MPEG Audio Frame Header

#include "StdAfx.h"
#include ".\musicmatchtag.h"

#include <math.h> // for floor

// information from documentation "musicmatch.txt" in id3lib's doc folder

CMusicMatchTag* CMusicMatchTag::FindTag(CMPAStream* pStream, bool bAppended, DWORD dwBegin, DWORD dwEnd)
{
	// check for footer at the end of file
	DWORD dwOffset = dwEnd - 48;
	BYTE* pBuffer = pStream->ReadBytes(32, dwOffset, true);
	if (memcmp("Brava Software Inc.             ", pBuffer, 32)==0)
		return new CMusicMatchTag(pStream, dwOffset);
	return NULL;
}


CMusicMatchTag::CMusicMatchTag(CMPAStream* pStream, DWORD dwOffset) :
	CTag(pStream, _T("MM"), true, dwOffset)
{
	// get version info (from footer)
	char szVersion[5];
	memcpy(szVersion, (char*)pStream->ReadBytes(4, dwOffset, false, true), 4);
	szVersion[4] = '\0';

	m_fVersion = (float)atof(szVersion);
	
	// data offsets in the 20 bytes before footer
	dwOffset -= 52;
	DWORD dwImgExtOffset = pStream->ReadLEValue(4, dwOffset);
	dwOffset += 4;
	//DWORD dwImgOffset = pStream->ReadLEValue(4, dwOffset);

	// next 4 bytes unused
	dwOffset += 4;
	DWORD dwVersionOffset = pStream->ReadLEValue(4, dwOffset);

	dwOffset += 4;
//	DWORD dwMetadataOffset = pStream->ReadLEValue(4, dwOffset);

	DWORD dwMetadataSize = 0;
	BYTE* pBuffer;

	if (m_fVersion > 3.00)
	{
		// three possible lengths for metadata section: 7936, 8004, and 8132 bytes
		DWORD dwPossSizes[3] = {7936, 8004, 8132};
		DWORD dwSyncOffset;
		
		for (int nIndex = 0; nIndex < 3; nIndex++)
		{
			// check for sync in version section
			dwSyncOffset = dwOffset - 20 - dwPossSizes[nIndex] - 256;
			pBuffer = pStream->ReadBytes(8, dwSyncOffset, false, true);

			if (memcmp("18273645", pBuffer, 8) == 0)
			{
				dwMetadataSize = dwPossSizes[nIndex];
				break;
			}
		}
	}
	else
		dwMetadataSize = 7868;	// fixed size up to version 3

	if (dwMetadataSize == 0)	// could not find correct metadata size
		throw NULL;

	// total size = footer + data offsets + metadata + version + img (incl. unused)
	m_dwSize = 48 + 20 + dwMetadataSize + 256 + (dwVersionOffset - dwImgExtOffset);

	// is header there?
	dwOffset -= (m_dwSize - 48) + 256;
	pBuffer = pStream->ReadBytes(8, dwOffset, false);
	if (memcmp("18273645", pBuffer, 8) == 0)
	{
		m_dwSize += 256;	// header exists
		m_dwOffset = dwOffset;
	}
	else
		m_dwOffset = dwOffset + 256;
}

CMusicMatchTag::~CMusicMatchTag(void)
{
}