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Efficient lookup for an ID3v2 tag in MPEG files with garbage.

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This looks for an ID3v2 tag until reaching the first valid MPEG frame in O(n) time.
This commit is contained in:
Tsuda Kageyu 2017-01-20 21:14:38 +09:00
parent d64c833359
commit d2e0e55223
1 changed files with 30 additions and 17 deletions
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47
taglib/mpeg/mpegfile.cpp
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@ -488,28 +488,41 @@ long MPEG::File::findID3v2()
const ByteVector headerID = ID3v2::Header::fileIdentifier();
seek(0);
const ByteVector data = readBlock(headerID.size());
if(data.size() < headerID.size())
return -1;
if(data == headerID)
if(readBlock(headerID.size()) == headerID)
return 0;
if(firstSyncByte(data[0]) && secondSynchByte(data[1]))
Header firstHeader(this, 0, true);
if(firstHeader.isValid())
return -1;
// Look for the entire file, if neither an MEPG frame or ID3v2 tag was found
// at the beginning of the file.
// We don't care about the inefficiency of the code, since this is a seldom case.
// Look for an ID3v2 tag until reaching the first valid MPEG frame.
const long tagOffset = find(headerID);
if(tagOffset < 0)
return -1;
char frameSyncBytes[2] = {};
char tagHeaderBytes[4] = {};
long position = 0;
const long frameOffset = firstFrameOffset();
if(frameOffset < tagOffset)
return -1;
while(true) {
seek(position);
const ByteVector buffer = readBlock(bufferSize());
if(buffer.isEmpty())
return -1;
return tagOffset;
for(unsigned int i = 0; i < buffer.size(); ++i) {
frameSyncBytes[0] = frameSyncBytes[1];
frameSyncBytes[1] = buffer[i];
if(firstSyncByte(frameSyncBytes[0]) && secondSynchByte(frameSyncBytes[1])) {
Header header(this, position + i - 1, true);
if(header.isValid())
return -1;
}
tagHeaderBytes[0] = tagHeaderBytes[1];
tagHeaderBytes[1] = tagHeaderBytes[2];
tagHeaderBytes[2] = buffer[i];
if(headerID == tagHeaderBytes)
return position + i - 2;
}
position += bufferSize();
}
}