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.

AK/CircularBuffer.h

@@ -9,6 +9,7 @@
 #include <AK/ByteBuffer.h>
 #include <AK/Error.h>
 #include <AK/Noncopyable.h>
+#include <AK/Vector.h>
 
 namespace AK {
 
@@ -36,6 +37,15 @@ public:
 
     ErrorOr<size_t> copy_from_seekback(size_t distance, size_t length);
 
+    struct Match {
+        size_t distance;
+        size_t length;
+    };
+    /// This searches the seekback buffer (between read head and limit) for occurrences where it matches the next `length` bytes from the read buffer.
+    /// Supplying any hints will only consider those distances, in case existing offsets need to be validated.
+    /// Note that, since we only start searching at the read head, the length between read head and write head is excluded from the distance.
+    ErrorOr<Vector<Match>> find_copy_in_seekback(size_t maximum_length, size_t minimum_length = 2, Optional<Vector<size_t> const&> distance_hints = {}) const;
+
     [[nodiscard]] size_t empty_space() const;
     [[nodiscard]] size_t used_space() const;
     [[nodiscard]] size_t capacity() const;