AK: Use hashing to accelerate searching a CircularBuffer

AK/CircularBuffer.cpp

@@ -296,10 +296,34 @@ ErrorOr<SearchableCircularBuffer> SearchableCircularBuffer::create_initialized(B
 
     circular_buffer.m_used_space = circular_buffer.m_buffer.size();
 
+    for (size_t i = 0; i + HASH_CHUNK_SIZE <= circular_buffer.m_buffer.size(); i++)
+        TRY(circular_buffer.insert_location_hash(circular_buffer.m_buffer.span().slice(i, HASH_CHUNK_SIZE), i));
+
     return circular_buffer;
 }
 
-ErrorOr<Vector<SearchableCircularBuffer::Match>> SearchableCircularBuffer::find_copy_in_seekback(size_t maximum_length, size_t minimum_length) const
+ErrorOr<Bytes> SearchableCircularBuffer::read(Bytes bytes)
+{
+    auto read_bytes_span = CircularBuffer::read(bytes);
+    TRY(hash_last_bytes(read_bytes_span.size()));
+    return read_bytes_span;
+}
+
+ErrorOr<void> SearchableCircularBuffer::discard(size_t discarded_bytes)
+{
+    TRY(CircularBuffer::discard(discarded_bytes));
+    TRY(hash_last_bytes(discarded_bytes));
+    return {};
+}
+
+ErrorOr<size_t> SearchableCircularBuffer::flush_to_stream(Stream& stream)
+{
+    auto flushed_byte_count = TRY(CircularBuffer::flush_to_stream(stream));
+    TRY(hash_last_bytes(flushed_byte_count));
+    return flushed_byte_count;
+}
+
+Optional<SearchableCircularBuffer::Match> SearchableCircularBuffer::find_copy_in_seekback(size_t maximum_length, size_t minimum_length)
 {
     VERIFY(minimum_length > 0);
 
@@ -308,71 +332,137 @@ ErrorOr<Vector<SearchableCircularBuffer::Match>> SearchableCircularBuffer::find_
         maximum_length = m_used_space;
 
     if (maximum_length < minimum_length)
-        return Vector<Match> {};
+        return {};
 
-    Vector<Match> matches;
+    Optional<Match> best_match;
 
-    // Use memmem to find the initial matches.
-    size_t haystack_offset_from_start = 0;
-    Vector<ReadonlyBytes, 2> haystack;
-    haystack.append(next_search_span(search_limit()));
-    if (haystack[0].size() < search_limit())
-        haystack.append(next_search_span(search_limit() - haystack[0].size()));
+    Array<u8, HASH_CHUNK_SIZE> needle_storage;
+    auto needle = needle_storage.span().trim(min(HASH_CHUNK_SIZE, maximum_length));
 
-    auto needle = next_read_span().trim(minimum_length);
+    {
+        auto needle_read_bytes = MUST(read_with_seekback(needle, used_space()));
+        VERIFY(needle_read_bytes.size() == needle.size());
+    }
 
-    auto memmem_match = AK::memmem(haystack.begin(), haystack.end(), needle);
-    while (memmem_match.has_value()) {
-        auto match_offset = memmem_match.release_value();
+    // Try an efficient hash-based search first.
+    if (needle.size() >= HASH_CHUNK_SIZE) {
+        auto needle_hash = StringView { needle }.hash();
 
-        // 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 maybe_starting_offset = m_hash_location_map.get(needle_hash);
 
-        auto size_to_discard = match_offset + 1;
+        if (maybe_starting_offset.has_value()) {
+            Optional<size_t> previous_buffer_offset;
+            auto current_buffer_offset = maybe_starting_offset.value();
 
-        // 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;
+            while (true) {
+                auto current_search_offset = (capacity() + m_reading_head - current_buffer_offset) % capacity();
+
+                // Validate the hash. In case it is invalid, we can discard the rest of the chain, as the data (and everything older) got updated.
+                Array<u8, HASH_CHUNK_SIZE> hash_chunk_at_offset;
+                auto hash_chunk_at_offset_span = MUST(read_with_seekback(hash_chunk_at_offset, current_search_offset + used_space()));
+                VERIFY(hash_chunk_at_offset_span.size() == HASH_CHUNK_SIZE);
+                auto found_chunk_hash = StringView { hash_chunk_at_offset }.hash();
+                if (needle_hash != found_chunk_hash) {
+                    if (!previous_buffer_offset.has_value())
+                        m_hash_location_map.remove(needle_hash);
+                    else
+                        m_location_chain_map.remove(*previous_buffer_offset);
+                    break;
+                }
+
+                // Validate the match through the set-distance-based implementation.
+                auto maybe_new_match = find_copy_in_seekback(Array { current_search_offset }, maximum_length, HASH_CHUNK_SIZE);
+
+                // If we found a match, record it.
+                // If we haven't found a match, we simply got a hash collision, so skip.
+                if (maybe_new_match.has_value()) {
+                    auto new_match = maybe_new_match.release_value();
+
+                    if (!best_match.has_value() || best_match->length < new_match.length) {
+                        best_match = new_match;
+
+                        // If we already found a result with the best possible length, then stop searching.
+                        if (best_match->length >= maximum_length)
+                            break;
+                    }
+                }
+
+                // Get the next location with the same hash from the location chain.
+                auto maybe_next_buffer_offset = m_location_chain_map.get(current_buffer_offset);
+
+                // End of the chain, nothing more to check.
+                if (!maybe_next_buffer_offset.has_value())
+                    break;
+
+                previous_buffer_offset = current_buffer_offset;
+                current_buffer_offset = maybe_next_buffer_offset.release_value();
             }
+
+            // If we found a match, return it now.
+            if (best_match.has_value())
+                return best_match;
         }
-
-        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;
+    // Try a plain memory search for smaller values.
+    // Note: This overlaps with the hash search for chunks of size HASH_CHUNK_SIZE for the purpose of validation.
+    if (minimum_length <= HASH_CHUNK_SIZE) {
+        size_t haystack_offset_from_start = 0;
+        Vector<ReadonlyBytes, 2> haystack;
+        haystack.append(next_search_span(search_limit()));
+        if (haystack[0].size() < search_limit())
+            haystack.append(next_search_span(search_limit() - haystack[0].size()));
 
-    for (size_t offset = minimum_length; offset < maximum_length; offset++) {
-        auto needle_data = m_buffer[(capacity() + m_reading_head + offset) % capacity()];
+        // TODO: `memmem` searches the memory in "natural" order, which means that it finds matches with a greater distance first.
+        //       Hash-based searching finds the shortest distances first, which is most likely better for encoding and memory efficiency.
+        //       Look into creating a `memmem_reverse`, which starts searching from the end.
+        auto memmem_match = AK::memmem(haystack.begin(), haystack.end(), needle);
+        while (memmem_match.has_value()) {
+            auto match_offset = memmem_match.release_value();
+            auto corrected_match_distance = search_limit() - haystack_offset_from_start - match_offset;
 
-        for (auto const& match : matches) {
-            auto haystack_data = m_buffer[(capacity() + m_reading_head - match.distance + offset) % capacity()];
+            // Validate the match through the set-distance-based implementation and extend it to the largest size possible.
+            auto maybe_new_match = find_copy_in_seekback(Array { corrected_match_distance }, min(maximum_length, HASH_CHUNK_SIZE), minimum_length);
 
-            if (haystack_data != needle_data)
-                continue;
+            // If we weren't able to validate the match at all, either our memmem search returned garbage or our validation function is incorrect. Investigate.
+            VERIFY(maybe_new_match.has_value());
 
-            TRY(next_matches.try_empend(match.distance, match.length + 1));
+            auto new_match = maybe_new_match.release_value();
+
+            if (!best_match.has_value() || best_match->length < new_match.length) {
+                best_match = new_match;
+
+                // If we already found a result with the best possible length, then stop searching.
+                if (best_match->length >= maximum_length)
+                    break;
+            }
+
+            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);
         }
 
-        if (next_matches.size() == 0)
-            return matches;
-
-        swap(matches, next_matches);
-        next_matches.clear_with_capacity();
+        // If we found a match of size HASH_CHUNK_SIZE, we should have already found that using the hash search. Investigate.
+        VERIFY(!best_match.has_value() || best_match->length < HASH_CHUNK_SIZE);
     }
 
-    return matches;
+    return best_match;
 }
 
 Optional<SearchableCircularBuffer::Match> SearchableCircularBuffer::find_copy_in_seekback(ReadonlySpan<size_t> distances, size_t maximum_length, size_t minimum_length) const
@@ -422,4 +512,75 @@ Optional<SearchableCircularBuffer::Match> SearchableCircularBuffer::find_copy_in
     return best_match;
 }
 
+ErrorOr<void> SearchableCircularBuffer::insert_location_hash(ReadonlyBytes value, size_t raw_offset)
+{
+    VERIFY(value.size() == HASH_CHUNK_SIZE);
+
+    auto value_hash = StringView { value }.hash();
+
+    // Discard any old entries for this offset first. This should eliminate accidental loops by breaking the chain.
+    // The actual cleanup is done on access, since we can only remove invalid references when actually walking the chain.
+    m_location_chain_map.remove(raw_offset);
+
+    // Check if we have any existing entries for this hash.
+    // 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.
+    auto existing_entry = m_hash_location_map.get(value_hash);
+
+    if (existing_entry.has_value())
+        TRY(m_location_chain_map.try_set(raw_offset, existing_entry.value()));
+
+    TRY(m_hash_location_map.try_set(value_hash, raw_offset));
+
+    return {};
+}
+
+ErrorOr<void> SearchableCircularBuffer::hash_last_bytes(size_t count)
+{
+    // Stop early if we don't have enough data overall to hash a full chunk.
+    if (search_limit() < HASH_CHUNK_SIZE)
+        return {};
+
+    auto remaining_recalculations = count;
+    while (remaining_recalculations > 0) {
+        // 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).
+        //       The number of recalculations stays the same, since we now have up to HASH_CHUNK_SIZE - 1 bytes that we can't hash now.
+        auto recalculation_span = next_search_span(min(remaining_recalculations + HASH_CHUNK_SIZE - 1, search_limit()));
+
+        // If the span is smaller than a hash chunk, we need to manually craft some consecutive data to do the hashing.
+        if (recalculation_span.size() < HASH_CHUNK_SIZE) {
+            auto auxiliary_span = next_seekback_span(remaining_recalculations);
+
+            // Ensure that our math is correct and that both spans are "adjacent".
+            VERIFY(recalculation_span.data() + recalculation_span.size() == m_buffer.data() + m_buffer.size());
+            VERIFY(auxiliary_span.data() == m_buffer.data());
+
+            while (recalculation_span.size() > 0 && recalculation_span.size() + auxiliary_span.size() >= HASH_CHUNK_SIZE) {
+                Array<u8, HASH_CHUNK_SIZE> temporary_hash_chunk;
+
+                auto copied_from_recalculation_span = recalculation_span.copy_to(temporary_hash_chunk);
+                VERIFY(copied_from_recalculation_span == recalculation_span.size());
+
+                auto copied_from_auxiliary_span = auxiliary_span.copy_to(temporary_hash_chunk.span().slice(copied_from_recalculation_span));
+                VERIFY(copied_from_recalculation_span + copied_from_auxiliary_span == HASH_CHUNK_SIZE);
+
+                TRY(insert_location_hash(temporary_hash_chunk, recalculation_span.data() - m_buffer.data()));
+
+                recalculation_span = recalculation_span.slice(1);
+                remaining_recalculations--;
+            }
+
+            continue;
+        }
+
+        for (size_t i = 0; i + HASH_CHUNK_SIZE <= recalculation_span.size(); i++) {
+            auto value = recalculation_span.slice(i, HASH_CHUNK_SIZE);
+            auto raw_offset = value.data() - m_buffer.data();
+            TRY(insert_location_hash(value, raw_offset));
+            remaining_recalculations--;
+        }
+    }
+
+    return {};
+}
+
 }