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LibAudio: Accurately skip MP3 frames using the actual header size

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Prevously, the header size was used to calculate the `slot_count` field
of `MP3::Header`, but `build_seek_table()` just used the maximum size
of the header instead, causing it not to seek far enough, and in cases
where a possible sync code occurred two bytes before the next frame, it
would read that possible sync code as if it was a real frame. It would
then either reject it due to bad field values, or could possibly skip
over the next real frame due to a larger calculated frame size in the
bogus frame.

By fixing this issue, we now properly calculate the duration of MP3
files where these fake sync codes occur. In the case of the raw file
for this podcast:

https://changelog.com/podcast/554

the duration goes from 1:21:57 to 1:22:47, which is the real duration
according to the player user interface.
This commit is contained in:
Zaggy1024 2023-08-25 20:20:46 -05:00 • committed by Tim Flynn
parent 5b8895fff0
commit e087d35cd8
2 changed files with 7 additions and 3 deletions
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9
Userland/Libraries/LibAudio/MP3Loader.cpp
View file

@ -192,7 +192,7 @@ MaybeLoaderError MP3LoaderPlugin::build_seek_table()
frame_count++;
sample_count += MP3::frame_size;
TRY(m_stream->seek(error_or_header.value().frame_size - 6, SeekMode::FromCurrentPosition));
TRY(m_stream->seek(error_or_header.value().frame_size - error_or_header.value().header_size, SeekMode::FromCurrentPosition));
// TODO: This is just here to clear the bitstream buffer.
// Bitstream should have a method to sync its state to the underlying stream.
@ -223,10 +223,13 @@ ErrorOr<MP3::Header, LoaderError> MP3LoaderPlugin::read_header()
header.copyright_bit = TRY(m_bitstream->read_bit());
header.original_bit = TRY(m_bitstream->read_bit());
header.emphasis = static_cast<MP3::Emphasis>(TRY(m_bitstream->read_bits(2)));
if (!header.protection_bit)
header.header_size = 4;
if (!header.protection_bit) {
header.crc16 = TRY(m_bitstream->read_bits<u16>(16));
header.header_size += 2;
}
header.frame_size = 144 * header.bitrate * 1000 / header.samplerate + header.padding_bit;
header.slot_count = header.frame_size - ((header.channel_count() == 2 ? 32 : 17) + (header.protection_bit ? 0 : 2) + 4);
header.slot_count = header.frame_size - ((header.channel_count() == 2 ? 32 : 17) + header.header_size);
return header;
}

1
Userland/Libraries/LibAudio/MP3Types.h
View file

@ -58,6 +58,7 @@ struct Header {
bool original_bit { false };
Emphasis emphasis { Emphasis::None };
u16 crc16 { 0 };
size_t header_size { 0 };
size_t frame_size { 0 };
size_t slot_count { 0 };