NAL unit 解码（二）

实际完成NAL解析工作的函数：

/** 
 * Parse an AVC AnnexB Bytestream bs to extract a single nalUnit 
 * while accumulating bytestream statistics into stats. 
 * 
 * If EOF occurs while trying to extract a NALunit, an exception 
 * of std::ios_base::failure is thrown.  The contsnts of stats will 
 * be correct at this point. 
 */  
static void  
_byteStreamNALUnit(  
  InputByteStream& bs,  
  vector& nalUnit,  
  AnnexBStats& stats)  
{  
  /* At the beginning of the decoding process, the decoder initialises its 
   * current position in the byte stream to the beginning of the byte stream. 
   * It then extracts and discards each leading_zero_8bits syntax element (if 
   * present), moving the current position in the byte stream forward one 
   * byte at a time, until the current position in the byte stream is such 
   * that the next four bytes in the bitstream form the four-byte sequence 
   * 0x00000001. 
   */  
  while ((bs.eofBeforeNBytes(24/8) || bs.peekBytes(24/8) != 0x000001)  
  &&     (bs.eofBeforeNBytes(32/8) || bs.peekBytes(32/8) != 0x00000001))  
  {  
    uint8_t leading_zero_8bits = bs.readByte(); //!< 提取并丢弃leading_zero_8bits句法元素  
    assert(leading_zero_8bits == 0);  
    stats.m_numLeadingZero8BitsBytes++;  
  }  
  
  /* 1. When the next four bytes in the bitstream form the four-byte sequence 
   * 0x00000001, the next byte in the byte stream (which is a zero_byte 
   * syntax element) is extracted and discarded and the current position in 
   * the byte stream is set equal to the position of the byte following this 
   * discarded byte. 
   */  
  /* NB, the previous step guarantees this will succeed -- if EOF was 
   * encountered, an exception will stop execution getting this far */  
  if (bs.peekBytes(24/8) != 0x000001)  
  {  
    uint8_t zero_byte = bs.readByte(); //!< 提取并丢弃zero_byte句法元素  
    assert(zero_byte == 0);  
    stats.m_numZeroByteBytes++;  
  }  
  
  /* 2. The next three-byte sequence in the byte stream (which is a 
   * start_code_prefix_one_3bytes) is extracted and discarded and the current 
   * position in the byte stream is set equal to the position of the byte 
   * following this three-byte sequence. 
   */  
  /* NB, (1) guarantees that the next three bytes are 0x00 00 01 */  
  uint32_t start_code_prefix_one_3bytes = bs.readBytes(24/8); //!< 提取并丢弃start_code_prefix_one_3bytes句法元素  
  assert(start_code_prefix_one_3bytes == 0x000001);  
  stats.m_numStartCodePrefixBytes += 3;  
  
  /* 3. NumBytesInNALunit is set equal to the number of bytes starting with 
   * the byte at the current position in the byte stream up to and including 
   * the last byte that precedes the location of any of the following 
   * conditions: 
   *   a. A subsequent byte-aligned three-byte sequence equal to 0x000000, or 
   *   b. A subsequent byte-aligned three-byte sequence equal to 0x000001, or 
   *   c. The end of the byte stream, as determined by unspecified means. 
   */  
  /* 4. NumBytesInNALunit bytes are removed from the bitstream and the 
   * current position in the byte stream is advanced by NumBytesInNALunit 
   * bytes. This sequence of bytes is nal_unit( NumBytesInNALunit ) and is 
   * decoded using the NAL unit decoding process 
   */  
  /* NB, (unsigned)x > 2 implies n!=0 && n!=1 */  
  while (bs.eofBeforeNBytes(24/8) || bs.peekBytes(24/8) > 2)   
  {  
    nalUnit.push_back(bs.readByte()); //!< 将NAL的负载部分保存起来  
  }  
    
  /* 5. When the current position in the byte stream is: 
   *  - not at the end of the byte stream (as determined by unspecified means) 
   *  - and the next bytes in the byte stream do not start with a three-byte 
   *    sequence equal to 0x000001 
   *  - and the next bytes in the byte stream do not start with a four byte 
   *    sequence equal to 0x00000001, 
   * the decoder extracts and discards each trailing_zero_8bits syntax 
   * element, moving the current position in the byte stream forward one byte 
   * at a time, until the current position in the byte stream is such that: 
   *  - the next bytes in the byte stream form the four-byte sequence 
   *    0x00000001 or 
   *  - the end of the byte stream has been encountered (as determined by 
   *    unspecified means). 
   */  
  /* NB, (3) guarantees there are at least three bytes available or none */  
  while ((bs.eofBeforeNBytes(24/8) || bs.peekBytes(24/8) != 0x000001)  
  &&     (bs.eofBeforeNBytes(32/8) || bs.peekBytes(32/8) != 0x00000001))  
  {  
    uint8_t trailing_zero_8bits = bs.readByte(); //!< 提取并丢弃trailing_zero_8bits句法元素  
    assert(trailing_zero_8bits == 0);  
    stats.m_numTrailingZero8BitsBytes++;  
  }  
}  

实际上，代码中的注释部分中，标记了1,2,3,4,5序号的注释，对应于draft附录B.2的5点，详细判断规则代码中已经说得很清楚了，这里就不再罗嗦了。