/**
* Creates a bit field for the MPEG frame header.
*
* @return the bit field
* @throws IOException if an error occurs
*/
private HeaderBitField createHeaderField() throws IOException {
HeaderBitField field = new HeaderBitField();
field.add(nextByte());
field.add(nextByte());
field.add(nextByte());
return field;
}
/**
* Searches for the next MPEG frame header from the current stream position on. This method
* advances the underlying input stream until it finds a valid frame header or the end of the
* stream is reached. In the former case a corresponding {@code AudioFrame} object is created. In
* the latter case there are no more headers, so the end of the stream is probably reached.
*
* @return the next {@code AudioFrame} or <b>null</b>
* @throws IOException if an IO error occurs
*/
public AudioFrame nextFrame() throws IOException {
AudioFrame frame = null;
while (!endOfStream && frame == null) {
findFrameSyncByte();
if (!endOfStream) {
HeaderBitField headerField = createHeaderField();
if (!endOfStream) {
frame = createHeader(headerField);
if (frame == null) {
pushBack(headerField);
} else {
System.arraycopy(headerField.toArray(), 0, header, 0, 3);
}
}
}
}
currentHeader = frame;
return frame;
}
/**
* Creates an {@code AudioFrame} object based on the given header field. If the header field
* contains invalid values, result is <b>null</b>.
*
* @param bits the header bit field
* @return the {@code AudioFrame}
*/
private static AudioFrame createHeader(HeaderBitField bits) {
if (bits.get(21, 23) != 7) {
return null;
}
int mpegVer = bits.get(19, 20);
int layer = bits.get(17, 18);
int bitRateCode = bits.get(12, 15);
int sampleRateCode = bits.get(10, 11);
int padding = bits.get(9);
if (mpegVer == 1
|| layer == 0
|| bitRateCode == 0
|| bitRateCode == 15
|| sampleRateCode == 3) {
// invalid header values
return null;
}
int bitRate = calculateBitRate(mpegVer, layer, bitRateCode);
int sampleRate = calculateSampleRate(mpegVer, sampleRateCode);
int length = calculateFrameLength(layer, bitRate, sampleRate, padding);
float duration = calculateDuration(layer, sampleRate);
int channels = calculateChannels(bits.get(6, 7));
return new AudioFrame(mpegVer, layer, bitRate, sampleRate, channels, length, duration);
}
/**
* Pushes the given header field back in the stream so that the bytes are read again. This method
* is called if an invalid header was detected. Then search has to continue at the next byte after
* the frame sync byte.
*
* @param field the header bit field with the invalid frame header
* @throws IOException if an error occurs
*/
private void pushBack(HeaderBitField field) throws IOException {
unread(field.toArray());
}
