Fix DASH Live edge calculation.

Also added clamping to getSegmentNum in one case where
it was not already implemented, and defined this behavior
property in the getSegmentNum javadoc.

Issue: #262

library/src/main/java/com/google/android/exoplayer/dash/DashChunkSource.java

@@ -349,7 +349,7 @@ public class DashChunkSource implements ChunkSource {
     int segmentNum;
     if (queue.isEmpty()) {
       if (currentManifest.dynamic) {
-        seekPositionUs = getLiveSeekPosition(nowUs, indexUnbounded);
+        seekPositionUs = getLiveSeekPosition(nowUs, indexUnbounded, segmentIndex.isExplicit());
       }
       segmentNum = segmentIndex.getSegmentNum(seekPositionUs);
     } else {
@@ -476,9 +476,10 @@ public class DashChunkSource implements ChunkSource {
    *
    * @param nowUs An estimate of the current server time, in microseconds.
    * @param indexUnbounded True if the segment index for this source is unbounded. False otherwise.
+   * @param indexExplicit True if the segment index is explicit. False otherwise.
    * @return The seek position in microseconds.
    */
-  private long getLiveSeekPosition(long nowUs, boolean indexUnbounded) {
+  private long getLiveSeekPosition(long nowUs, boolean indexUnbounded, boolean indexExplicit) {
     long liveEdgeTimestampUs;
     if (indexUnbounded) {
       liveEdgeTimestampUs = nowUs - currentManifest.availabilityStartTime * 1000;
@@ -491,6 +492,12 @@ public class DashChunkSource implements ChunkSource {
             + segmentIndex.getDurationUs(lastSegmentNum);
         liveEdgeTimestampUs = Math.max(liveEdgeTimestampUs, indexLiveEdgeTimestampUs);
       }
+      if (!indexExplicit) {
+        // Some segments defined by the index may not be available yet. Bound the calculated live
+        // edge based on the elapsed time since the manifest became available.
+        liveEdgeTimestampUs = Math.min(liveEdgeTimestampUs,
+            nowUs - currentManifest.availabilityStartTime * 1000);
+      }
     }
     return liveEdgeTimestampUs - liveEdgeLatencyUs;
   }

library/src/main/java/com/google/android/exoplayer/dash/DashSegmentIndex.java

@@ -28,6 +28,11 @@ public interface DashSegmentIndex {
 
   /**
    * Returns the segment number of the segment containing a given media time.
+   * <p>
+   * If the given media time is outside the range of the index, then the returned segment number is
+   * clamped to {@link #getFirstSegmentNum()} (if the given media time is earlier the start of the
+   * first segment) or {@link #getLastSegmentNum()} (if the given media time is later then the end
+   * of the last segment).
    *
    * @param timeUs The time in microseconds.
    * @return The segment number of the corresponding segment.
@@ -78,4 +83,18 @@ public interface DashSegmentIndex {
    */
   int getLastSegmentNum();
 
+  /**
+   * Returns true if segments are defined explicitly by the index.
+   * <p>
+   * If true is returned, each segment is defined explicitly by the index data, and all of the
+   * listed segments are guaranteed to be available at the time when the index was obtained.
+   * <p>
+   * If false is returned then segment information was derived from properties such as a fixed
+   * segment duration. If the presentation is dynamic, it's possible that only a subset of the
+   * segments are available.
+   *
+   * @return True if segments are defined explicitly by the index. False otherwise.
+   */
+  boolean isExplicit();
+
 }

library/src/main/java/com/google/android/exoplayer/dash/DashWrappingSegmentIndex.java

@@ -74,4 +74,9 @@ public class DashWrappingSegmentIndex implements DashSegmentIndex {
     return Util.binarySearchFloor(segmentIndex.timesUs, timeUs, true, true);
   }
 
+  @Override
+  public boolean isExplicit() {
+    return true;
+  }
+
 }

library/src/main/java/com/google/android/exoplayer/dash/mpd/Representation.java

@@ -277,6 +277,11 @@ public abstract class Representation {
       return segmentBase.getLastSegmentNum();
     }
 
+    @Override
+    public boolean isExplicit() {
+      return segmentBase.isExplicit();
+    }
+
   }
 
 }

library/src/main/java/com/google/android/exoplayer/dash/mpd/SegmentBase.java

@@ -128,15 +128,22 @@ public abstract class SegmentBase {
       this.segmentTimeline = segmentTimeline;
     }
 
+    /**
+     * @see DashSegmentIndex#getSegmentNum(long)
+     */
     public int getSegmentNum(long timeUs) {
+      int lowIndex = getFirstSegmentNum();
+      int highIndex = getLastSegmentNum();
       if (segmentTimeline == null) {
         // All segments are of equal duration (with the possible exception of the last one).
         long durationUs = (duration * C.MICROS_PER_SECOND) / timescale;
-        return startNumber + (int) (timeUs / durationUs);
+        int segmentNum = startNumber + (int) (timeUs / durationUs);
+        // Ensure we stay within bounds.
+        return segmentNum < lowIndex ? lowIndex
+            : highIndex != DashSegmentIndex.INDEX_UNBOUNDED && segmentNum > highIndex ? highIndex
+            : segmentNum;
       } else {
-        // Identify the segment using binary search.
-        int lowIndex = getFirstSegmentNum();
-        int highIndex = getLastSegmentNum();
+        // The high index cannot be unbounded. Identify the segment using binary search.
         while (lowIndex <= highIndex) {
           int midIndex = (lowIndex + highIndex) / 2;
           long midTimeUs = getSegmentTimeUs(midIndex);
@@ -152,6 +159,9 @@ public abstract class SegmentBase {
       }
     }
 
+    /**
+     * @see DashSegmentIndex#getDurationUs(int)
+     */
     public final long getSegmentDurationUs(int sequenceNumber) {
       if (segmentTimeline != null) {
         long duration = segmentTimeline.get(sequenceNumber - startNumber).duration;
@@ -163,6 +173,9 @@ public abstract class SegmentBase {
       }
     }
 
+    /**
+     * @see DashSegmentIndex#getTimeUs(int)
+     */
     public final long getSegmentTimeUs(int sequenceNumber) {
       long unscaledSegmentTime;
       if (segmentTimeline != null) {
@@ -174,14 +187,33 @@ public abstract class SegmentBase {
       return Util.scaleLargeTimestamp(unscaledSegmentTime, C.MICROS_PER_SECOND, timescale);
     }
 
+    /**
+     * Returns a {@link RangedUri} defining the location of a segment for the given index in the
+     * given representation.
+     *
+     * @see DashSegmentIndex#getSegmentUrl(int)
+     */
     public abstract RangedUri getSegmentUrl(Representation representation, int index);
 
+    /**
+     * @see DashSegmentIndex#getFirstSegmentNum()
+     */
     public int getFirstSegmentNum() {
       return startNumber;
     }
 
+    /**
+     * @see DashSegmentIndex#getLastSegmentNum()
+     */
     public abstract int getLastSegmentNum();
 
+    /**
+     * @see DashSegmentIndex#isExplicit()
+     */
+    public boolean isExplicit() {
+      return segmentTimeline != null;
+    }
+
   }
 
   /**
@@ -225,6 +257,11 @@ public abstract class SegmentBase {
       return startNumber + mediaSegments.size() - 1;
     }
 
+    @Override
+    public boolean isExplicit() {
+      return true;
+    }
+
   }
 
   /**