Limit CEA-608 captions to 32 chars per line

ANSI/CTA-608-E R-2014 spec defines exactly 32 columns on the screen,
and limits all lines to this length.

See 3.2.2 definition of 'Column'.

issue:#7341
PiperOrigin-RevId: 311549881

RELEASENOTES.md

@@ -123,6 +123,8 @@
     *   Stop parsing unsupported WebVTT CSS properties. The spec provides an
         [exhaustive list](https://www.w3.org/TR/webvtt1/#the-cue-pseudo-element)
         of which are supported.
+    *   Ignore excess characters in CEA-608 lines (max length is 32)
+        ([#7341](https://github.com/google/ExoPlayer/issues/7341)).
 *   DRM:
     *   Add support for attaching DRM sessions to clear content in the demo app.
     *   Remove `DrmSessionManager` references from all renderers.
@@ -188,8 +190,7 @@
 *   IMA extension: Upgrade to IMA SDK version 3.18.2, and migrate to new
     preloading APIs ([#6429](https://github.com/google/ExoPlayer/issues/6429)).
 *   IMA extension:
-    *    Upgrade to IMA SDK version 3.19.0, and migrate to new
+    *   Upgrade to IMA SDK version 3.19.0, and migrate to new preloading APIs
-         preloading APIs
         ([#6429](https://github.com/google/ExoPlayer/issues/6429)).
     *   Add support for timing out ad preloading, to avoid playback getting
         stuck if an ad group unexpectedly fails to load.

library/common/src/main/java/com/google/android/exoplayer2/util/Util.java

@@ -1291,6 +1291,36 @@ public final class Util {
     return (toUnsignedLong(mostSignificantBits) << 32) | toUnsignedLong(leastSignificantBits);
   }
 
+  /**
+   * Truncates a sequence of ASCII characters to a maximum length.
+   *
+   * <p><b>Note:</b> This is not safe to use in general on Unicode text because it may separate
+   * characters from combining characters or split up surrogate pairs.
+   *
+   * @param sequence The character sequence to truncate.
+   * @param maxLength The max length to truncate to.
+   * @return {@code sequence} directly if {@code sequence.length() <= maxLength}, otherwise {@code
+   *     sequence.subsequence(0, maxLength}.
+   */
+  public static CharSequence truncateAscii(CharSequence sequence, int maxLength) {
+    return sequence.length() <= maxLength ? sequence : sequence.subSequence(0, maxLength);
+  }
+
+  /**
+   * Truncates a string of ASCII characters to a maximum length.
+   *
+   * <p><b>Note:</b> This is not safe to use in general on Unicode text because it may separate
+   * characters from combining characters or split up surrogate pairs.
+   *
+   * @param string The string to truncate.
+   * @param maxLength The max length to truncate to.
+   * @return {@code string} directly if {@code string.length() <= maxLength}, otherwise {@code
+   *     string.substring(0, maxLength}.
+   */
+  public static String truncateAscii(String string, int maxLength) {
+    return string.length() <= maxLength ? string : string.substring(0, maxLength);
+  }
+
   /**
    * Returns a byte array containing values parsed from the hex string provided.
    *

library/common/src/test/java/com/google/android/exoplayer2/util/UtilTest.java

@@ -24,9 +24,14 @@ import static com.google.android.exoplayer2.util.Util.parseXsDuration;
 import static com.google.android.exoplayer2.util.Util.unescapeFileName;
 import static com.google.common.truth.Truth.assertThat;
 
+import android.text.SpannableString;
+import android.text.Spanned;
+import android.text.style.StrikethroughSpan;
+import android.text.style.UnderlineSpan;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.testutil.TestUtil;
+import com.google.android.exoplayer2.testutil.truth.SpannedSubject;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 import java.util.ArrayList;
@@ -713,6 +718,40 @@ public class UtilTest {
     assertThat(Util.toLong(0xFEDCBA, 0x87654321)).isEqualTo(0xFEDCBA_87654321L);
   }
 
+  @Test
+  public void truncateAscii_shortInput_returnsInput() {
+    String input = "a short string";
+
+    assertThat(Util.truncateAscii(input, 100)).isSameInstanceAs(input);
+    assertThat(Util.truncateAscii((CharSequence) input, 100)).isSameInstanceAs(input);
+  }
+
+  @Test
+  public void truncateAscii_longInput_truncated() {
+    String input = "a much longer string";
+
+    assertThat(Util.truncateAscii(input, 5)).isEqualTo("a muc");
+    assertThat(Util.truncateAscii((CharSequence) input, 5).toString()).isEqualTo("a muc");
+  }
+
+  @Test
+  public void truncateAscii_preservesStylingSpans() {
+    SpannableString input = new SpannableString("a short string");
+    input.setSpan(new UnderlineSpan(), 0, 10, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+    input.setSpan(new StrikethroughSpan(), 4, 10, Spanned.SPAN_INCLUSIVE_EXCLUSIVE);
+
+    CharSequence result = Util.truncateAscii(input, 7);
+
+    assertThat(result).isInstanceOf(SpannableString.class);
+    assertThat(result.toString()).isEqualTo("a short");
+    SpannedSubject.assertThat((Spanned) result)
+        .hasUnderlineSpanBetween(0, 7)
+        .withFlags(Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+    SpannedSubject.assertThat((Spanned) result)
+        .hasStrikethroughSpanBetween(4, 7)
+        .withFlags(Spanned.SPAN_INCLUSIVE_EXCLUSIVE);
+  }
+
   @Test
   public void toHexString_returnsHexString() {
     byte[] bytes = TestUtil.createByteArray(0x12, 0xFC, 0x06);

library/core/src/main/java/com/google/android/exoplayer2/text/cea/Cea608Decoder.java

@@ -37,6 +37,7 @@ import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.Log;
 import com.google.android.exoplayer2.util.MimeTypes;
 import com.google.android.exoplayer2.util.ParsableByteArray;
+import com.google.android.exoplayer2.util.Util;
 import java.nio.ByteBuffer;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -868,8 +869,12 @@ public final class Cea608Decoder extends CeaDecoder {
     }
 
     public void append(char text) {
+      // Don't accept more than 32 chars. We'll trim further, considering indent & tabOffset, in
+      // build().
+      if (captionStringBuilder.length() < SCREEN_CHARWIDTH) {
         captionStringBuilder.append(text);
       }
+    }
 
     public void rollUp() {
       rolledUpCaptions.add(buildCurrentLine());
@@ -883,14 +888,17 @@ public final class Cea608Decoder extends CeaDecoder {
 
     @Nullable
     public Cue build(@Cue.AnchorType int forcedPositionAnchor) {
+      // The number of empty columns before the start of the text, in the range [0-31].
+      int startPadding = indent + tabOffset;
+      int maxTextLength = SCREEN_CHARWIDTH - startPadding;
       SpannableStringBuilder cueString = new SpannableStringBuilder();
       // Add any rolled up captions, separated by new lines.
       for (int i = 0; i < rolledUpCaptions.size(); i++) {
-        cueString.append(rolledUpCaptions.get(i));
+        cueString.append(Util.truncateAscii(rolledUpCaptions.get(i), maxTextLength));
         cueString.append('\n');
       }
       // Add the current line.
-      cueString.append(buildCurrentLine());
+      cueString.append(Util.truncateAscii(buildCurrentLine(), maxTextLength));
 
       if (cueString.length() == 0) {
         // The cue is empty.
@@ -898,8 +906,7 @@ public final class Cea608Decoder extends CeaDecoder {
       }
 
       int positionAnchor;
-      // The number of empty columns before the start of the text, in the range [0-31].
+
-      int startPadding = indent + tabOffset;
       // The number of empty columns after the end of the text, in the same range.
       int endPadding = SCREEN_CHARWIDTH - startPadding - cueString.length();
       int startEndPaddingDelta = startPadding - endPadding;