AK: Add an optional starting offset to CircularBuffer::offset_of

This parameter allows to start searching after an offset. For example,
to resume a search.

It is unfortunately a breaking change in API so this patch also modifies
one user and one test.

AK/CircularBuffer.cpp

@@ -52,19 +52,29 @@ 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> until) const
+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();
 
-    if (spans[0].size() > read_until)
-        spans[0] = spans[0].trim(read_until);
-    else if (is_wrapping_around())
-        spans[1] = m_buffer.span().slice(0, read_until - spans[0].size());
+    if (read_from > 0)
+        spans[0] = spans[0].slice(min(spans[0].size(), read_from));
 
-    return AK::memmem(spans.begin(), spans.end(), needle.bytes());
+    if (spans[0].size() + read_from > read_until)
+        spans[0] = spans[0].trim(read_until - read_from);
+
+    if (is_wrapping_around())
+        spans[1] = m_buffer.span().slice(max(spans[0].size(), read_from) - spans[0].size(), min(read_until, m_used_space) - spans[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()

AK/CircularBuffer.h

@@ -36,7 +36,7 @@ class CircularBuffer {
     [[nodiscard]] size_t used_space() const;
     [[nodiscard]] size_t capacity() const;
 
-    Optional<size_t> offset_of(StringView needle, Optional<size_t> until = {}) const;
+    Optional<size_t> offset_of(StringView needle, Optional<size_t> from = {}, Optional<size_t> until = {}) const;
 
     void clear();
 

Tests/AK/TestCircularBuffer.cpp

@@ -272,6 +272,44 @@ TEST_CASE(offset_of)
     EXPECT(result.has_value());
     EXPECT_EQ(result.value(), 13ul);
 
-    result = circular_buffer.offset_of("!Well"sv, 12);
+    result = circular_buffer.offset_of("!Well"sv, {}, 12);
     EXPECT(!result.has_value());
+
+    result = circular_buffer.offset_of("e"sv, 2);
+    EXPECT(result.has_value());
+    EXPECT_EQ(result.value(), 9ul);
+}
+
+TEST_CASE(offset_of_with_until_and_after)
+{
+    auto const source = "Well Hello Friends!"sv;
+    auto byte_buffer_or_error = ByteBuffer::copy(source.bytes());
+    EXPECT(!byte_buffer_or_error.is_error());
+    auto byte_buffer = byte_buffer_or_error.release_value();
+
+    auto circular_buffer_or_error = CircularBuffer::create_initialized(byte_buffer);
+    EXPECT(!circular_buffer_or_error.is_error());
+    auto circular_buffer = circular_buffer_or_error.release_value();
+
+    auto result = circular_buffer.offset_of("Well Hello Friends!"sv, 0, 19);
+    EXPECT_EQ(result.value_or(42), 0ul);
+
+    result = circular_buffer.offset_of(" Hello"sv, 4, 10);
+    EXPECT_EQ(result.value_or(42), 4ul);
+
+    result = circular_buffer.offset_of("el"sv, 3, 10);
+    EXPECT_EQ(result.value_or(42), 6ul);
+
+    safe_discard(circular_buffer, 5);
+    auto written_bytes = circular_buffer.write(byte_buffer.span().trim(5));
+    EXPECT_EQ(written_bytes, 5ul);
+
+    result = circular_buffer.offset_of("Hello Friends!Well "sv, 0, 19);
+    EXPECT_EQ(result.value_or(42), 0ul);
+
+    result = circular_buffer.offset_of("o Frie"sv, 4, 10);
+    EXPECT_EQ(result.value_or(42), 4ul);
+
+    result = circular_buffer.offset_of("el"sv, 3, 14);
+    EXPECT_EQ(result.value_or(42), 15ul);
 }

Userland/Libraries/LibCore/Stream.h

@@ -719,7 +719,7 @@ class BufferedHelper {
         Optional<size_t> longest_match;
         size_t match_size = 0;
         for (auto& candidate : candidates) {
-            auto const result = m_buffer.offset_of(candidate, readable_size);
+            auto const result = m_buffer.offset_of(candidate, {}, readable_size);
             if (result.has_value()) {
                 auto previous_match = longest_match.value_or(*result);
                 if ((previous_match < *result) || (previous_match == *result && match_size < candidate.length())) {