Squashed 'third_party/immer/' content from commit ad3e3556d

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immer/memory_policy.hpp

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+//
+// 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/heap/cpp_heap.hpp>
+#include <immer/heap/heap_policy.hpp>
+#include <immer/refcount/no_refcount_policy.hpp>
+#include <immer/refcount/refcount_policy.hpp>
+#include <immer/refcount/unsafe_refcount_policy.hpp>
+#include <immer/transience/gc_transience_policy.hpp>
+#include <immer/transience/no_transience_policy.hpp>
+#include <type_traits>
+
+namespace immer {
+
+/*!
+ * Metafunction that returns the best *transience policy* to use for a
+ * given *refcount policy*.
+ */
+template <typename RefcountPolicy>
+struct get_transience_policy
+    : std::conditional<std::is_same<RefcountPolicy, no_refcount_policy>::value,
+                       gc_transience_policy,
+                       no_transience_policy>
+{};
+
+template <typename T>
+using get_transience_policy_t = typename get_transience_policy<T>::type;
+
+/*!
+ * Metafunction that returns wether to *prefer fewer bigger objects*
+ * to use for a given *heap policy*.
+ */
+template <typename HeapPolicy>
+struct get_prefer_fewer_bigger_objects
+    : std::integral_constant<
+          bool,
+          std::is_same<HeapPolicy, heap_policy<cpp_heap>>::value>
+{};
+
+template <typename T>
+constexpr auto get_prefer_fewer_bigger_objects_v =
+    get_prefer_fewer_bigger_objects<T>::value;
+
+/*!
+ * Metafunction that returns wether to use *transient R-Values*
+ * for a given *refcount policy*.
+ */
+template <typename RefcountPolicy>
+struct get_use_transient_rvalues
+    : std::integral_constant<
+          bool,
+          !std::is_same<RefcountPolicy, no_refcount_policy>::value>
+{};
+
+template <typename T>
+constexpr auto get_use_transient_rvalues_v =
+    get_use_transient_rvalues<T>::value;
+
+/*!
+ * This is a default implementation of a *memory policy*.  A memory
+ * policy is just a bag of other policies plus some flags with hints
+ * to the user about the best way to use these strategies.
+ *
+ * @tparam HeapPolicy A *heap policy*, for example, @ref heap_policy.
+ * @tparam RefcountPolicy A *reference counting policy*, for example,
+ *         @ref refcount_policy.
+ * @tparam TransiencePolicy A *transience policy*, for example,
+ *         @ref no_transience_policy.
+ * @tparam PreferFewerBiggerObjects Boolean flag indicating whether
+ *         the user should prefer to allocate memory in bigger chungs
+ *         --e.g. by putting various objects in the same memory
+ *         region-- or not.
+ * @tparam UseTransientRValues Boolean flag indicating whether
+ *         immutable containers should try to modify contents in-place
+ *         when manipulating an r-value reference.
+ */
+template <typename HeapPolicy,
+          typename RefcountPolicy,
+          typename TransiencePolicy = get_transience_policy_t<RefcountPolicy>,
+          bool PreferFewerBiggerObjects =
+              get_prefer_fewer_bigger_objects_v<HeapPolicy>,
+          bool UseTransientRValues =
+              get_use_transient_rvalues_v<RefcountPolicy>>
+struct memory_policy
+{
+    using heap       = HeapPolicy;
+    using refcount   = RefcountPolicy;
+    using transience = TransiencePolicy;
+
+    static constexpr bool prefer_fewer_bigger_objects =
+        PreferFewerBiggerObjects;
+
+    static constexpr bool use_transient_rvalues = UseTransientRValues;
+
+    using transience_t = typename transience::template apply<heap>::type;
+};
+
+/*!
+ * The default *heap policy* just uses the standard heap with a
+ * @ref free_list_heap_policy.  If `IMMER_NO_FREE_LIST` is defined to `1`
+ * then it just uses the standard heap.
+ */
+#if IMMER_NO_FREE_LIST
+using default_heap_policy = heap_policy<debug_size_heap<cpp_heap>>;
+#else
+#if IMMER_NO_THREAD_SAFETY
+using default_heap_policy     = unsafe_free_list_heap_policy<cpp_heap>;
+#else
+using default_heap_policy = free_list_heap_policy<cpp_heap>;
+#endif
+#endif
+
+/*!
+ * By default we use thread safe reference counting.
+ */
+#if IMMER_NO_THREAD_SAFETY
+using default_refcount_policy = unsafe_refcount_policy;
+#else
+using default_refcount_policy = refcount_policy;
+#endif
+
+/*!
+ * The default memory policy.
+ */
+using default_memory_policy =
+    memory_policy<default_heap_policy, default_refcount_policy>;
+
+} // namespace immer