WebVTT parser improvements.

* Split findNextCueHeader and validateWebVttHeader into static methods.
  This is a step toward WebVTT in HLS, where we'll need to re-use these
  to peek at the top of the WebVTT file (they'll be moved into a util
  class).
* Made parser robust against bad cue headers + added a test.
* Removed spurious looking assertion in WebvttSubtitle.

library/src/androidTest/assets/webvtt/with_bad_cue_header

@@ -0,0 +1,10 @@
+WEBVTT # This comment is allowed
+
+00:00.000 --> 00:01.234
+This is the first subtitle.
+
+00:02.badbadbadbadbadbad --> 00:03.456
+This is the second subtitle.
+
+00:04.000 --> 00:05.000
+This is the third subtitle.

library/src/androidTest/java/com/google/android/exoplayer/text/webvtt/WebvttParserTest.java

@@ -33,6 +33,7 @@ public class WebvttParserTest extends InstrumentationTestCase {
   private static final String TYPICAL_WITH_IDS_FILE = "webvtt/typical_with_identifiers";
   private static final String TYPICAL_WITH_COMMENTS_FILE = "webvtt/typical_with_comments";
   private static final String WITH_POSITIONING_FILE = "webvtt/with_positioning";
+  private static final String WITH_BAD_CUE_HEADER_FILE = "webvtt/with_bad_cue_header";
   private static final String WITH_TAGS_FILE = "webvtt/with_tags";
   private static final String EMPTY_FILE = "webvtt/empty";
 
@@ -132,6 +133,20 @@ public class WebvttParserTest extends InstrumentationTestCase {
         Cue.ANCHOR_TYPE_END, 0.1f);
   }
 
+  public void testParseWithBadCueHeader() throws IOException {
+    WebvttParser parser = new WebvttParser();
+    InputStream inputStream =
+        getInstrumentation().getContext().getResources().getAssets().open(WITH_BAD_CUE_HEADER_FILE);
+    WebvttSubtitle subtitle = parser.parse(inputStream);
+
+    // test event count
+    assertEquals(4, subtitle.getEventTimeCount());
+
+    // test cues
+    assertCue(subtitle, 0, 0, 1234000, "This is the first subtitle.");
+    assertCue(subtitle, 2, 4000000, 5000000, "This is the third subtitle.");
+  }
+
   private static void assertCue(WebvttSubtitle subtitle, int eventTimeIndex, long startTimeUs,
       int endTimeUs, String text) {
     assertCue(subtitle, eventTimeIndex, startTimeUs, endTimeUs, text, null, Cue.DIMEN_UNSET,

library/src/main/java/com/google/android/exoplayer/text/webvtt/WebvttParser.java

@@ -23,6 +23,7 @@ import com.google.android.exoplayer.util.MimeTypes;
 
 import android.text.Html;
 import android.text.Layout.Alignment;
+import android.text.TextUtils;
 import android.util.Log;
 
 import java.io.BufferedReader;
@@ -43,9 +44,9 @@ public final class WebvttParser implements SubtitleParser {
   private static final String TAG = "WebvttParser";
 
   private static final Pattern HEADER = Pattern.compile("^\uFEFF?WEBVTT((\u0020|\u0009).*)?$");
-  private static final Pattern COMMENT_BLOCK = Pattern.compile("^NOTE((\u0020|\u0009).*)?$");
+  private static final Pattern COMMENT = Pattern.compile("^NOTE((\u0020|\u0009).*)?$");
   private static final Pattern CUE_HEADER = Pattern.compile("^(\\S+)\\s+-->\\s+(\\S+)(.*)?$");
-  private static final Pattern CUE_SETTING = Pattern.compile("\\S+?:\\S+");
+  private static final Pattern CUE_SETTING = Pattern.compile("(\\S+?):(\\S+)");
 
   private final PositionHolder positionHolder;
   private final StringBuilder textBuilder;
@@ -62,50 +63,25 @@ public final class WebvttParser implements SubtitleParser {
 
   @Override
   public final WebvttSubtitle parse(InputStream inputStream) throws IOException {
-    ArrayList<WebvttCue> subtitles = new ArrayList<>();
-
     BufferedReader webvttData = new BufferedReader(new InputStreamReader(inputStream, C.UTF8_NAME));
-    String line;
 
-    // File should start with "WEBVTT".
+    // Validate the first line of the header, and skip the remainder.
-    line = webvttData.readLine();
+    validateWebvttHeaderLine(webvttData);
-    if (line == null || !HEADER.matcher(line).matches()) {
+    while (!TextUtils.isEmpty(webvttData.readLine())) {}
-      throw new ParserException("Expected WEBVTT. Got " + line);
-    }
-
-    // Parse the remainder of the header.
-    while (true) {
-      line = webvttData.readLine();
-      if (line == null) {
-        // We reached EOF before finishing the header.
-        throw new ParserException("Expected an empty line after webvtt header");
-      } else if (line.isEmpty()) {
-        // We read the newline that separates the header from the body.
-        break;
-      }
-    }
-
-    // Process the cues and text.
-    while ((line = webvttData.readLine()) != null) {
-      // Skip a comment block, if present.
-      Matcher matcher = COMMENT_BLOCK.matcher(line);
-      if (matcher.find()) {
-        // Skip until the end of the comment block.
-        while ((line = webvttData.readLine()) != null && !line.isEmpty()) {
-          // Ignore comment text.
-        }
-        continue;
-      }
-
-      // Skip anything other than a cue header.
-      matcher = CUE_HEADER.matcher(line);
-      if (!matcher.matches()) {
-        continue;
-      }
 
+    ArrayList<WebvttCue> subtitles = new ArrayList<>();
+    Matcher cueHeaderMatcher;
+    while ((cueHeaderMatcher = findNextCueHeader(webvttData)) != null) {
+      long cueStartTime;
+      long cueEndTime;
+      try {
         // Parse the cue start and end times.
-      long cueStartTime = parseTimestampUs(matcher.group(1));
+        cueStartTime = parseTimestampUs(cueHeaderMatcher.group(1));
-      long cueEndTime = parseTimestampUs(matcher.group(2));
+        cueEndTime = parseTimestampUs(cueHeaderMatcher.group(2));
+      } catch (NumberFormatException e) {
+        Log.w(TAG, "Skipping cue with bad header: " + cueHeaderMatcher.group());
+        continue;
+      }
 
       // Default cue settings.
       Alignment cueTextAlignment = null;
@@ -117,12 +93,10 @@ public final class WebvttParser implements SubtitleParser {
       float cueWidth = Cue.DIMEN_UNSET;
 
       // Parse the cue settings list.
-      matcher = CUE_SETTING.matcher(matcher.group(3));
+      Matcher cueSettingMatcher = CUE_SETTING.matcher(cueHeaderMatcher.group(3));
-      while (matcher.find()) {
+      while (cueSettingMatcher.find()) {
-        String match = matcher.group();
+        String name = cueSettingMatcher.group(1);
-        String[] parts = match.split(":", 2);
+        String value = cueSettingMatcher.group(2);
-        String name = parts[0];
-        String value = parts[1];
         try {
           if ("line".equals(name)) {
             parseLineAttribute(value, positionHolder);
@@ -141,7 +115,7 @@ public final class WebvttParser implements SubtitleParser {
             Log.w(TAG, "Unknown cue setting " + name + ":" + value);
           }
         } catch (NumberFormatException e) {
-          Log.w(TAG, e.getMessage() + ": " + match);
+          Log.w(TAG, "Skipping bad cue setting: " + cueSettingMatcher.group());
         }
       }
 
@@ -153,6 +127,7 @@ public final class WebvttParser implements SubtitleParser {
 
       // Parse the cue text.
       textBuilder.setLength(0);
+      String line;
       while ((line = webvttData.readLine()) != null && !line.isEmpty()) {
         if (textBuilder.length() > 0) {
           textBuilder.append("<br>");
@@ -169,6 +144,45 @@ public final class WebvttParser implements SubtitleParser {
     return new WebvttSubtitle(subtitles);
   }
 
+  /**
+   * Reads and validates the first line of a WebVTT file.
+   *
+   * @param input The input from which the line should be read.
+   * @throws ParserException If the line isn't the start of a valid WebVTT file.
+   * @throws IOException If an error occurs reading from the input.
+   */
+  private static void validateWebvttHeaderLine(BufferedReader input) throws IOException {
+    String line = input.readLine();
+    if (line == null || !HEADER.matcher(line).matches()) {
+      throw new ParserException("Expected WEBVTT. Got " + line);
+    }
+  }
+
+  /**
+   * Reads lines up to and including the next WebVTT cue header.
+   *
+   * @param input The input from which lines should be read.
+   * @throws IOException If an error occurs reading from the input.
+   * @return A {@link Matcher} for the WebVTT cue header, or null if the end of the input was
+   *     reached without a cue header being found. In the case that a cue header is found, groups 1,
+   *     2 and 3 of the returned matcher contain the start time, end time and settings list.
+   */
+  private static Matcher findNextCueHeader(BufferedReader input) throws IOException {
+    String line;
+    while ((line = input.readLine()) != null) {
+      if (COMMENT.matcher(line).matches()) {
+        // Skip until the end of the comment block.
+        while ((line = input.readLine()) != null && !line.isEmpty()) {}
+      } else {
+        Matcher cueHeaderMatcher = CUE_HEADER.matcher(line);
+        if (cueHeaderMatcher.matches()) {
+          return cueHeaderMatcher;
+        }
+      }
+    }
+    return null;
+  }
+
   private static long parseTimestampUs(String s) throws NumberFormatException {
     long value = 0;
     String[] parts = s.split("\\.", 2);

library/src/main/java/com/google/android/exoplayer/text/webvtt/WebvttSubtitle.java

@@ -56,7 +56,6 @@ public final class WebvttSubtitle implements Subtitle {
 
   @Override
   public int getNextEventTimeIndex(long timeUs) {
-    Assertions.checkArgument(timeUs >= 0);
     int index = Util.binarySearchCeil(sortedCueTimesUs, timeUs, false, false);
     return index < sortedCueTimesUs.length ? index : -1;
   }