1 files changed, 141 insertions, 0 deletions
diff --git a/third_party/immer/immer/heap/heap_policy.hpp b/third_party/immer/immer/heap/heap_policy.hpp
new file mode 100644
index 0000000000..582c113f33
--- /dev/null
+++ b/third_party/immer/immer/heap/heap_policy.hpp
@@ -0,0 +1,141 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <immer/config.hpp>
+#include <immer/heap/debug_size_heap.hpp>
+#include <immer/heap/free_list_heap.hpp>
+#include <immer/heap/split_heap.hpp>
+#include <immer/heap/thread_local_free_list_heap.hpp>
+
+#include <algorithm>
+#include <cstdlib>
+
+namespace immer {
+
+/*!
+ * Heap policy that unconditionally uses its `Heap` argument.
+ */
+template <typename Heap>
+struct heap_policy
+{
+    using type = Heap;
+
+    template <std::size_t>
+    struct optimized
+    {
+        using type = Heap;
+    };
+};
+
+template <typename Deriv, typename HeapPolicy>
+struct enable_optimized_heap_policy
+{
+    static void* operator new(std::size_t size)
+    {
+        using heap_type =
+            typename HeapPolicy ::template optimized<sizeof(Deriv)>::type;
+
+        return heap_type::allocate(size);
+    }
+
+    static void operator delete(void* data, std::size_t size)
+    {
+        using heap_type =
+            typename HeapPolicy ::template optimized<sizeof(Deriv)>::type;
+
+        heap_type::deallocate(size, data);
+    }
+};
+
+/*!
+ * Heap policy that returns a heap with a free list of objects
+ * of `max_size = max(Sizes...)` on top an underlying `Heap`.  Note
+ * these two properties of the resulting heap:
+ *
+ * - Allocating an object that is bigger than `max_size` may trigger
+ *   *undefined behavior*.
+ *
+ * - Allocating an object of size less than `max_size` still
+ *   returns an object of `max_size`.
+ *
+ * Basically, this heap will always return objects of `max_size`.
+ * When an object is freed, it does not directly invoke `std::free`,
+ * but it keeps the object in a global linked list instead.  When a
+ * new object is requested, it does not need to call `std::malloc` but
+ * it can directly pop and return the other object from the global
+ * list, a much faster operation.
+ *
+ * This actually creates a hierarchy with two free lists:
+ *
+ * - A `thread_local` free list is used first.  It does not need any
+ *   kind of synchronization and is very fast.  When the thread
+ *   finishes, its contents are returned to the next free list.
+ *
+ * - A global free list using lock-free access via atomics.
+ *
+ * @tparam Heap Heap to be used when the free list is empty.
+ *
+ * @rst
+ *
+ * .. tip:: For many applications that use immutable data structures
+ *    significantly, this is actually the best heap policy, and it
+ *    might become the default in the future.
+ *
+ *    Note that most our data structures internally use trees with the
+ *    same big branching factors.  This means that all *vectors*,
+ *    *maps*, etc. can just allocate elements from the same free-list
+ *    optimized heap.  Not only does this lowers the allocation time,
+ *    but also makes up for more efficient *cache utilization*.  When
+ *    a new node is needed, there are high chances the allocator will
+ *    return a node that was just accessed.  When batches of immutable
+ *    updates are made, this can make a significant difference.
+ *
+ * @endrst
+ */
+template <typename Heap, std::size_t Limit = default_free_list_size>
+struct free_list_heap_policy
+{
+    using type = debug_size_heap<Heap>;
+
+    template <std::size_t Size>
+    struct optimized
+    {
+        using type =
+            split_heap<Size,
+                       with_free_list_node<thread_local_free_list_heap<
+                           Size,
+                           Limit,
+                           free_list_heap<Size, Limit, debug_size_heap<Heap>>>>,
+                       debug_size_heap<Heap>>;
+    };
+};
+
+/*!
+ * Similar to @ref free_list_heap_policy, but it assumes no
+ * multi-threading, so a single global free list with no concurrency
+ * checks is used.
+ */
+template <typename Heap, std::size_t Limit = default_free_list_size>
+struct unsafe_free_list_heap_policy
+{
+    using type = Heap;
+
+    template <std::size_t Size>
+    struct optimized
+    {
+        using type = split_heap<
+            Size,
+            with_free_list_node<
+                unsafe_free_list_heap<Size, Limit, debug_size_heap<Heap>>>,
+            debug_size_heap<Heap>>;
+    };
+};
+
+} // namespace immer