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LibCore: Do short forward seeks by discarding bytes from the buffer

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This saves us an actual seek and rereading already stored buffer data in
cases where the seek is entirely covered by the currently buffered data.
This is especially important since we implement `discard` using `seek`
for seekable streams.
This commit is contained in:
Tim Schumacher 2023-01-16 16:46:18 +01:00 committed by Jelle Raaijmakers
parent bdf991fe76
commit add2e2c076
Notes: sideshowbarker 2024-07-17 06:38:11 +09:00 
Author: https://github.com/timschumi
Commit: https://github.com/SerenityOS/serenity/commit/add2e2c076
Pull-request: https://github.com/SerenityOS/serenity/pull/17042
Reviewed-by: https://github.com/LucasChollet ✅
2 changed files with 30 additions and 3 deletions
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19
Tests/LibCore/TestLibCoreStream.cpp
View file

@ -470,8 +470,9 @@ TEST_CASE(buffered_small_file_read)
TEST_CASE(buffered_file_tell_and_seek)
{
// We choose a buffer size of 12 bytes to cover half of the input file.
auto file = Core::Stream::File::open("/usr/Tests/LibCore/small.txt"sv, Core::Stream::OpenMode::Read).release_value();
auto buffered_file = Core::Stream::BufferedFile::create(move(file)).release_value();
auto buffered_file = Core::Stream::BufferedFile::create(move(file), 12).release_value();
// Initial state.
{
@ -529,13 +530,27 @@ TEST_CASE(buffered_file_tell_and_seek)
EXPECT_EQ(current_offset, 0);
}
// Read the first character.
// Read the first character. This should prime the buffer if it hasn't happened already.
{
auto character = buffered_file->read_value<char>().release_value();
EXPECT_EQ(character, 'W');
auto current_offset = buffered_file->tell().release_value();
EXPECT_EQ(current_offset, 1);
}
// Seek beyond the buffer size, which should invalidate the buffer.
{
auto current_offset = buffered_file->seek(12, Core::Stream::SeekMode::SetPosition).release_value();
EXPECT_EQ(current_offset, 12);
}
// Ensure that we still read the correct contents from the new offset with a (presumably) freshly filled buffer.
{
auto character = buffered_file->read_value<char>().release_value();
EXPECT_EQ(character, 'r');
auto current_offset = buffered_file->tell().release_value();
EXPECT_EQ(current_offset, 13);
}
}
constexpr auto buffered_sent_data = "Well hello friends!\n:^)\nThis shouldn't be present. :^("sv;

14
Userland/Libraries/LibCore/Stream.h
View file

@ -819,6 +819,11 @@ public:
m_buffer.clear();
}
ErrorOr<void> discard_bytes(size_t count)
{
return m_buffer.discard(count);
}
private:
ErrorOr<size_t> populate_read_buffer()
{
@ -875,8 +880,15 @@ public:
virtual void close() override { m_helper.stream().close(); }
virtual ErrorOr<off_t> seek(i64 offset, SeekMode mode) override
{
if (mode == SeekMode::FromCurrentPosition)
if (mode == SeekMode::FromCurrentPosition) {
// If possible, seek using the buffer alone.
if (0 <= offset && static_cast<u64>(offset) <= m_helper.buffered_data_size()) {
MUST(m_helper.discard_bytes(offset));
return TRY(m_helper.stream().tell()) - m_helper.buffered_data_size();
}
offset = offset - m_helper.buffered_data_size();
}
auto result = TRY(m_helper.stream().seek(offset, mode));
m_helper.clear_buffer();