Snap to frame boundary in ConstantBitrateSeeker

- This change snaps the seek position for constant bitrate MP3s
  to the nearest frame boundary, avoiding the need to skip one
  byte at a time to re-synchronize (this may still happen if the
  MP3 does not really have fixed size frames).
- Tweaked both ConstantBitrateSeeker and WavHeader to ensure the
  returned positions are valid.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177441798

library/core/src/androidTest/assets/mp3/play-trimmed.mp3.1.dump

@@ -25,5 +25,9 @@ track 0:
     language = null
     drmInitData = -
     initializationData:
-  sample count = 0
+  sample count = 1
+  sample 0:
+    time = 0
+    flags = 1
+    data = length 418, hash B819987
 tracksEnded = true

library/core/src/androidTest/assets/mp3/play-trimmed.mp3.2.dump

@@ -25,5 +25,9 @@ track 0:
     language = null
     drmInitData = -
     initializationData:
-  sample count = 0
+  sample count = 1
+  sample 0:
+    time = 0
+    flags = 1
+    data = length 418, hash B819987
 tracksEnded = true

library/core/src/androidTest/assets/mp3/play-trimmed.mp3.3.dump

@@ -25,5 +25,9 @@ track 0:
     language = null
     drmInitData = -
     initializationData:
-  sample count = 0
+  sample count = 1
+  sample 0:
+    time = 0
+    flags = 1
+    data = length 418, hash B819987
 tracksEnded = true

library/core/src/main/java/com/google/android/exoplayer2/extractor/mp3/ConstantBitrateSeeker.java

@@ -26,27 +26,47 @@ import com.google.android.exoplayer2.util.Util;
   private static final int BITS_PER_BYTE = 8;
 
   private final long firstFramePosition;
+  private final long dataSize;
+  private final int frameSize;
   private final int bitrate;
   private final long durationUs;
 
-  public ConstantBitrateSeeker(long firstFramePosition, int bitrate, long inputLength) {
+  /**
+   * @param firstFramePosition The position (byte offset) of the first frame.
+   * @param inputLength The length of the stream.
+   * @param frameSize The size of a single frame in the stream.
+   * @param bitrate The stream's bitrate.
+   */
+  public ConstantBitrateSeeker(long firstFramePosition, long inputLength, int frameSize,
+      int bitrate) {
     this.firstFramePosition = firstFramePosition;
+    this.frameSize = frameSize;
     this.bitrate = bitrate;
-    durationUs = inputLength == C.LENGTH_UNSET ? C.TIME_UNSET : getTimeUs(inputLength);
+    if (inputLength == C.LENGTH_UNSET) {
+      dataSize = C.LENGTH_UNSET;
+      durationUs = C.TIME_UNSET;
+    } else {
+      dataSize = inputLength - firstFramePosition;
+      durationUs = getTimeUs(inputLength);
+    }
   }
 
   @Override
   public boolean isSeekable() {
-    return durationUs != C.TIME_UNSET;
+    return dataSize != C.LENGTH_UNSET;
   }
 
   @Override
   public long getPosition(long timeUs) {
-    if (durationUs == C.TIME_UNSET) {
+    if (dataSize == C.LENGTH_UNSET) {
       return firstFramePosition;
     }
-    timeUs = Util.constrainValue(timeUs, 0, durationUs);
+    long positionOffset = (timeUs * bitrate) / (C.MICROS_PER_SECOND * BITS_PER_BYTE);
-    return firstFramePosition + (timeUs * bitrate) / (C.MICROS_PER_SECOND * BITS_PER_BYTE);
+    // Constrain to nearest preceding frame offset.
+    positionOffset = (positionOffset / frameSize) * frameSize;
+    positionOffset = Util.constrainValue(positionOffset, 0, dataSize - frameSize);
+    // Add data start position.
+    return firstFramePosition + positionOffset;
   }
 
   @Override

library/core/src/main/java/com/google/android/exoplayer2/extractor/mp3/Mp3Extractor.java

@@ -393,8 +393,8 @@ public final class Mp3Extractor implements Extractor {
     input.peekFully(scratch.data, 0, 4);
     scratch.setPosition(0);
     MpegAudioHeader.populateHeader(scratch.readInt(), synchronizedHeader);
-    return new ConstantBitrateSeeker(input.getPosition(), synchronizedHeader.bitrate,
+    return new ConstantBitrateSeeker(input.getPosition(), input.getLength(),
-        input.getLength());
+        synchronizedHeader.frameSize, synchronizedHeader.bitrate);
   }
 
   /**

library/core/src/main/java/com/google/android/exoplayer2/extractor/wav/WavHeader.java

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.extractor.wav;
 
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.extractor.SeekMap;
+import com.google.android.exoplayer2.util.Util;
 
 /** Header for a WAV file. */
 /* package */ final class WavHeader implements SeekMap {
@@ -83,10 +84,12 @@ import com.google.android.exoplayer2.extractor.SeekMap;
 
   @Override
   public long getPosition(long timeUs) {
-    long unroundedPosition = (timeUs * averageBytesPerSecond) / C.MICROS_PER_SECOND;
+    long positionOffset = (timeUs * averageBytesPerSecond) / C.MICROS_PER_SECOND;
-    // Round down to nearest frame.
+    // Constrain to nearest preceding frame offset.
-    long position = (unroundedPosition / blockAlignment) * blockAlignment;
+    positionOffset = (positionOffset / blockAlignment) * blockAlignment;
-    return Math.min(position, dataSize - blockAlignment) + dataStartPosition;
+    positionOffset = Util.constrainValue(positionOffset, 0, dataSize - blockAlignment);
+    // Add data start position.
+    return dataStartPosition + positionOffset;
   }
 
   // Misc getters.