bitreserve.java

一个详细的javaLayer的源码

/*
 * 12/12/99 0.0.7	Implementation stores single bits 
 *					as ints for better performance. [e-mail].
 *
 * Java Conversion by E.B, [e-mail], JavaLayer
 *
 *---------------------------------------------------
 * bit_res.h
 *
 * 	Declarations for Bit Reservoir for Layer III
 *
 *  Adapted from the public c code by Jeff Tsay.
 *---------------------------------------------------
 */

package javazoom.jl.decoder;
	
/**
 * Implementation of Bit Reservoir for Layer III.
 * <p>
 * The implementation stores single bits as a word in the buffer. If
 * a bit is set, the corresponding word in the buffer will be non-zero.
 * If a bit is clear, the corresponding word is zero. Although this
 * may seem waseful, this can be a factor of two quicker than 
 * packing 8 bits to a byte and extracting. 
 * <p> 
 */

// REVIEW: there is no range checking, so buffer underflow or overflow
// can silently occur.
final class BitReserve
{
   /**
    * Size of the internal buffer to store the reserved bits.
    * Must be a power of 2. And x8, as each bit is stored as a single
    * entry.
    */
	private static final int		BUFSIZE = 4096*8;
	
	/**
	 * Mask that can be used to quickly implement the
	 * modulus operation on BUFSIZE.
	 */
	private static final int		BUFSIZE_MASK = BUFSIZE-1;
	
	private int 					offset, totbit, buf_byte_idx;
	private final int[] 			buf = new int[BUFSIZE];
	private int 					buf_bit_idx;
	
   BitReserve()
   {
	  
	  offset = 0;
      totbit = 0;
      buf_byte_idx = 0;	  
   }
      
   
   /**
    * Return totbit Field.
	*/
   public int hsstell() 
   { 
	   return(totbit); 
   }

   /**
    * Read a number bits from the bit stream.
    * @param N the number of
	*/
   public int hgetbits(int N)
   {
   	 totbit += N;
	 
	 int val = 0;
	 
	 int pos = buf_byte_idx;
	 if (pos+N < BUFSIZE)
	 {
		while (N-- > 0)
		{
			val <<= 1;
			val |= ((buf[pos++]!=0) ? 1 : 0);		 
		}
	}
	 else
	 {	 
		 while (N-- > 0)
		{
			 val <<= 1;			 
			 val |= ((buf[pos]!=0) ? 1 : 0);
			 pos = (pos+1) & BUFSIZE_MASK;
		}		 
	 }	
	 buf_byte_idx = pos;
	 return val;
   }
   
	 
   
   /**
    * Read 1 bit from the bit stream.
	*/
/*
   public int hget1bit_old()
   {
   	  int val;
	  totbit++;
	  if (buf_bit_idx == 0)
	  {
         buf_bit_idx = 8;
	     buf_byte_idx++;		 
	  }
      // BUFSIZE = 4096 = 2^12, so
      // buf_byte_idx%BUFSIZE == buf_byte_idx & 0xfff
      val = buf[buf_byte_idx & BUFSIZE_MASK] & putmask[buf_bit_idx];
      buf_bit_idx--;
	  val = val >>> buf_bit_idx;
      return val;   
   }
 */
   /**
    * Returns next bit from reserve.
    * @returns 0 if next bit is reset, or 1 if next bit is set.
    */
   public int hget1bit()
   {   	  
	  totbit++;	  
	  int val = buf[buf_byte_idx];
	  buf_byte_idx = (buf_byte_idx+1) & BUFSIZE_MASK;
      return val;
   }
   
   /**
    * Retrieves bits from the reserve.     
    */
/*   
   public int readBits(int[] out, int len)
   {
		if (buf_bit_idx == 0)
		{
		   buf_bit_idx = 8;
		   buf_byte_idx++;
		   current = buf[buf_byte_idx & BUFSIZE_MASK];
		}      
		
		
		
		// save total number of bits returned
		len = buf_bit_idx;
		buf_bit_idx = 0;
		  
		int b = current;
		int count = len-1;
		  
		while (count >= 0)
		{
		    out[count--] = (b & 0x1);
		    b >>>= 1;
		}
	  
		totbit += len;
		return len;
   }
  */
   
   /**
    * Write 8 bits into the bit stream.
	*/
   public void hputbuf(int val)
   {   	  
	   int ofs = offset;
	   buf[ofs++] = val & 0x80;
	   buf[ofs++] = val & 0x40;
	   buf[ofs++] = val & 0x20;
	   buf[ofs++] = val & 0x10;
	   buf[ofs++] = val & 0x08;
	   buf[ofs++] = val & 0x04;
	   buf[ofs++] = val & 0x02;
	   buf[ofs++] = val & 0x01;
	   
	   if (ofs==BUFSIZE)
			offset = 0;
	   else
			offset = ofs;
	   
   }
 
   /**
    * Rewind N bits in Stream.
	*/
   public void rewindNbits(int N)
   {
 	  totbit -= N;	  	  
	  buf_byte_idx -= N;
	  if (buf_byte_idx<0)
		  buf_byte_idx += BUFSIZE;
   }
	
   /**
    * Rewind N bytes in Stream.
	*/
   public void rewindNbytes(int N)
   {
      int bits = (N << 3);
	  totbit -= bits;
	  buf_byte_idx -= bits;	  
	  if (buf_byte_idx<0)
		  buf_byte_idx += BUFSIZE;
   }
}