylist.java

@@ -21,6 +21,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 
 /** Represents an HLS master playlist. */
 public final class HlsMasterPlaylist extends HlsPlaylist {
@@ -35,7 +36,8 @@ public final class HlsMasterPlaylist extends HlsPlaylist {
           /* subtitles= */ Collections.emptyList(),
           /* muxedAudioFormat= */ null,
           /* muxedCaptionFormats= */ Collections.emptyList(),
-          /* hasIndependentSegments= */ false);
+          /* hasIndependentSegments= */ false,
+          /* variableDefinitions= */ Collections.emptyMap());
 
   public static final int GROUP_INDEX_VARIANT = 0;
   public static final int GROUP_INDEX_AUDIO = 1;
@@ -110,6 +112,8 @@ public HlsUrl(String url, Format format) {
    * captions information.
    */
   public final List<Format> muxedCaptionFormats;
+  /** Contains variable definitions, as defined by the #EXT-X-DEFINE tag. */
+  public final Map<String, String> variableDefinitions;
 
   /**
    * @param baseUri See {@link #baseUri}.
@@ -120,6 +124,7 @@ public HlsUrl(String url, Format format) {
    * @param muxedAudioFormat See {@link #muxedAudioFormat}.
    * @param muxedCaptionFormats See {@link #muxedCaptionFormats}.
    * @param hasIndependentSegments See {@link #hasIndependentSegments}.
+   * @param variableDefinitions See {@link #variableDefinitions}.
    */
   public HlsMasterPlaylist(
       String baseUri,
@@ -129,14 +134,16 @@ public HlsMasterPlaylist(
       List<HlsUrl> subtitles,
       Format muxedAudioFormat,
       List<Format> muxedCaptionFormats,
-      boolean hasIndependentSegments) {
+      boolean hasIndependentSegments,
+      Map<String, String> variableDefinitions) {
     super(baseUri, tags, hasIndependentSegments);
     this.variants = Collections.unmodifiableList(variants);
     this.audios = Collections.unmodifiableList(audios);
     this.subtitles = Collections.unmodifiableList(subtitles);
     this.muxedAudioFormat = muxedAudioFormat;
     this.muxedCaptionFormats = muxedCaptionFormats != null
         ? Collections.unmodifiableList(muxedCaptionFormats) : null;
+    this.variableDefinitions = Collections.unmodifiableMap(variableDefinitions);
   }
 
   @Override
@@ -149,7 +156,8 @@ public HlsMasterPlaylist copy(List<StreamKey> streamKeys) {
         copyRenditionsList(subtitles, GROUP_INDEX_SUBTITLE, streamKeys),
         muxedAudioFormat,
         muxedCaptionFormats,
-        hasIndependentSegments);
+        hasIndependentSegments,
+        variableDefinitions);
   }
 
   /**
@@ -169,7 +177,8 @@ public static HlsMasterPlaylist createSingleVariantMasterPlaylist(String variant
         emptyList,
         /* muxedAudioFormat= */ null,
         /* muxedCaptionFormats= */ null,
-        /* hasIndependentSegments= */ false);
+        /* hasIndependentSegments= */ false,
+        /* variableDefinitions= */ Collections.emptyMap());
   }
 
   private static List<HlsUrl> copyRenditionsList(