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ID3SyncSafeInteger.javaAPI DocJaudiotagger 2.0.44201Wed Mar 30 16:12:06 BST 2011org.jaudiotagger.tag.id3

ID3SyncSafeInteger

public class ID3SyncSafeInteger extends Object
Peforms encoding/decoding of an syncsafe integer

Syncsafe integers are used for the size in the tag header of v23 and v24 tags, and in the frame size in the frame header of v24 frames.

In some parts of the tag it is inconvenient to use the unsychronisation scheme because the size of unsynchronised data is not known in advance, which is particularly problematic with size descriptors. The solution in ID3v2 is to use synchsafe integers, in which there can never be any false synchs. Synchsafe integers are integers that keep its highest bit (bit 7) zeroed, making seven bits out of eight available. Thus a 32 bit synchsafe integer can store 28 bits of information.

Example:

255 (%11111111) encoded as a 16 bit synchsafe integer is 383 (%00000001 01111111).

Fields Summary
public static final int
INTEGRAL_SIZE
public static final int
MAX_SAFE_SIZE
Sizes equal or smaller than this are the same whether held as sync safe integer or normal integer so it doesnt matter.
Constructors Summary
Methods Summary
private static intbufferToValue(byte[] buffer)
Read syncsafe value from byteArray in format specified in spec and convert to int.

param
buffer syncsafe integer
return
decoded int


                              
        
    
        //Note Need to && with 0xff otherwise if value is greater than 128 we get a negative number
        //when cast byte to int
        return ((buffer[0] & 0xff) << 21) + ((buffer[1] & 0xff) << 14) + ((buffer[2] & 0xff) << 7) + ((buffer[3]) & 0xff);
protected static intbufferToValue(java.nio.ByteBuffer buffer)
Read syncsafe value from buffer in format specified in spec and convert to int.

The buffers position is moved to just after the location of the syncsafe integer

param
buffer syncsafe integer
return
decoded int

        byte byteBuffer[] = new byte[INTEGRAL_SIZE];
        buffer.get(byteBuffer, 0, INTEGRAL_SIZE);
        return bufferToValue(byteBuffer);
protected static booleanisBufferEmpty(byte[] buffer)
Checks if the buffer just contains zeros

This can be used to identify when accessing padding of a tag

param
buffer
return
true if buffer only contains zeros

        for (byte aBuffer : buffer)
        {
            if (aBuffer != 0)
            {
                return false;
            }
        }
        return true;
protected static booleanisBufferNotSyncSafe(java.nio.ByteBuffer buffer)
Is buffer holding a value that is definently not syncsafe

We cannot guarantee a buffer is holding a syncsafe integer but there are some checks we can do to show that it definently is not.

The buffer is NOT moved after reading.

This function is useful for reading ID3v24 frames created in iTunes because iTunes does not use syncsafe integers in its frames.

param
buffer
return
true if this buffer is definently not holding a syncsafe integer

        int position = buffer.position();

        //Check Bit7 not set
        for (int i = 0; i < INTEGRAL_SIZE; i++)
        {
            byte nextByte = buffer.get(position + i);
            if ((nextByte & 0x80) > 0)
            {
                return true;
            }
        }
        return false;
protected static byte[]valueToBuffer(int size)
Convert int value to syncsafe value held in bytearray

param
size
return
buffer syncsafe integer

        byte[] buffer = new byte[4];
        buffer[0] = (byte) ((size & 0x0FE00000) >> 21);
        buffer[1] = (byte) ((size & 0x001FC000) >> 14);
        buffer[2] = (byte) ((size & 0x00003F80) >> 7);
        buffer[3] = (byte) (size & 0x0000007F);
        return buffer;