ladybird/AK/CircularBuffer.cpp
Tim Schumacher 221b91ff61 AK: Add CircularBuffer::find_copy_in_seekback()
This is useful for compressors, which quite frequently need to find a
matching span of data within the seekback.
2023-05-17 09:08:53 +02:00

356 lines
11 KiB
C++

/*
* Copyright (c) 2022, Lucas Chollet <lucas.chollet@free.fr>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#include <AK/CircularBuffer.h>
#include <AK/MemMem.h>
#include <AK/Stream.h>
namespace AK {
CircularBuffer::CircularBuffer(ByteBuffer buffer)
: m_buffer(move(buffer))
{
}
ErrorOr<CircularBuffer> CircularBuffer::create_empty(size_t size)
{
auto temporary_buffer = TRY(ByteBuffer::create_uninitialized(size));
CircularBuffer circular_buffer { move(temporary_buffer) };
return circular_buffer;
}
ErrorOr<CircularBuffer> CircularBuffer::create_initialized(ByteBuffer buffer)
{
CircularBuffer circular_buffer { move(buffer) };
circular_buffer.m_used_space = circular_buffer.m_buffer.size();
return circular_buffer;
}
size_t CircularBuffer::empty_space() const
{
return capacity() - m_used_space;
}
size_t CircularBuffer::used_space() const
{
return m_used_space;
}
size_t CircularBuffer::capacity() const
{
return m_buffer.size();
}
size_t CircularBuffer::seekback_limit() const
{
return m_seekback_limit;
}
bool CircularBuffer::is_wrapping_around() const
{
return capacity() <= m_reading_head + m_used_space;
}
Optional<size_t> CircularBuffer::offset_of(StringView needle, Optional<size_t> from, Optional<size_t> until) const
{
auto const read_from = from.value_or(0);
auto const read_until = until.value_or(m_used_space);
VERIFY(read_from <= read_until);
Array<ReadonlyBytes, 2> spans {};
spans[0] = next_read_span();
auto const original_span_0_size = spans[0].size();
if (read_from > 0)
spans[0] = spans[0].slice(min(spans[0].size(), read_from));
if (spans[0].size() + read_from > read_until)
spans[0] = spans[0].trim(read_until - read_from);
else if (is_wrapping_around())
spans[1] = m_buffer.span().slice(max(original_span_0_size, read_from) - original_span_0_size, min(read_until, m_used_space) - original_span_0_size);
auto maybe_found = AK::memmem(spans.begin(), spans.end(), needle.bytes());
if (maybe_found.has_value())
*maybe_found += read_from;
return maybe_found;
}
void CircularBuffer::clear()
{
m_reading_head = 0;
m_used_space = 0;
m_seekback_limit = 0;
}
Bytes CircularBuffer::next_write_span()
{
if (is_wrapping_around())
return m_buffer.span().slice(m_reading_head + m_used_space - capacity(), capacity() - m_used_space);
return m_buffer.span().slice(m_reading_head + m_used_space, capacity() - (m_reading_head + m_used_space));
}
ReadonlyBytes CircularBuffer::next_read_span() const
{
return m_buffer.span().slice(m_reading_head, min(capacity() - m_reading_head, m_used_space));
}
ReadonlyBytes CircularBuffer::next_read_span_with_seekback(size_t distance) const
{
VERIFY(m_seekback_limit <= capacity());
VERIFY(distance <= m_seekback_limit);
// Note: We are adding the capacity once here to ensure that we can wrap around the negative space by using modulo.
auto read_offset = (capacity() + m_reading_head + m_used_space - distance) % capacity();
return m_buffer.span().slice(read_offset, min(capacity() - read_offset, distance));
}
size_t CircularBuffer::write(ReadonlyBytes bytes)
{
auto remaining = bytes.size();
while (remaining > 0) {
auto const next_span = next_write_span();
if (next_span.size() == 0)
break;
auto const written_bytes = bytes.slice(bytes.size() - remaining).copy_trimmed_to(next_span);
m_used_space += written_bytes;
m_seekback_limit += written_bytes;
if (m_seekback_limit > capacity())
m_seekback_limit = capacity();
remaining -= written_bytes;
}
return bytes.size() - remaining;
}
Bytes CircularBuffer::read(Bytes bytes)
{
auto remaining = bytes.size();
while (remaining > 0) {
auto const next_span = next_read_span();
if (next_span.size() == 0)
break;
auto written_bytes = next_span.copy_trimmed_to(bytes.slice(bytes.size() - remaining));
m_used_space -= written_bytes;
m_reading_head += written_bytes;
if (m_reading_head >= capacity())
m_reading_head -= capacity();
remaining -= written_bytes;
}
return bytes.trim(bytes.size() - remaining);
}
ErrorOr<Bytes> CircularBuffer::read_with_seekback(Bytes bytes, size_t distance)
{
if (distance > m_seekback_limit)
return Error::from_string_literal("Tried a seekback read beyond the seekback limit");
auto remaining = bytes.size();
while (remaining > 0) {
auto const next_span = next_read_span_with_seekback(distance);
if (next_span.size() == 0)
break;
auto written_bytes = next_span.copy_trimmed_to(bytes.slice(bytes.size() - remaining));
distance -= written_bytes;
remaining -= written_bytes;
}
return bytes.trim(bytes.size() - remaining);
}
ErrorOr<void> CircularBuffer::discard(size_t discarding_size)
{
if (m_used_space < discarding_size)
return Error::from_string_literal("Can not discard more data than what the buffer contains");
m_used_space -= discarding_size;
m_reading_head = (m_reading_head + discarding_size) % capacity();
return {};
}
ErrorOr<size_t> CircularBuffer::fill_from_stream(Stream& stream)
{
auto next_span = next_write_span();
if (next_span.size() == 0)
return 0;
auto bytes = TRY(stream.read_some(next_span));
m_used_space += bytes.size();
m_seekback_limit += bytes.size();
if (m_seekback_limit > capacity())
m_seekback_limit = capacity();
return bytes.size();
}
ErrorOr<size_t> CircularBuffer::flush_to_stream(Stream& stream)
{
auto next_span = next_read_span();
if (next_span.size() == 0)
return 0;
auto written_bytes = TRY(stream.write_some(next_span));
m_used_space -= written_bytes;
m_reading_head += written_bytes;
if (m_reading_head >= capacity())
m_reading_head -= capacity();
return written_bytes;
}
ErrorOr<size_t> CircularBuffer::copy_from_seekback(size_t distance, size_t length)
{
if (distance > m_seekback_limit)
return Error::from_string_literal("Tried a seekback copy beyond the seekback limit");
auto remaining_length = length;
while (remaining_length > 0) {
if (empty_space() == 0)
break;
auto next_span = next_read_span_with_seekback(distance);
if (next_span.size() == 0)
break;
auto length_written = write(next_span.trim(remaining_length));
remaining_length -= length_written;
// If we copied right from the end of the seekback area (i.e. our length is larger than the distance)
// and the last copy was one complete "chunk", we can now double the distance to copy twice as much data in one go.
if (remaining_length > distance && length_written == distance)
distance *= 2;
}
return length - remaining_length;
}
ErrorOr<Vector<CircularBuffer::Match>> CircularBuffer::find_copy_in_seekback(size_t maximum_length, size_t minimum_length, Optional<Vector<size_t> const&> distance_hints) const
{
VERIFY(minimum_length > 0);
// Clip the maximum length to the amount of data that we actually store.
if (maximum_length > m_used_space)
maximum_length = m_used_space;
if (maximum_length < minimum_length)
return Vector<Match> {};
Vector<Match> matches;
if (distance_hints.has_value()) {
// If we have any hints, verify and use those.
for (auto const& distance : distance_hints.value()) {
// TODO: This does not yet support looping repetitions.
if (distance < minimum_length)
continue;
auto needle_offset = (capacity() + m_reading_head) % capacity();
auto haystack_offset = (capacity() + m_reading_head - distance) % capacity();
for (size_t i = 0; i < minimum_length; i++) {
if (m_buffer[needle_offset] != m_buffer[haystack_offset])
break;
needle_offset = (needle_offset + 1) % capacity();
haystack_offset = (haystack_offset + 1) % capacity();
if (i + 1 == minimum_length)
TRY(matches.try_empend(distance, minimum_length));
}
}
} else {
// Otherwise, use memmem to find the initial matches.
// Note: We have the read head as our reference point, but `next_read_span_with_seekback` isn't aware of that and continues to use the write head.
// Therefore, we need to make sure to slice off the extraneous bytes from the end of the span and shift the returned distances by the correct amount.
size_t haystack_offset_from_start = 0;
Vector<ReadonlyBytes, 2> haystack;
haystack.append(next_read_span_with_seekback(m_seekback_limit));
if (haystack[0].size() < m_seekback_limit - used_space())
haystack.append(next_read_span_with_seekback(m_seekback_limit - haystack[0].size()));
haystack.last() = haystack.last().trim(haystack.last().size() - used_space());
auto needle = next_read_span().trim(minimum_length);
auto memmem_match = AK::memmem(haystack.begin(), haystack.end(), needle);
while (memmem_match.has_value()) {
auto match_offset = memmem_match.release_value();
// Add the match to the list of matches to work with.
TRY(matches.try_empend(m_seekback_limit - used_space() - haystack_offset_from_start - match_offset, minimum_length));
auto size_to_discard = match_offset + 1;
// Trim away the already processed bytes from the haystack.
haystack_offset_from_start += size_to_discard;
while (size_to_discard > 0) {
if (haystack[0].size() < size_to_discard) {
size_to_discard -= haystack[0].size();
haystack.remove(0);
} else {
haystack[0] = haystack[0].slice(size_to_discard);
break;
}
}
if (haystack.size() == 0)
break;
// Try and find the next match.
memmem_match = AK::memmem(haystack.begin(), haystack.end(), needle);
}
}
// From now on, all matches that we have stored have at least a length of `minimum_length` and they all refer to the same value.
// For the remaining part, we will keep checking the next byte incrementally and keep eliminating matches until we eliminated all of them.
Vector<Match> next_matches;
for (size_t offset = minimum_length; offset < maximum_length; offset++) {
auto needle_data = m_buffer[(capacity() + m_reading_head + offset) % capacity()];
for (auto const& match : matches) {
auto haystack_data = m_buffer[(capacity() + m_reading_head - match.distance + offset) % capacity()];
if (haystack_data != needle_data)
continue;
TRY(next_matches.try_empend(match.distance, match.length + 1));
}
if (next_matches.size() == 0)
return matches;
swap(matches, next_matches);
next_matches.clear_with_capacity();
}
return matches;
}
}