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diff --git a/Source/WebCore/html/track/BufferedLineReader.cpp b/Source/WebCore/html/track/BufferedLineReader.cpp
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+++ b/Source/WebCore/html/track/BufferedLineReader.cpp
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+/*
+ * Copyright (C) 2013, Opera Software ASA. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "BufferedLineReader.h"
+
+#include <wtf/unicode/CharacterNames.h>
+
+namespace WebCore {
+
+std::optional<String> BufferedLineReader::nextLine()
+{
+    if (m_maybeSkipLF) {
+        // We ran out of data after a CR (U+000D), which means that we may be
+        // in the middle of a CRLF pair. If the next character is a LF (U+000A)
+        // then skip it, and then (unconditionally) return the buffered line.
+        if (!m_buffer.isEmpty()) {
+            if (m_buffer.currentCharacter() == newlineCharacter)
+                m_buffer.advancePastNewline();
+            m_maybeSkipLF = false;
+        }
+        // If there was no (new) data available, then keep m_maybeSkipLF set,
+        // and fall through all the way down to the EOS check at the end of the function.
+    }
+
+    bool shouldReturnLine = false;
+    bool checkForLF = false;
+    while (!m_buffer.isEmpty()) {
+        UChar character = m_buffer.currentCharacter();
+        m_buffer.advance();
+
+        if (character == newlineCharacter || character == carriageReturn) {
+            // We found a line ending. Return the accumulated line.
+            shouldReturnLine = true;
+            checkForLF = (character == carriageReturn);
+            break;
+        }
+
+        // NULs are transformed into U+FFFD (REPLACEMENT CHAR.) in step 1 of
+        // the WebVTT parser algorithm.
+        if (character == '\0')
+            character = replacementCharacter;
+
+        m_lineBuffer.append(character);
+    }
+
+    if (checkForLF) {
+        // May be in the middle of a CRLF pair.
+        if (!m_buffer.isEmpty()) {
+            if (m_buffer.currentCharacter() == newlineCharacter)
+                m_buffer.advancePastNewline();
+        } else {
+            // Check for the newline on the next call (unless we reached EOS, in
+            // which case we'll return the contents of the line buffer, and
+            // reset state for the next line.)
+            m_maybeSkipLF = true;
+        }
+    }
+
+    if (isAtEndOfStream()) {
+        // We've reached the end of the stream proper. Emit a line if the
+        // current line buffer is non-empty. (Note that if shouldReturnLine is
+        // set already, we want to return a line nonetheless.)
+        shouldReturnLine |= !m_lineBuffer.isEmpty();
+    }
+
+    if (shouldReturnLine) {
+        auto line = m_lineBuffer.toString();
+        m_lineBuffer.clear();
+        return WTFMove(line);
+    }
+
+    ASSERT(m_buffer.isEmpty());
+    return std::nullopt;
+}
+
+} // namespace WebCore