// Copyright (c) 2015 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 NET_QUIC_STREAM_SEQUENCER_BUFFER_H_
#define NET_QUIC_STREAM_SEQUENCER_BUFFER_H_

// StreamSequencerBuffer implements QuicStreamSequencerBufferInterface.
// It is a circular stream buffer with random write and
// in-sequence read. It consists of a vector of pointers pointing
// to memory blocks created as needed and a list of Gaps to indicate
// the missing data between the data already written into the buffer.
// - Data are written in with offset indicating where it should be in the
// stream, and the buffer grown as needed (up to the maximum buffer capacity),
// without expensive copying (extra blocks are allocated).
// - Data can be read from the buffer if there is no gap before it,
// and the buffer shrinks as the data are consumed.
// - An upper limit on the number of blocks in the buffer provides an upper
//   bound on memory use.
//
// This class is thread-unsafe.
//
// StreamSequencerBuffer maintains a concept of the readable region, which
// contains all written data that has not been read.
// It promises stability of the underlying memory addresses in the readable
// region, so pointers into it can be maintained, and the offset of a pointer
// from the start of the read region can be calculated.
//
// Expected Use:
//  StreamSequencerBuffer buffer(2.5 * 8 * 1024);
//  std::string source(1024, 'a');
//  base::StringPiece std::string_piece(source.data(), source.size());
//  size_t written = 0;
//  buffer.OnStreamData(800, std::string_piece, GetEpollClockNow(), &written);
//  source = std::string{800, 'b'};
//  base::StringPiece std::string_piece1(source.data(), 800);
//  // Try to write to [1, 801), but should fail due to overlapping,
//  // res should be QUIC_INVALID_STREAM_DATA
//  auto res = buffer.OnStreamData(1, std::string_piece1, &written));
//  // write to [0, 800), res should be QUIC_NO_ERROR
//  auto res = buffer.OnStreamData(0, std::string_piece1, GetEpollClockNow(),
//                                  &written);
//
//  // Read into a iovec array with total capacity of 120 bytes.
//  char dest[120];
//  iovec iovecs[3]{iovec{dest, 40}, iovec{dest + 40, 40},
//                  iovec{dest + 80, 40}};
//  size_t read = buffer.Readv(iovecs, 3);
//
//  // Get single readable region with timestamp.
//  QuicTime t;
//  iovec iov;
//  buffer.GetReadableRegion(iov, &t);
//
//  // Get readable regions from [256, 1024) and consume some of it.
//  iovec iovs[2];
//  int iov_count = buffer.GetReadableRegions(iovs, 2);
//  // Consume some bytes in iovs, returning number of bytes having been
//  consumed.
//  size_t consumed = consume_iovs(iovs, iov_count);
//  buffer.MarkConsumed(consumed);

#include <functional>
#include <list>
#include <memory>

#include "base/macros.h"
#include "net/quic/quic_protocol.h"
#include "net/quic/quic_stream_sequencer_buffer_interface.h"

namespace net {

namespace test {
class StreamSequencerBufferPeer;
}  // namespace test

class NET_EXPORT_PRIVATE StreamSequencerBuffer
    : public QuicStreamSequencerBufferInterface {
 public:
  // A Gap indicates a missing chunk of bytes between
  // [begin_offset, end_offset) in the stream
  struct Gap {
    Gap(QuicStreamOffset begin_offset, QuicStreamOffset end_offset);
    QuicStreamOffset begin_offset;
    QuicStreamOffset end_offset;
  };

  // A FrameInfo stores the length of a frame and the time it arrived.
  struct NET_EXPORT_PRIVATE FrameInfo {
    FrameInfo();
    FrameInfo(size_t length, QuicTime timestamp);

    size_t length;
    QuicTime timestamp;
  };

  // Size of blocks used by this buffer.
  // Choose 8K to make block large enough to hold multiple frames, each of
  // which could be up to 1.5 KB.
  static const size_t kBlockSizeBytes = 8 * 1024;  // 8KB

  // The basic storage block used by this buffer.
  struct BufferBlock {
    char buffer[kBlockSizeBytes];
  };

  explicit StreamSequencerBuffer(size_t max_capacity_bytes);

  ~StreamSequencerBuffer() override;

  // QuicStreamSequencerBufferInterface implementation.
  void Clear() override;
  bool Empty() const override;
  QuicErrorCode OnStreamData(QuicStreamOffset offset,
                             base::StringPiece data,
                             QuicTime timestamp,
                             size_t* bytes_buffered) override;
  size_t Readv(const struct iovec* dest_iov, size_t dest_count) override;
  int GetReadableRegions(struct iovec* iov, int iov_len) const override;
  bool GetReadableRegion(iovec* iov, QuicTime* timestamp) const override;
  bool MarkConsumed(size_t bytes_buffered) override;
  size_t FlushBufferedFrames() override;
  bool HasBytesToRead() const override;
  QuicStreamOffset BytesConsumed() const override;
  size_t BytesBuffered() const override;

 private:
  friend class test::StreamSequencerBufferPeer;

  // Dispose the given buffer block.
  // After calling this method, blocks_[index] is set to nullptr
  // in order to indicate that no memory set is allocated for that block.
  void RetireBlock(size_t index);

  // Should only be called after the indexed block is read till the end of the
  // block or a gap has been reached.
  // If the block at |block_index| contains no buffered data, then the block is
  // retired.
  void RetireBlockIfEmpty(size_t block_index);

  // Called within OnStreamData() to update the gap OnStreamData() writes into
  // (remove, split or change begin/end offset).
  void UpdateGapList(std::list<Gap>::iterator gap_with_new_data_written,
                     QuicStreamOffset start_offset,
                     size_t bytes_written);

  // Calculate the capacity of block at specified index.
  // Return value should be either kBlockSizeBytes for non-trailing blocks and
  // max_buffer_capacity % kBlockSizeBytes for trailing block.
  size_t GetBlockCapacity(size_t index) const;

  // Does not check if offset is within reasonable range.
  size_t GetBlockIndex(QuicStreamOffset offset) const;

  // Given an offset in the stream, return the offset from the beginning of the
  // block which contains this data.
  size_t GetInBlockOffset(QuicStreamOffset offset) const;

  // Get offset relative to index 0 in logical 1st block to start next read.
  size_t ReadOffset() const;

  // Get the index of the logical 1st block to start next read.
  size_t NextBlockToRead() const;

  // Returns number of bytes available to be read out.
  size_t ReadableBytes() const;

  // Called after Readv() and MarkConsumed() to keep frame_arrival_time_map_
  // up to date.
  // |offset| is the byte next read should start from. All frames before it
  // should be removed from the map.
  void UpdateFrameArrivalMap(QuicStreamOffset offset);

  // The maximum total capacity of this buffer in byte, as constructed.
  const size_t max_buffer_capacity_bytes_;

  // How many blocks this buffer would need when it reaches full capacity.
  const size_t blocks_count_;

  // Number of bytes read out of buffer.
  QuicStreamOffset total_bytes_read_;

  // Contains Gaps which represents currently missing data.
  std::list<Gap> gaps_;

  // An ordered, variable-length list of blocks, with the length limited
  // such that the number of blocks never exceeds blocks_count_.
  // Each list entry can hold up to kBlockSizeBytes bytes.
  std::vector<BufferBlock*> blocks_;

  // Number of bytes in buffer.
  size_t num_bytes_buffered_;

  // Stores all the buffered frames' start offset, length and arrival time.
  std::map<QuicStreamOffset, FrameInfo> frame_arrival_time_map_;

  DISALLOW_COPY_AND_ASSIGN(StreamSequencerBuffer);
};
}  // namespace net

#endif  // NET_QUIC_STREAM_SEQUENCER_BUFFER_H_