mp3dec.c

著名的 helix realplayer 基于手机 symbian 系统的 播放器全套源代码

/* ***** BEGIN LICENSE BLOCK ***** 
 * Version: RCSL 1.0/RPSL 1.0 
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 * Portions Copyright (c) 1995-2002 RealNetworks, Inc. All Rights Reserved. 
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 * the file under the RealNetworks Community Source License Version 1.0 
 * (the "RCSL") available at http://www.helixcommunity.org/content/rcsl, 
 * in which case the RCSL will apply. You may also obtain the license terms 
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 * contents of the file.  
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 * This file is part of the Helix DNA Technology. RealNetworks is the 
 * developer of the Original Code and owns the copyrights in the portions 
 * it created. 
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 * Technology Compatibility Kit Test Suite(s) Location: 
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 * Contributor(s): 
 *  
 * ***** END LICENSE BLOCK ***** */ 

/**************************************************************************************
 * Fixed-point MP3 decoder
 * Jon Recker ([e-mail]), Ken Cooke ([e-mail])
 * June 2003
 *
 * mp3dec.c - platform-independent top level MP3 decoder API
 **************************************************************************************/

#include "hlxclib/string.h"		/* for memmove, memcpy (can replace with different implementations if desired) */
#include "mp3common.h"	/* includes mp3dec.h (public API) and internal, platform-independent API */

/**************************************************************************************
 * Function:    MP3InitDecoder
 *
 * Description: allocate memory for platform-specific data
 *              clear all the user-accessible fields
 *
 * Inputs:      none
 *
 * Outputs:     none
 *
 * Return:      handle to mp3 decoder instance, 0 if malloc fails
 **************************************************************************************/
HMP3Decoder MP3InitDecoder(void)
{
	MP3DecInfo *mp3DecInfo;

	mp3DecInfo = AllocateBuffers();

	return (HMP3Decoder)mp3DecInfo;
}

/**************************************************************************************
 * Function:    MP3FreeDecoder
 *
 * Description: free platform-specific data allocated by InitMP3Decoder
 *              zero out the contents of MP3DecInfo struct
 *
 * Inputs:      valid MP3 decoder instance pointer (HMP3Decoder)
 *
 * Outputs:     none
 *
 * Return:      none
 **************************************************************************************/
void MP3FreeDecoder(HMP3Decoder hMP3Decoder)
{
	MP3DecInfo *mp3DecInfo = (MP3DecInfo *)hMP3Decoder;

	if (!mp3DecInfo)
		return;

	FreeBuffers(mp3DecInfo);
}

/**************************************************************************************
 * Function:    MP3FindSyncWord
 *
 * Description: locate the next byte-alinged sync word in the raw mp3 stream
 *
 * Inputs:      buffer to search for sync word
 *              max number of bytes to search in buffer
 *
 * Outputs:     none
 *
 * Return:      offset to first sync word (bytes from start of buf)
 *              -1 if sync not found after searching nBytes
 **************************************************************************************/
int MP3FindSyncWord(unsigned char *buf, int nBytes)
{
	int i;

	/* find byte-aligned syncword - need 12 (MPEG 1,2) or 11 (MPEG 2.5) matching bits */
	for (i = 0; i < nBytes - 1; i++) {
		if ( (buf[i+0] & SYNCWORDH) == SYNCWORDH && (buf[i+1] & SYNCWORDL) == SYNCWORDL )
			return i;
	}
	
	return -1;
}

/**************************************************************************************
 * Function:    MP3FindFreeSync
 *
 * Description: figure out number of bytes between adjacent sync words in "free" mode
 *
 * Inputs:      buffer to search for next sync word
 *              the 4-byte frame header starting at the current sync word
 *              max number of bytes to search in buffer
 *
 * Outputs:     none
 *
 * Return:      offset to next sync word, minus any pad byte (i.e. nSlots)
 *              -1 if sync not found after searching nBytes
 *
 * Notes:       this checks that the first 22 bits of the next frame header are the
 *                same as the current frame header, but it's still not foolproof
 *                (could accidentally find a sequence in the bitstream which 
 *                 appears to match but is not actually the next frame header)
 *              this could be made more error-resilient by checking several frames
 *                in a row and verifying that nSlots is the same in each case
 *              since free mode requires CBR (see spec) we generally only call
 *                this function once (first frame) then store the result (nSlots)
 *                and just use it from then on
 **************************************************************************************/
static int MP3FindFreeSync(unsigned char *buf, unsigned char firstFH[4], int nBytes)
{
	int offset = 0;
	unsigned char *bufPtr = buf;

	/* loop until we either: 
	 *  - run out of nBytes (FindMP3SyncWord() returns -1)
	 *  - find the next valid frame header (sync word, version, layer, CRC flag, bitrate, and sample rate
	 *      in next header must match current header)
	 */
	while (1) {
		offset = MP3FindSyncWord(bufPtr, nBytes);
		bufPtr += offset;
		if (offset < 0) {
			return -1;
		} else if ( (bufPtr[0] == firstFH[0]) && (bufPtr[1] == firstFH[1]) && ((bufPtr[2] & 0xfc) == (firstFH[2] & 0xfc)) ) {
			/* want to return number of bytes per frame, NOT counting the padding byte, so subtract one if padFlag == 1 */
			if ((firstFH[2] >> 1) & 0x01)
				bufPtr--;
			return bufPtr - buf;
		}
		bufPtr += 3;
		nBytes -= (offset + 3);
	};

	return -1;
}

/**************************************************************************************
 * Function:    MP3GetLastFrameInfo
 *
 * Description: get info about last MP3 frame decoded (number of sampled decoded, 
 *                sample rate, bitrate, etc.)
 *
 * Inputs:      valid MP3 decoder instance pointer (HMP3Decoder)
 *              pointer to MP3FrameInfo struct
 *
 * Outputs:     filled-in MP3FrameInfo struct
 *
 * Return:      none
 *
 * Notes:       call this right after calling MP3Decode
 **************************************************************************************/
void MP3GetLastFrameInfo(HMP3Decoder hMP3Decoder, MP3FrameInfo *mp3FrameInfo)
{
	MP3DecInfo *mp3DecInfo = (MP3DecInfo *)hMP3Decoder;

	if (!mp3DecInfo || mp3DecInfo->layer != 3) {
		mp3FrameInfo->bitrate = 0;
		mp3FrameInfo->nChans = 0;
		mp3FrameInfo->samprate = 0;
		mp3FrameInfo->bitsPerSample = 0;
		mp3FrameInfo->outputSamps = 0;
		mp3FrameInfo->layer = 0;
		mp3FrameInfo->version = 0;
	} else {
		mp3FrameInfo->bitrate = mp3DecInfo->bitrate;
		mp3FrameInfo->nChans = mp3DecInfo->nChans;
		mp3FrameInfo->samprate = mp3DecInfo->samprate;
		mp3FrameInfo->bitsPerSample = 16;
		mp3FrameInfo->outputSamps = mp3DecInfo->nChans * (int)samplesPerFrameTab[mp3DecInfo->version][mp3DecInfo->layer - 1];
		mp3FrameInfo->layer = mp3DecInfo->layer;
		mp3FrameInfo->version = mp3DecInfo->version;
	}
}