h264_bit_reader.h

// Copyright 2014 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.
//
// This file contains an implementation of an H264 Annex-B video stream parser.

#ifndef MEDIA_FILTERS_H264_BIT_READER_H_
#define MEDIA_FILTERS_H264_BIT_READER_H_

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

using namespace std;

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();
        
        // 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(const uint8_t* data, off_t size);
        
        // 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 1-32, inclusive.
        // Return false if the given number of bits cannot be read (not enough
        // bits in the stream), true otherwise.
        bool ReadBits(int num_bits, int* out);
        
        // Return the number of bits left in the stream.
        off_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.
        const uint8_t* data_;
        
        // Bytes left in the stream (without the curr_byte_).
        off_t bytes_left_;
        
        // Contents of the current byte; first unread bit starting at position
        // 8 - num_remaining_bits_in_curr_byte_ from MSB.
        int curr_byte_;
        
        // Number of bits remaining in curr_byte_
        int num_remaining_bits_in_curr_byte_;
        
        // Used in emulation prevention three byte detection (see spec).
        // Initially set to 0xffff to accept all initial two-byte sequences.
        int prev_two_bytes_;
        
        // Number of emulation preventation bytes (0x000003) we met.
        size_t emulation_prevention_bytes_;
        
        DISALLOW_COPY_AND_ASSIGN(H264BitReader);
    };
    
}  // namespace media

#endif  // MEDIA_FILTERS_H264_BIT_READER_H_