Squashed 'third_party/immer/' content from commit ad3e3556d

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diff --git a/immer/array_transient.hpp b/immer/array_transient.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/detail/arrays/with_capacity.hpp>
+#include <immer/memory_policy.hpp>
+
+namespace immer {
+
+template <typename T, typename MemoryPolicy>
+class array;
+
+/*!
+ * Mutable version of `immer::array`.
+ *
+ * @rst
+ *
+ * Refer to :doc:`transients` to learn more about when and how to use
+ * the mutable versions of immutable containers.
+ *
+ * @endrst
+ */
+template <typename T, typename MemoryPolicy = default_memory_policy>
+class array_transient : MemoryPolicy::transience_t::owner
+{
+    using impl_t             = detail::arrays::with_capacity<T, MemoryPolicy>;
+    using impl_no_capacity_t = detail::arrays::no_capacity<T, MemoryPolicy>;
+    using owner_t            = typename MemoryPolicy::transience_t::owner;
+
+public:
+    using value_type      = T;
+    using reference       = const T&;
+    using size_type       = std::size_t;
+    using difference_type = std::ptrdiff_t;
+    using const_reference = const T&;
+
+    using iterator         = const T*;
+    using const_iterator   = iterator;
+    using reverse_iterator = std::reverse_iterator<iterator>;
+
+    using memory_policy   = MemoryPolicy;
+    using persistent_type = array<T, MemoryPolicy>;
+
+    /*!
+     * Default constructor.  It creates a mutable array of `size() ==
+     * 0`.  It does not allocate memory and its complexity is
+     * @f$ O(1) @f$.
+     */
+    array_transient() = default;
+
+    /*!
+     * Returns an iterator pointing at the first element of the
+     * collection. It does not allocate memory and its complexity is
+     * @f$ O(1) @f$.
+     */
+    IMMER_NODISCARD iterator begin() const { return impl_.data(); }
+
+    /*!
+     * Returns an iterator pointing just after the last element of the
+     * collection. It does not allocate and its complexity is @f$ O(1) @f$.
+     */
+    IMMER_NODISCARD iterator end() const { return impl_.data() + impl_.size; }
+
+    /*!
+     * Returns an iterator that traverses the collection backwards,
+     * pointing at the first element of the reversed collection. It
+     * does not allocate memory and its complexity is @f$ O(1) @f$.
+     */
+    IMMER_NODISCARD reverse_iterator rbegin() const
+    {
+        return reverse_iterator{end()};
+    }
+
+    /*!
+     * Returns an iterator that traverses the collection backwards,
+     * pointing after the last element of the reversed collection. It
+     * does not allocate memory and its complexity is @f$ O(1) @f$.
+     */
+    IMMER_NODISCARD reverse_iterator rend() const
+    {
+        return reverse_iterator{begin()};
+    }
+
+    /*!
+     * Returns the number of elements in the container.  It does
+     * not allocate memory and its complexity is @f$ O(1) @f$.
+     */
+    IMMER_NODISCARD std::size_t size() const { return impl_.size; }
+
+    /*!
+     * Returns `true` if there are no elements in the container.  It
+     * does not allocate memory and its complexity is @f$ O(1) @f$.
+     */
+    IMMER_NODISCARD bool empty() const { return impl_.size == 0; }
+
+    /*!
+     * Access the raw data.
+     */
+    IMMER_NODISCARD const T* data() const { return impl_.data(); }
+
+    /*!
+     * Provide mutable access to the raw underlaying data.
+     */
+    IMMER_NODISCARD T* data_mut() { return impl_.data_mut(*this); }
+
+    /*!
+     * Access the last element.
+     */
+    IMMER_NODISCARD const T& back() const { return data()[size() - 1]; }
+
+    /*!
+     * Access the first element.
+     */
+    IMMER_NODISCARD const T& front() const { return data()[0]; }
+
+    /*!
+     * Returns a `const` reference to the element at position `index`.
+     * It is undefined when @f$ 0 index \geq size() @f$.  It does not
+     * allocate memory and its complexity is *effectively* @f$ O(1)
+     * @f$.
+     */
+    reference operator[](size_type index) const { return impl_.get(index); }
+
+    /*!
+     * Returns a `const` reference to the element at position
+     * `index`. It throws an `std::out_of_range` exception when @f$
+     * index \geq size() @f$.  It does not allocate memory and its
+     * complexity is *effectively* @f$ O(1) @f$.
+     */
+    reference at(size_type index) const { return impl_.get_check(index); }
+
+    /*!
+     * Inserts `value` at the end.  It may allocate memory and its
+     * complexity is *effectively* @f$ O(1) @f$.
+     */
+    void push_back(value_type value)
+    {
+        impl_.push_back_mut(*this, std::move(value));
+    }
+
+    /*!
+     * Sets to the value `value` at position `idx`.
+     * Undefined for `index >= size()`.
+     * It may allocate memory and its complexity is
+     * *effectively* @f$ O(1) @f$.
+     */
+    void set(size_type index, value_type value)
+    {
+        impl_.assoc_mut(*this, index, std::move(value));
+    }
+
+    /*!
+     * Updates the array to contain the result of the expression
+     * `fn((*this)[idx])` at position `idx`.
+     * Undefined for `0 >= size()`.
+     * It may allocate memory and its complexity is
+     * *effectively* @f$ O(1) @f$.
+     */
+    template <typename FnT>
+    void update(size_type index, FnT&& fn)
+    {
+        impl_.update_mut(*this, index, std::forward<FnT>(fn));
+    }
+
+    /*!
+     * Resizes the array to only contain the first `min(elems, size())`
+     * elements. It may allocate memory and its complexity is
+     * *effectively* @f$ O(1) @f$.
+     */
+    void take(size_type elems) { impl_.take_mut(*this, elems); }
+
+    /*!
+     * Returns an @a immutable form of this container, an
+     * `immer::array`.
+     */
+    IMMER_NODISCARD persistent_type persistent() &
+    {
+        this->owner_t::operator=(owner_t{});
+        return persistent_type{impl_};
+    }
+    IMMER_NODISCARD persistent_type persistent() &&
+    {
+        return persistent_type{std::move(impl_)};
+    }
+
+private:
+    friend persistent_type;
+
+    array_transient(impl_t impl)
+        : impl_(std::move(impl))
+    {}
+
+    impl_t impl_ = impl_t::empty();
+};
+
+} // namespace immer