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AK: Use hashing to accelerate searching a CircularBuffer
This commit is contained in:
Tim Schumacher
Linus Groh
parent
4a10cf1506
commit
60ac254df6
Notes:
sideshowbarker
2024-07-18 05:01:22 +09:00
Author: https://github.com/timschumi Commit: https://github.com/SerenityOS/serenity/commit/60ac254df6 Pull-request: https://github.com/SerenityOS/serenity/pull/19332 Reviewed-by: https://github.com/linusg ✅
@ -296,10 +296,34 @@ ErrorOr<SearchableCircularBuffer> SearchableCircularBuffer::create_initialized(B
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circular_buffer.m_used_space = circular_buffer.m_buffer.size();
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for (size_t i = 0; i + HASH_CHUNK_SIZE <= circular_buffer.m_buffer.size(); i++)
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TRY(circular_buffer.insert_location_hash(circular_buffer.m_buffer.span().slice(i, HASH_CHUNK_SIZE), i));
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return circular_buffer;
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}
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ErrorOr<Vector<SearchableCircularBuffer::Match>> SearchableCircularBuffer::find_copy_in_seekback(size_t maximum_length, size_t minimum_length) const
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ErrorOr<Bytes> SearchableCircularBuffer::read(Bytes bytes)
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{
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auto read_bytes_span = CircularBuffer::read(bytes);
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TRY(hash_last_bytes(read_bytes_span.size()));
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return read_bytes_span;
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}
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ErrorOr<void> SearchableCircularBuffer::discard(size_t discarded_bytes)
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{
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TRY(CircularBuffer::discard(discarded_bytes));
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TRY(hash_last_bytes(discarded_bytes));
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return {};
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}
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ErrorOr<size_t> SearchableCircularBuffer::flush_to_stream(Stream& stream)
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{
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auto flushed_byte_count = TRY(CircularBuffer::flush_to_stream(stream));
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TRY(hash_last_bytes(flushed_byte_count));
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return flushed_byte_count;
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}
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Optional<SearchableCircularBuffer::Match> SearchableCircularBuffer::find_copy_in_seekback(size_t maximum_length, size_t minimum_length)
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{
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VERIFY(minimum_length > 0);
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@ -308,25 +332,110 @@ ErrorOr<Vector<SearchableCircularBuffer::Match>> SearchableCircularBuffer::find_
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maximum_length = m_used_space;
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if (maximum_length < minimum_length)
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return Vector<Match> {};
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return {};
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Vector<Match> matches;
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Optional<Match> best_match;
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// Use memmem to find the initial matches.
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Array<u8, HASH_CHUNK_SIZE> needle_storage;
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auto needle = needle_storage.span().trim(min(HASH_CHUNK_SIZE, maximum_length));
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{
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auto needle_read_bytes = MUST(read_with_seekback(needle, used_space()));
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VERIFY(needle_read_bytes.size() == needle.size());
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}
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// Try an efficient hash-based search first.
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if (needle.size() >= HASH_CHUNK_SIZE) {
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auto needle_hash = StringView { needle }.hash();
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auto maybe_starting_offset = m_hash_location_map.get(needle_hash);
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if (maybe_starting_offset.has_value()) {
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Optional<size_t> previous_buffer_offset;
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auto current_buffer_offset = maybe_starting_offset.value();
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while (true) {
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auto current_search_offset = (capacity() + m_reading_head - current_buffer_offset) % capacity();
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// Validate the hash. In case it is invalid, we can discard the rest of the chain, as the data (and everything older) got updated.
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Array<u8, HASH_CHUNK_SIZE> hash_chunk_at_offset;
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auto hash_chunk_at_offset_span = MUST(read_with_seekback(hash_chunk_at_offset, current_search_offset + used_space()));
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VERIFY(hash_chunk_at_offset_span.size() == HASH_CHUNK_SIZE);
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auto found_chunk_hash = StringView { hash_chunk_at_offset }.hash();
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if (needle_hash != found_chunk_hash) {
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if (!previous_buffer_offset.has_value())
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m_hash_location_map.remove(needle_hash);
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else
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m_location_chain_map.remove(*previous_buffer_offset);
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break;
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}
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// Validate the match through the set-distance-based implementation.
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auto maybe_new_match = find_copy_in_seekback(Array { current_search_offset }, maximum_length, HASH_CHUNK_SIZE);
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// If we found a match, record it.
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// If we haven't found a match, we simply got a hash collision, so skip.
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if (maybe_new_match.has_value()) {
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auto new_match = maybe_new_match.release_value();
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if (!best_match.has_value() || best_match->length < new_match.length) {
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best_match = new_match;
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// If we already found a result with the best possible length, then stop searching.
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if (best_match->length >= maximum_length)
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break;
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}
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}
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// Get the next location with the same hash from the location chain.
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auto maybe_next_buffer_offset = m_location_chain_map.get(current_buffer_offset);
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// End of the chain, nothing more to check.
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if (!maybe_next_buffer_offset.has_value())
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break;
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previous_buffer_offset = current_buffer_offset;
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current_buffer_offset = maybe_next_buffer_offset.release_value();
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}
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// If we found a match, return it now.
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if (best_match.has_value())
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return best_match;
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}
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}
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// Try a plain memory search for smaller values.
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// Note: This overlaps with the hash search for chunks of size HASH_CHUNK_SIZE for the purpose of validation.
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if (minimum_length <= HASH_CHUNK_SIZE) {
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size_t haystack_offset_from_start = 0;
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Vector<ReadonlyBytes, 2> haystack;
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haystack.append(next_search_span(search_limit()));
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if (haystack[0].size() < search_limit())
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haystack.append(next_search_span(search_limit() - haystack[0].size()));
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auto needle = next_read_span().trim(minimum_length);
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// TODO: `memmem` searches the memory in "natural" order, which means that it finds matches with a greater distance first.
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// Hash-based searching finds the shortest distances first, which is most likely better for encoding and memory efficiency.
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// Look into creating a `memmem_reverse`, which starts searching from the end.
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auto memmem_match = AK::memmem(haystack.begin(), haystack.end(), needle);
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while (memmem_match.has_value()) {
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auto match_offset = memmem_match.release_value();
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auto corrected_match_distance = search_limit() - haystack_offset_from_start - match_offset;
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// Add the match to the list of matches to work with.
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TRY(matches.try_empend(m_seekback_limit - used_space() - haystack_offset_from_start - match_offset, minimum_length));
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// Validate the match through the set-distance-based implementation and extend it to the largest size possible.
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auto maybe_new_match = find_copy_in_seekback(Array { corrected_match_distance }, min(maximum_length, HASH_CHUNK_SIZE), minimum_length);
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// If we weren't able to validate the match at all, either our memmem search returned garbage or our validation function is incorrect. Investigate.
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VERIFY(maybe_new_match.has_value());
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auto new_match = maybe_new_match.release_value();
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if (!best_match.has_value() || best_match->length < new_match.length) {
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best_match = new_match;
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// If we already found a result with the best possible length, then stop searching.
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if (best_match->length >= maximum_length)
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break;
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}
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auto size_to_discard = match_offset + 1;
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@ -349,30 +458,11 @@ ErrorOr<Vector<SearchableCircularBuffer::Match>> SearchableCircularBuffer::find_
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memmem_match = AK::memmem(haystack.begin(), haystack.end(), needle);
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}
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// 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.
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// For the remaining part, we will keep checking the next byte incrementally and keep eliminating matches until we eliminated all of them.
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Vector<Match> next_matches;
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for (size_t offset = minimum_length; offset < maximum_length; offset++) {
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auto needle_data = m_buffer[(capacity() + m_reading_head + offset) % capacity()];
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for (auto const& match : matches) {
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auto haystack_data = m_buffer[(capacity() + m_reading_head - match.distance + offset) % capacity()];
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if (haystack_data != needle_data)
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continue;
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TRY(next_matches.try_empend(match.distance, match.length + 1));
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// If we found a match of size HASH_CHUNK_SIZE, we should have already found that using the hash search. Investigate.
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VERIFY(!best_match.has_value() || best_match->length < HASH_CHUNK_SIZE);
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}
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if (next_matches.size() == 0)
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return matches;
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swap(matches, next_matches);
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next_matches.clear_with_capacity();
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}
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return matches;
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return best_match;
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}
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Optional<SearchableCircularBuffer::Match> SearchableCircularBuffer::find_copy_in_seekback(ReadonlySpan<size_t> distances, size_t maximum_length, size_t minimum_length) const
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@ -422,4 +512,75 @@ Optional<SearchableCircularBuffer::Match> SearchableCircularBuffer::find_copy_in
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return best_match;
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}
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ErrorOr<void> SearchableCircularBuffer::insert_location_hash(ReadonlyBytes value, size_t raw_offset)
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{
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VERIFY(value.size() == HASH_CHUNK_SIZE);
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auto value_hash = StringView { value }.hash();
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// Discard any old entries for this offset first. This should eliminate accidental loops by breaking the chain.
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// The actual cleanup is done on access, since we can only remove invalid references when actually walking the chain.
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m_location_chain_map.remove(raw_offset);
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// Check if we have any existing entries for this hash.
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// If so, we need to add it to the location chain map instead, as we will soon replace the entry in the hash location map.
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auto existing_entry = m_hash_location_map.get(value_hash);
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if (existing_entry.has_value())
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TRY(m_location_chain_map.try_set(raw_offset, existing_entry.value()));
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TRY(m_hash_location_map.try_set(value_hash, raw_offset));
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return {};
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}
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ErrorOr<void> SearchableCircularBuffer::hash_last_bytes(size_t count)
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{
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// Stop early if we don't have enough data overall to hash a full chunk.
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if (search_limit() < HASH_CHUNK_SIZE)
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return {};
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auto remaining_recalculations = count;
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while (remaining_recalculations > 0) {
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// Note: We offset everything by HASH_CHUNK_SIZE because we have up to HASH_CHUNK_SIZE - 1 bytes that we couldn't hash before (as we had missing data).
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// The number of recalculations stays the same, since we now have up to HASH_CHUNK_SIZE - 1 bytes that we can't hash now.
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auto recalculation_span = next_search_span(min(remaining_recalculations + HASH_CHUNK_SIZE - 1, search_limit()));
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// If the span is smaller than a hash chunk, we need to manually craft some consecutive data to do the hashing.
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if (recalculation_span.size() < HASH_CHUNK_SIZE) {
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auto auxiliary_span = next_seekback_span(remaining_recalculations);
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// Ensure that our math is correct and that both spans are "adjacent".
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VERIFY(recalculation_span.data() + recalculation_span.size() == m_buffer.data() + m_buffer.size());
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VERIFY(auxiliary_span.data() == m_buffer.data());
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while (recalculation_span.size() > 0 && recalculation_span.size() + auxiliary_span.size() >= HASH_CHUNK_SIZE) {
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Array<u8, HASH_CHUNK_SIZE> temporary_hash_chunk;
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auto copied_from_recalculation_span = recalculation_span.copy_to(temporary_hash_chunk);
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VERIFY(copied_from_recalculation_span == recalculation_span.size());
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auto copied_from_auxiliary_span = auxiliary_span.copy_to(temporary_hash_chunk.span().slice(copied_from_recalculation_span));
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VERIFY(copied_from_recalculation_span + copied_from_auxiliary_span == HASH_CHUNK_SIZE);
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TRY(insert_location_hash(temporary_hash_chunk, recalculation_span.data() - m_buffer.data()));
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recalculation_span = recalculation_span.slice(1);
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remaining_recalculations--;
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}
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continue;
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}
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for (size_t i = 0; i + HASH_CHUNK_SIZE <= recalculation_span.size(); i++) {
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auto value = recalculation_span.slice(i, HASH_CHUNK_SIZE);
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auto raw_offset = value.data() - m_buffer.data();
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TRY(insert_location_hash(value, raw_offset));
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remaining_recalculations--;
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}
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}
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return {};
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}
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}
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@ -8,6 +8,7 @@
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#include <AK/ByteBuffer.h>
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#include <AK/Error.h>
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#include <AK/HashMap.h>
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#include <AK/Noncopyable.h>
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#include <AK/Vector.h>
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@ -67,6 +68,11 @@ public:
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[[nodiscard]] size_t search_limit() const;
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// These functions update the read pointer, so we need to hash any data that we have processed.
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ErrorOr<Bytes> read(Bytes bytes);
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ErrorOr<void> discard(size_t discarded_bytes);
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ErrorOr<size_t> flush_to_stream(Stream& stream);
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struct Match {
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size_t distance;
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size_t length;
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@ -74,15 +80,26 @@ public:
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/// This searches the seekback buffer (between read head and limit) for occurrences where it matches the next `length` bytes from the read buffer.
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/// Supplying any hints will only consider those distances, in case existing offsets need to be validated.
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/// Note that, since we only start searching at the read head, the length between read head and write head is excluded from the distance.
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ErrorOr<Vector<Match>> find_copy_in_seekback(size_t maximum_length, size_t minimum_length = 2) const;
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Optional<Match> find_copy_in_seekback(size_t maximum_length, size_t minimum_length = 2);
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Optional<Match> find_copy_in_seekback(ReadonlySpan<size_t> distances, size_t maximum_length, size_t minimum_length = 2) const;
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// The chunk size for which the hash table holds hashes.
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// This is nice for users to know, as picking a minimum match length that is
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// equal or greater than this allows us to completely skip a slow memory search.
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static constexpr size_t HASH_CHUNK_SIZE = 3;
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private:
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// Note: This function has a similar purpose as next_seekback_span, but they differ in their reference point.
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// Seekback operations start counting their distance at the write head, while search operations start counting their distance at the read head.
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[[nodiscard]] ReadonlyBytes next_search_span(size_t distance) const;
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SearchableCircularBuffer(ByteBuffer);
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HashMap<unsigned, size_t> m_hash_location_map;
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HashMap<size_t, size_t> m_location_chain_map;
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ErrorOr<void> insert_location_hash(ReadonlyBytes value, size_t raw_offset);
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ErrorOr<void> hash_last_bytes(size_t count);
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};
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}
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@ -342,72 +342,62 @@ TEST_CASE(find_copy_in_seekback)
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auto written_needle_bytes = buffer.write(needle);
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VERIFY(written_needle_bytes == needle.size());
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// Note: As of now, the preference during a tie is determined by which algorithm found the match.
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// Hash-based matching finds the shortest distance first, while memmem finds the greatest distance first.
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// A matching TODO can be found in CircularBuffer.cpp.
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{
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// Find the largest matches with a length between 1 and 1 (all "A").
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auto matches = MUST(buffer.find_copy_in_seekback(1, 1));
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EXPECT_EQ(matches.size(), 4ul);
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EXPECT_EQ(matches[0].distance, 11ul);
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EXPECT_EQ(matches[0].length, 1ul);
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EXPECT_EQ(matches[1].distance, 9ul);
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EXPECT_EQ(matches[1].length, 1ul);
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EXPECT_EQ(matches[2].distance, 6ul);
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EXPECT_EQ(matches[2].length, 1ul);
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EXPECT_EQ(matches[3].distance, 2ul);
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EXPECT_EQ(matches[3].length, 1ul);
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// Find the largest match with a length between 1 and 1 (all "A").
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auto match = buffer.find_copy_in_seekback(1, 1);
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EXPECT(match.has_value());
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EXPECT_EQ(match.value().distance, 11ul);
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EXPECT_EQ(match.value().length, 1ul);
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}
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{
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// Find the largest matches with a length between 1 and 2 (all "AB", everything smaller gets eliminated).
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auto matches = MUST(buffer.find_copy_in_seekback(2, 1));
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EXPECT_EQ(matches.size(), 4ul);
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EXPECT_EQ(matches[0].distance, 11ul);
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EXPECT_EQ(matches[0].length, 2ul);
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EXPECT_EQ(matches[1].distance, 9ul);
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EXPECT_EQ(matches[1].length, 2ul);
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EXPECT_EQ(matches[2].distance, 6ul);
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EXPECT_EQ(matches[2].length, 2ul);
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EXPECT_EQ(matches[3].distance, 2ul);
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EXPECT_EQ(matches[3].length, 2ul);
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// Find the largest match with a length between 1 and 2 (all "AB", everything smaller gets eliminated).
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auto match = buffer.find_copy_in_seekback(2, 1);
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EXPECT(match.has_value());
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EXPECT_EQ(match.value().distance, 11ul);
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EXPECT_EQ(match.value().length, 2ul);
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}
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{
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// Find the largest matches with a length between 1 and 3 (all "ABC", everything smaller gets eliminated).
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auto matches = MUST(buffer.find_copy_in_seekback(3, 1));
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EXPECT_EQ(matches.size(), 2ul);
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EXPECT_EQ(matches[0].distance, 9ul);
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EXPECT_EQ(matches[0].length, 3ul);
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EXPECT_EQ(matches[1].distance, 6ul);
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EXPECT_EQ(matches[1].length, 3ul);
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// Find the largest match with a length between 1 and 3 (all "ABC", everything smaller gets eliminated).
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auto match = buffer.find_copy_in_seekback(3, 1);
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EXPECT(match.has_value());
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EXPECT_EQ(match.value().distance, 6ul);
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EXPECT_EQ(match.value().length, 3ul);
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}
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{
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// Find the largest matches with a length between 1 and 4 (all "ABCD", everything smaller gets eliminated).
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auto matches = MUST(buffer.find_copy_in_seekback(4, 1));
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EXPECT_EQ(matches.size(), 1ul);
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EXPECT_EQ(matches[0].distance, 6ul);
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EXPECT_EQ(matches[0].length, 4ul);
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// Find the largest match with a length between 1 and 4 (all "ABCD", everything smaller gets eliminated).
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auto match = buffer.find_copy_in_seekback(4, 1);
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EXPECT(match.has_value());
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EXPECT_EQ(match.value().distance, 6ul);
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EXPECT_EQ(match.value().length, 4ul);
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}
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{
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// Find the largest matches with a length between 1 and 5 (all "ABCD", everything smaller gets eliminated, and nothing larger exists).
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auto matches = MUST(buffer.find_copy_in_seekback(5, 1));
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EXPECT_EQ(matches.size(), 1ul);
|
||||
EXPECT_EQ(matches[0].distance, 6ul);
|
||||
EXPECT_EQ(matches[0].length, 4ul);
|
||||
// Find the largest match with a length between 1 and 5 (all "ABCD", everything smaller gets eliminated, and nothing larger exists).
|
||||
auto match = buffer.find_copy_in_seekback(5, 1);
|
||||
EXPECT(match.has_value());
|
||||
EXPECT_EQ(match.value().distance, 6ul);
|
||||
EXPECT_EQ(match.value().length, 4ul);
|
||||
}
|
||||
|
||||
{
|
||||
// Find the largest matches with a length between 4 and 5 (all "ABCD", everything smaller never gets found, nothing larger exists).
|
||||
auto matches = MUST(buffer.find_copy_in_seekback(5, 4));
|
||||
EXPECT_EQ(matches.size(), 1ul);
|
||||
EXPECT_EQ(matches[0].distance, 6ul);
|
||||
EXPECT_EQ(matches[0].length, 4ul);
|
||||
// Find the largest match with a length between 4 and 5 (all "ABCD", everything smaller never gets found, nothing larger exists).
|
||||
auto match = buffer.find_copy_in_seekback(5, 4);
|
||||
EXPECT(match.has_value());
|
||||
EXPECT_EQ(match.value().distance, 6ul);
|
||||
EXPECT_EQ(match.value().length, 4ul);
|
||||
}
|
||||
|
||||
{
|
||||
// Find the largest matches with a length between 5 and 5 (nothing is found).
|
||||
auto matches = MUST(buffer.find_copy_in_seekback(5, 5));
|
||||
EXPECT_EQ(matches.size(), 0ul);
|
||||
// Find the largest match with a length between 5 and 5 (nothing is found).
|
||||
auto match = buffer.find_copy_in_seekback(5, 5);
|
||||
EXPECT(!match.has_value());
|
||||
}
|
||||
|
||||
{
|
||||
@ -432,8 +422,8 @@ TEST_CASE(find_copy_in_seekback)
|
||||
|
||||
{
|
||||
// Check that we don't find anything for a minimum length beyond the whole buffer size.
|
||||
auto matches = MUST(buffer.find_copy_in_seekback(12, 13));
|
||||
EXPECT_EQ(matches.size(), 0ul);
|
||||
auto match = buffer.find_copy_in_seekback(12, 13);
|
||||
EXPECT(!match.has_value());
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
@ -1007,17 +1007,17 @@ ErrorOr<void> LzmaCompressor::encode_once()
|
||||
}
|
||||
|
||||
// If we weren't able to find any viable existing offsets, we now have to search the rest of the dictionary for possible new offsets.
|
||||
auto new_distance_results = TRY(m_dictionary->find_copy_in_seekback(m_dictionary->used_space(), normalized_to_real_match_length_offset));
|
||||
auto new_distance_result = m_dictionary->find_copy_in_seekback(m_dictionary->used_space(), normalized_to_real_match_length_offset);
|
||||
|
||||
if (new_distance_results.size() > 0) {
|
||||
auto selected_match = new_distance_results[0];
|
||||
if (new_distance_result.has_value()) {
|
||||
auto selected_match = new_distance_result.release_value();
|
||||
TRY(encode_new_match(selected_match.distance, selected_match.length));
|
||||
return {};
|
||||
}
|
||||
|
||||
// If we weren't able to find any matches, we don't have any other choice than to encode the next byte as a literal.
|
||||
u8 next_byte { 0 };
|
||||
m_dictionary->read({ &next_byte, sizeof(next_byte) });
|
||||
TRY(m_dictionary->read({ &next_byte, sizeof(next_byte) }));
|
||||
TRY(encode_literal(next_byte));
|
||||
return {};
|
||||
}
|
||||
|
||||
Loading…
Reference in New Issue
Block a user