// Copyright 2014 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//
// This file contains an implementation of an H264 Annex-B video stream parser.

#ifndef MEDIA_PARSERS_H264_BIT_READER_H_
#define MEDIA_PARSERS_H264_BIT_READER_H_

#include <stddef.h>
#include <stdint.h>
#include <sys/types.h>

#include "base/memory/raw_span.h"
#include "media/base/media_export.h"

namespace media {

// A class to provide bit-granularity reading of H.264 streams.
// This is not a generic bit reader class, as it takes into account
// H.264 stream-specific constraints, such as skipping emulation-prevention
// bytes and stop bits. See spec for more details.
class MEDIA_EXPORT H264BitReader {
 public:
  H264BitReader();

  H264BitReader(const H264BitReader&) = delete;
  H264BitReader& operator=(const H264BitReader&) = delete;

  ~H264BitReader();

  // Initialize the reader to start reading at |data|, |size| being size
  // of |data| in bytes.
  // Return false on insufficient size of stream..
  // TODO(posciak,fischman): consider replacing Initialize() with
  // heap-allocating and creating bit readers on demand instead.
  bool Initialize(base::span<const uint8_t> data);

  // Read |num_bits| next bits from stream and return in |*out|, first bit
  // from the stream starting at |num_bits| position in |*out|.
  // |num_bits| may be 1-31, inclusive.
  // Return false if the given number of bits cannot be read (not enough
  // bits in the stream), true otherwise.
  bool ReadBits(size_t num_bits, uint32_t* out);

  // Return the number of bits left in the stream.
  size_t NumBitsLeft();

  // See the definition of more_rbsp_data() in spec.
  bool HasMoreRBSPData();

  // Return the number of emulation prevention bytes already read.
  size_t NumEmulationPreventionBytesRead();

 private:
  // Advance to the next byte, loading it into curr_byte_.
  // Return false on end of stream.
  bool UpdateCurrByte();

  // Pointer to the next unread (not in curr_byte_) byte in the stream.
  base::raw_span<const uint8_t, DanglingUntriaged> data_;

  // Contents of the current byte; first unread bit starting at position
  // 8 - num_remaining_bits_in_curr_byte_ from MSB.
  uint32_t curr_byte_ = 0;

  // Number of bits remaining in curr_byte_
  uint32_t num_remaining_bits_in_curr_byte_ = 0;

  // Used in emulation prevention three byte detection (see spec).
  // Initially set to 0xffff to accept all initial two-byte sequences.
  uint32_t prev_two_bytes_ = 0;

  // Number of emulation preventation bytes (0x000003) we met.
  uint32_t emulation_prevention_bytes_ = 0;
};

}  // namespace media

#endif  // MEDIA_PARSERS_H264_BIT_READER_H_