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BMFF Performance boost: don't read boxes we're not interested in #1974

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Oct 20, 2021
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BMFF Performance boost: don't read boxes we're not interested in #1974

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22 changes: 21 additions & 1 deletion src/bmffimage.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -125,6 +125,14 @@ namespace Exiv2
return box == TAG_meta || box == TAG_iinf || box == TAG_iloc;
}

static bool skipBox(uint32_t box)
{
// Allows boxHandler() to optimise the reading of files by identifying
// box types that we're not interested in. Box types listed here must
// not appear in the cases in switch (box_type) in boxHandler().
return box == TAG_mdat; // mdat is where the main image lives and can be huge
}

std::string BmffImage::mimeType() const
{
switch (fileType_) {
Expand Down Expand Up @@ -230,7 +238,18 @@ namespace Exiv2
long restore = io_->tell();
enforce(box_length >= hdrsize, Exiv2::kerCorruptedMetadata);
enforce(box_length - hdrsize <= static_cast<size_t>(pbox_end - restore), Exiv2::kerCorruptedMetadata);
DataBuf data(static_cast<long>(box_length - hdrsize));

const long buffer_size = static_cast<long>(box_length - hdrsize);
if (skipBox(box_type)) {
if (bTrace) {
out << std::endl;
}
// The enforce() above checks that restore + buffer_size won't
// exceed pbox_end, and by implication, won't excced LONG_MAX
return restore + buffer_size;
}

DataBuf data(buffer_size);
const long box_end = restore + data.size();
io_->read(data.data(), data.size());
io_->seek(restore, BasicIo::beg);
Expand All @@ -248,6 +267,7 @@ namespace Exiv2
}

switch (box_type) {
// See notes in skipBox()
case TAG_ftyp: {
enforce(data.size() >= 4, Exiv2::kerCorruptedMetadata);
fileType_ = data.read_uint32(0, endian_);
Expand Down