deque.hpp: Report resize requests in pop and steal

rt/include/typon/fundamental/deque.hpp

@@ -23,20 +23,16 @@ namespace typon::fdt::lock_free
     using u8 = typename ring_buffer<T>::u8;
     using u64 = typename ring_buffer<T>::u64;
 
-    static constexpr typename pop_type::template state<1> Abort {};
-    static constexpr typename pop_type::template state<2> Prune {};
+    static constexpr unsigned char Abort {1};
+    static constexpr unsigned char Resize {2};
 
     using enum std::memory_order;
 
-    const u8 _bits;
     std::atomic<u64> _top {1};
     std::atomic<u64> _bottom {1};
     std::atomic<array_type *> _array;
 
-    deque(u8 bits = 3) noexcept
-      : _bits(bits)
-      , _array(new array_type(bits))
-    {}
+    deque(u8 bits = 3) noexcept : _array(new array_type(bits)) {}
 
     ~deque()
     {
@@ -47,7 +43,7 @@ namespace typon::fdt::lock_free
     {
       u64 bottom = _bottom.load(relaxed);
       u64 top = _top.load(acquire);
-      array_type * array = _array.load(relaxed);
+      auto array = _array.load(relaxed);
       if (bottom - top > array->capacity() - 1)
       {
         array = array->grow(top, bottom);
@@ -58,68 +54,69 @@ namespace typon::fdt::lock_free
       _bottom.store(bottom + 1, relaxed);
     }
 
-    pop_type pop() noexcept
+    optional<T> pop() noexcept
     {
       u64 bottom = _bottom.load(relaxed) - 1;
-      array_type * array = _array.load(relaxed);
+      auto array = _array.load(relaxed);
       _bottom.store(bottom, relaxed);
       std::atomic_thread_fence(seq_cst);
       u64 top = _top.load(relaxed);
-      pop_type x {};
       if (top <= bottom)
       {
-        x = array->get(bottom);
         if (top == bottom)
         {
           if (!_top.compare_exchange_strong(top, top + 1, seq_cst, relaxed))
           {
-            x = {};
-          }
             _bottom.store(bottom + 1, relaxed);
+            return array->_next ? optional<T>(Resize) : optional<T>();
           }
-      }
-      else
-      {
           _bottom.store(bottom + 1, relaxed);
         }
-      u64 capacity = array->capacity();
-      if (capacity > u64(1) << _bits && capacity > (bottom - top) * 4)
+        T x = array->get(bottom);
+        if (array->_next && array->capacity() > (bottom - top) * 4)
         {
-        x.set_state(Prune);
-      }
-      return x;
+          return { x, Resize };
         }
-
-    array_type * shrink() noexcept
-    {
-      u64 bottom = _bottom.load(relaxed);
-      array_type * array = _array.load(relaxed);
-      u64 top = _top.load(relaxed);
-      array_type * next = array->shrink(top, bottom);
-      if (next)
-      {
-        _array.store(next, relaxed);
-        return array;
+        return { x };
       }
-      return nullptr;
+      _bottom.store(bottom + 1, relaxed);
+      return array->_next ? optional<T>(Resize) : optional<T>();
     }
 
-    pop_type steal() noexcept
+    optional<T> steal() noexcept
     {
       u64 top = _top.load(acquire);
       std::atomic_thread_fence(seq_cst);
       u64 bottom = _bottom.load(acquire);
+      // Use acquire instead of consume (semantics under revision).
+      auto array = _array.load(acquire);
       if (top < bottom)
       {
-        array_type * array = _array.load(consume);
         T x = array->get(top);
         if (!_top.compare_exchange_strong(top, top + 1, seq_cst, relaxed))
         {
           return { Abort };
         }
+        if (array->_next && array->capacity() > (bottom - top) * 4)
+        {
+          return { x, Resize };
+        }
         return { x };
       }
-      return {};
+      return array->_next ? optional<T>(Resize) : optional<T>();
+    }
+
+    array_type * shrink() noexcept
+    {
+      u64 bottom = _bottom.load(relaxed);
+      u64 top = _top.load(relaxed);
+      auto array = _array.load(relaxed);
+      if (auto next = array->shrink(top, bottom))
+      {
+        _array.store(next, relaxed);
+        return array;
+      }
+      return nullptr;
     }
   };