diff --git a/quic/core/quic_stream_sequencer.h b/quic/core/quic_stream_sequencer.h
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+++ b/quic/core/quic_stream_sequencer.h
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+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef QUICHE_QUIC_CORE_QUIC_STREAM_SEQUENCER_H_
+#define QUICHE_QUIC_CORE_QUIC_STREAM_SEQUENCER_H_
+
+#include <cstddef>
+#include <map>
+
+#include "base/macros.h"
+#include "net/third_party/quiche/src/quic/core/quic_packets.h"
+#include "net/third_party/quiche/src/quic/core/quic_stream_sequencer_buffer.h"
+#include "net/third_party/quiche/src/quic/platform/api/quic_export.h"
+#include "net/third_party/quiche/src/quic/platform/api/quic_string.h"
+
+namespace quic {
+
+namespace test {
+class QuicStreamSequencerPeer;
+}  // namespace test
+
+// Buffers frames until we have something which can be passed
+// up to the next layer.
+class QUIC_EXPORT_PRIVATE QuicStreamSequencer {
+ public:
+  // Interface that thie Sequencer uses to communicate with the Stream.
+  class StreamInterface {
+   public:
+    virtual ~StreamInterface() = default;
+
+    // Called when new data is available to be read from the sequencer.
+    virtual void OnDataAvailable() = 0;
+    // Called when the end of the stream has been read.
+    virtual void OnFinRead() = 0;
+    // Called when bytes have been consumed from the sequencer.
+    virtual void AddBytesConsumed(QuicByteCount bytes) = 0;
+    // TODO(rch): Clean up this interface via OnUnrecoverableError and
+    // remove PeerAddressOfLatestPacket().
+    // Called when an error has occurred which should result in the stream
+    // being reset.
+    virtual void Reset(QuicRstStreamErrorCode error) = 0;
+    // Called when an error has occurred which should result in the connection
+    // being closed.
+    virtual void CloseConnectionWithDetails(QuicErrorCode error,
+                                            const QuicString& details) = 0;
+
+    // Returns the stream id of this stream.
+    virtual QuicStreamId id() const = 0;
+    // Returns the peer address of the last packet received for this stream.
+    virtual const QuicSocketAddress& PeerAddressOfLatestPacket() const = 0;
+  };
+
+  explicit QuicStreamSequencer(StreamInterface* quic_stream);
+  QuicStreamSequencer(const QuicStreamSequencer&) = delete;
+  QuicStreamSequencer(QuicStreamSequencer&&) = default;
+  QuicStreamSequencer& operator=(const QuicStreamSequencer&) = delete;
+  virtual ~QuicStreamSequencer();
+
+  // If the frame is the next one we need in order to process in-order data,
+  // ProcessData will be immediately called on the stream until all buffered
+  // data is processed or the stream fails to consume data.  Any unconsumed
+  // data will be buffered. If the frame is not the next in line, it will be
+  // buffered.
+  void OnStreamFrame(const QuicStreamFrame& frame);
+
+  // If the frame is the next one we need in order to process in-order data,
+  // ProcessData will be immediately called on the crypto stream until all
+  // buffered data is processed or the crypto stream fails to consume data. Any
+  // unconsumed data will be buffered. If the frame is not the next in line, it
+  // will be buffered.
+  void OnCryptoFrame(const QuicCryptoFrame& frame);
+
+  // Once data is buffered, it's up to the stream to read it when the stream
+  // can handle more data.  The following three functions make that possible.
+
+  // Fills in up to iov_len iovecs with the next readable regions.  Returns the
+  // number of iovs used.  Non-destructive of the underlying data.
+  int GetReadableRegions(iovec* iov, size_t iov_len) const;
+
+  // Fills in one iovec with the next readable region.  Returns false if there
+  // is no readable region available.
+  bool GetReadableRegion(iovec* iov) const;
+
+  // Fill in one iovec with the next unread region for the quic spdy stream.
+  // Returns false if no readable region is available.
+  bool PrefetchNextRegion(iovec* iov);
+
+  // Copies the data into the iov_len buffers provided.  Returns the number of
+  // bytes read.  Any buffered data no longer in use will be released.
+  // TODO(rch): remove this method and instead implement it as a helper method
+  // based on GetReadableRegions and MarkConsumed.
+  int Readv(const struct iovec* iov, size_t iov_len);
+
+  // Consumes |num_bytes| data.  Used in conjunction with |GetReadableRegions|
+  // to do zero-copy reads.
+  void MarkConsumed(size_t num_bytes);
+
+  // Appends all of the readable data to |buffer| and marks all of the appended
+  // data as consumed.
+  void Read(QuicString* buffer);
+
+  // Returns true if the sequncer has bytes available for reading.
+  bool HasBytesToRead() const;
+
+  // Number of bytes available to read.
+  size_t ReadableBytes() const;
+
+  // Returns true if the sequencer has delivered the fin.
+  bool IsClosed() const;
+
+  // Calls |OnDataAvailable| on |stream_| if there is buffered data that can
+  // be processed, and causes |OnDataAvailable| to be called as new data
+  // arrives.
+  void SetUnblocked();
+
+  // Blocks processing of frames until |SetUnblocked| is called.
+  void SetBlockedUntilFlush();
+
+  // Sets the sequencer to discard all incoming data itself and not call
+  // |stream_->OnDataAvailable()|.  |stream_->OnFinRead()| will be called
+  // automatically when the FIN is consumed (which may be immediately).
+  void StopReading();
+
+  // Free the memory of underlying buffer.
+  void ReleaseBuffer();
+
+  // Free the memory of underlying buffer when no bytes remain in it.
+  void ReleaseBufferIfEmpty();
+
+  // Number of bytes in the buffer right now.
+  size_t NumBytesBuffered() const;
+
+  // Number of bytes has been consumed.
+  QuicStreamOffset NumBytesConsumed() const;
+
+  QuicStreamOffset close_offset() const { return close_offset_; }
+
+  int num_frames_received() const { return num_frames_received_; }
+
+  int num_duplicate_frames_received() const {
+    return num_duplicate_frames_received_;
+  }
+
+  bool ignore_read_data() const { return ignore_read_data_; }
+
+  void set_level_triggered(bool level_triggered) {
+    level_triggered_ = level_triggered;
+  }
+
+  bool level_triggered() const { return level_triggered_; }
+
+  void set_stream(StreamInterface* stream) { stream_ = stream; }
+
+  // Returns string describing internal state.
+  const QuicString DebugString() const;
+
+ private:
+  friend class test::QuicStreamSequencerPeer;
+
+  // Deletes and records as consumed any buffered data that is now in-sequence.
+  // (To be called only after StopReading has been called.)
+  void FlushBufferedFrames();
+
+  // Wait until we've seen 'offset' bytes, and then terminate the stream.
+  void CloseStreamAtOffset(QuicStreamOffset offset);
+
+  // If we've received a FIN and have processed all remaining data, then inform
+  // the stream of FIN, and clear buffers.
+  bool MaybeCloseStream();
+
+  // Shared implementation between OnStreamFrame and OnCryptoFrame.
+  void OnFrameData(QuicStreamOffset byte_offset,
+                   size_t data_len,
+                   const char* data_buffer);
+
+  // The stream which owns this sequencer.
+  StreamInterface* stream_;
+
+  // Stores received data in offset order.
+  QuicStreamSequencerBuffer buffered_frames_;
+
+  // The offset, if any, we got a stream termination for.  When this many bytes
+  // have been processed, the sequencer will be closed.
+  QuicStreamOffset close_offset_;
+
+  // If true, the sequencer is blocked from passing data to the stream and will
+  // buffer all new incoming data until FlushBufferedFrames is called.
+  bool blocked_;
+
+  // Count of the number of frames received.
+  int num_frames_received_;
+
+  // Count of the number of duplicate frames received.
+  int num_duplicate_frames_received_;
+
+  // If true, all incoming data will be discarded.
+  bool ignore_read_data_;
+
+  // If false, only call OnDataAvailable() when it becomes newly unblocked.
+  // Otherwise, call OnDataAvailable() when number of readable bytes changes.
+  bool level_triggered_;
+
+  // Latched value of quic_stop_reading_when_level_triggered flag.  When true,
+  // the sequencer will discard incoming data (but not FIN bits) after
+  // StopReading is called, even in level_triggered_ mode.
+  const bool stop_reading_when_level_triggered_;
+};
+
+}  // namespace quic
+
+#endif  // QUICHE_QUIC_CORE_QUIC_STREAM_SEQUENCER_H_