AK: Add CircularBuffer::read_with_seekback

Author: https://github.com/timschumi
Commit: https://github.com/SerenityOS/serenity/commit/d717a08003
Pull-request: https://github.com/SerenityOS/serenity/pull/16917
Reviewed-by: https://github.com/ADKaster ✅

AK/CircularBuffer.cpp

@@ -71,6 +71,7 @@ void CircularBuffer::clear()
 {
     m_reading_head = 0;
     m_used_space = 0;
+    m_seekback_limit = 0;
 }
 
 Bytes CircularBuffer::next_write_span()
@@ -85,6 +86,17 @@ 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, m_seekback_limit));
+}
+
 size_t CircularBuffer::write(ReadonlyBytes bytes)
 {
     auto remaining = bytes.size();
@@ -98,6 +110,10 @@ size_t CircularBuffer::write(ReadonlyBytes bytes)
 
         m_used_space += written_bytes;
 
+        m_seekback_limit += written_bytes;
+        if (m_seekback_limit > capacity())
+            m_seekback_limit = capacity();
+
         remaining -= written_bytes;
     }
 
@@ -127,6 +143,27 @@ Bytes CircularBuffer::read(Bytes 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)

AK/CircularBuffer.h

@@ -42,6 +42,10 @@ public:
     Bytes read(Bytes bytes);
     ErrorOr<void> discard(size_t discarded_bytes);
 
+    /// Compared to `read()`, this starts reading from an offset that is `distance` bytes
+    /// before the current write pointer and allows for reading already-read data.
+    ErrorOr<Bytes> read_with_seekback(Bytes bytes, size_t distance);
+
     [[nodiscard]] size_t empty_space() const;
     [[nodiscard]] size_t used_space() const;
     [[nodiscard]] size_t capacity() const;
@@ -57,11 +61,13 @@ private:
 
     [[nodiscard]] Bytes next_write_span();
     [[nodiscard]] ReadonlyBytes next_read_span() const;
+    [[nodiscard]] ReadonlyBytes next_read_span_with_seekback(size_t distance) const;
 
     ByteBuffer m_buffer {};
 
     size_t m_reading_head {};
     size_t m_used_space {};
+    size_t m_seekback_limit {};
 };
 
 }