deque.hpp: Improve shrink implementation

rt/include/typon/fundamental/ring_buffer.hpp

@@ -2,6 +2,7 @@
 #define TYPON_FUNDAMENTAL_RINGBUFFER_HPP_INCLUDED
 
 #include <atomic>
+#include <bit>
 
 
 namespace typon::fdt::lock_free
@@ -18,7 +19,7 @@ namespace typon::fdt::lock_free
 
     const u8 _bits;
     const u64 _mask;
-    ring_buffer * const _next;
+    ring_buffer * _next;
     std::atomic<T> * const _array;
 
     ring_buffer(u8 bits, ring_buffer * next = nullptr) noexcept
@@ -68,7 +69,24 @@ namespace typon::fdt::lock_free
 
     ring_buffer * shrink(u64 start, u64 end) noexcept
     {
-      return fill(std::exchange(_next, nullptr), start, end);
+      ring_buffer * last = nullptr;
+      ring_buffer * next = this;
+      u64 size = end - start;
+      while (auto buff = next->_next)
+      {
+        if (buff->capacity() < size * 4)
+        {
+          break;
+        }
+        last = next;
+        next = buff;
+      }
+      if (!last)
+      {
+        return nullptr;
+      }
+      last->_next = nullptr;
+      return fill(next, start, end);
     }
   };