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diff --git a/third_party/libwebrtc/rtc_base/bit_buffer.h b/third_party/libwebrtc/rtc_base/bit_buffer.h
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+++ b/third_party/libwebrtc/rtc_base/bit_buffer.h
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+/*
+ *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef RTC_BASE_BIT_BUFFER_H_
+#define RTC_BASE_BIT_BUFFER_H_
+
+#include <stddef.h>  // For size_t.
+#include <stdint.h>  // For integer types.
+
+namespace rtc {
+
+// A BitBuffer API for write operations. Supports symmetric write APIs to the
+// reading APIs of BitstreamReader.
+// Sizes/counts specify bits/bytes, for clarity.
+// Byte order is assumed big-endian/network.
+class BitBufferWriter {
+ public:
+  // Constructs a bit buffer for the writable buffer of `bytes`.
+  BitBufferWriter(uint8_t* bytes, size_t byte_count);
+
+  BitBufferWriter(const BitBufferWriter&) = delete;
+  BitBufferWriter& operator=(const BitBufferWriter&) = delete;
+
+  // Gets the current offset, in bytes/bits, from the start of the buffer. The
+  // bit offset is the offset into the current byte, in the range [0,7].
+  void GetCurrentOffset(size_t* out_byte_offset, size_t* out_bit_offset);
+
+  // The remaining bits in the byte buffer.
+  uint64_t RemainingBitCount() const;
+
+  // Moves current position `byte_count` bytes forward. Returns false if
+  // there aren't enough bytes left in the buffer.
+  bool ConsumeBytes(size_t byte_count);
+  // Moves current position `bit_count` bits forward. Returns false if
+  // there aren't enough bits left in the buffer.
+  bool ConsumeBits(size_t bit_count);
+
+  // Sets the current offset to the provied byte/bit offsets. The bit
+  // offset is from the given byte, in the range [0,7].
+  bool Seek(size_t byte_offset, size_t bit_offset);
+
+  // Writes byte-sized values from the buffer. Returns false if there isn't
+  // enough data left for the specified type.
+  bool WriteUInt8(uint8_t val);
+  bool WriteUInt16(uint16_t val);
+  bool WriteUInt32(uint32_t val);
+
+  // Writes bit-sized values to the buffer. Returns false if there isn't enough
+  // room left for the specified number of bits.
+  bool WriteBits(uint64_t val, size_t bit_count);
+
+  // Writes value in range [0, num_values - 1]
+  // See ReadNonSymmetric documentation for the format,
+  // Call SizeNonSymmetricBits to get number of bits needed to store the value.
+  // Returns false if there isn't enough room left for the value.
+  bool WriteNonSymmetric(uint32_t val, uint32_t num_values);
+  // Returns number of bits required to store `val` with NonSymmetric encoding.
+  static size_t SizeNonSymmetricBits(uint32_t val, uint32_t num_values);
+
+  // Writes the exponential golomb encoded version of the supplied value.
+  // Returns false if there isn't enough room left for the value.
+  bool WriteExponentialGolomb(uint32_t val);
+  // Writes the signed exponential golomb version of the supplied value.
+  // Signed exponential golomb values are just the unsigned values mapped to the
+  // sequence 0, 1, -1, 2, -2, etc. in order.
+  bool WriteSignedExponentialGolomb(int32_t val);
+
+ private:
+  // The buffer, as a writable array.
+  uint8_t* const writable_bytes_;
+  // The total size of `bytes_`.
+  const size_t byte_count_;
+  // The current offset, in bytes, from the start of `bytes_`.
+  size_t byte_offset_;
+  // The current offset, in bits, into the current byte.
+  size_t bit_offset_;
+};
+
+}  // namespace rtc
+
+#endif  // RTC_BASE_BIT_BUFFER_H_