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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_transient;
+
+/*!
+ * Immutable container that stores a sequence of elements in
+ * contiguous memory.
+ *
+ * @tparam T The type of the values to be stored in the container.
+ *
+ * @rst
+ *
+ * It supports the most efficient iteration and random access,
+ * equivalent to a ``std::vector`` or ``std::array``, but all
+ * manipulations are :math:`O(size)`.
+ *
+ * .. tip:: Don't be fooled by the bad complexity of this data
+ *    structure.  It is a great choice for short sequence or when it
+ *    is seldom or never changed.  This depends on the ``sizeof(T)``
+ *    and the expensiveness of its ``T``'s copy constructor, in case
+ *    of doubt, measure.  For basic types, using an `array` when
+ *    :math:`n < 100` is a good heuristic.
+ *
+ * @endrst
+ */
+template <typename T, typename MemoryPolicy = default_memory_policy>
+class array
+{
+    using impl_t =
+        std::conditional_t<MemoryPolicy::use_transient_rvalues,
+                           detail::arrays::with_capacity<T, MemoryPolicy>,
+                           detail::arrays::no_capacity<T, MemoryPolicy>>;
+
+    using move_t =
+        std::integral_constant<bool, MemoryPolicy::use_transient_rvalues>;
+
+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 transient_type = array_transient<T, MemoryPolicy>;
+
+    /*!
+     * Default constructor.  It creates an array of `size() == 0`.  It
+     * does not allocate memory and its complexity is @f$ O(1) @f$.
+     */
+    array() = default;
+
+    /*!
+     * Constructs an array containing the elements in `values`.
+     */
+    array(std::initializer_list<T> values)
+        : impl_{impl_t::from_initializer_list(values)}
+    {}
+
+    /*!
+     * Constructs a array containing the elements in the range
+     * defined by the forward iterator `first` and range sentinel `last`.
+     */
+    template <typename Iter,
+              typename Sent,
+              std::enable_if_t<detail::compatible_sentinel_v<Iter, Sent> &&
+                                   detail::is_forward_iterator_v<Iter>,
+                               bool> = true>
+    array(Iter first, Sent last)
+        : impl_{impl_t::from_range(first, last)}
+    {}
+
+    /*!
+     * Constructs a array containing the element `val` repeated `n`
+     * times.
+     */
+    array(size_type n, T v = {})
+        : impl_{impl_t::from_fill(n, v)}
+    {}
+
+    /*!
+     * Returns an iterator pointing at the first element of the
+     * collection. It does not allocate memory and its complexity is
+     * @f$ 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$.
+     */
+    IMMER_NODISCARD bool empty() const { return impl_.size == 0; }
+
+    /*!
+     * Access the raw data.
+     */
+    IMMER_NODISCARD const T* data() const { return impl_.data(); }
+
+    /*!
+     * Access the last element.
+     */
+    IMMER_NODISCARD const T& back() const { return data()[size() - 1]; }
+
+    /*!
+     * Access the first element.
+     */
+    IMMER_NODISCARD const T& front() const { return data()[0]; }
+
+    /*!
+     * Returns a `const` reference to the element at position `index`.
+     * It is undefined when @f$ 0 index \geq size() @f$.  It does not
+     * allocate memory and its complexity is *effectively* @f$ O(1)
+     * @f$.
+     */
+    IMMER_NODISCARD reference operator[](size_type index) const
+    {
+        return impl_.get(index);
+    }
+
+    /*!
+     * Returns a `const` reference to the element at position
+     * `index`. It throws an `std::out_of_range` exception when @f$
+     * index \geq size() @f$.  It does not allocate memory and its
+     * complexity is *effectively* @f$ O(1) @f$.
+     */
+    reference at(size_type index) const { return impl_.get_check(index); }
+
+    /*!
+     * Returns whether the vectors are equal.
+     */
+    IMMER_NODISCARD bool operator==(const array& other) const
+    {
+        return impl_.equals(other.impl_);
+    }
+    IMMER_NODISCARD bool operator!=(const array& other) const
+    {
+        return !(*this == other);
+    }
+
+    /*!
+     * Returns an array with `value` inserted at the end.  It may
+     * allocate memory and its complexity is @f$ O(size) @f$.
+     *
+     * @rst
+     *
+     * **Example**
+     *   .. literalinclude:: ../example/array/array.cpp
+     *      :language: c++
+     *      :dedent: 8
+     *      :start-after: push-back/start
+     *      :end-before:  push-back/end
+     *
+     * @endrst
+     */
+    IMMER_NODISCARD array push_back(value_type value) const&
+    {
+        return impl_.push_back(std::move(value));
+    }
+
+    IMMER_NODISCARD decltype(auto) push_back(value_type value) &&
+    {
+        return push_back_move(move_t{}, std::move(value));
+    }
+
+    /*!
+     * Returns an array containing value `value` at position `idx`.
+     * Undefined for `index >= size()`.
+     * It may allocate memory and its complexity is @f$ O(size) @f$.
+     *
+     * @rst
+     *
+     * **Example**
+     *   .. literalinclude:: ../example/array/array.cpp
+     *      :language: c++
+     *      :dedent: 8
+     *      :start-after: set/start
+     *      :end-before:  set/end
+     *
+     * @endrst
+     */
+    IMMER_NODISCARD array set(std::size_t index, value_type value) const&
+    {
+        return impl_.assoc(index, std::move(value));
+    }
+
+    IMMER_NODISCARD decltype(auto) set(size_type index, value_type value) &&
+    {
+        return set_move(move_t{}, index, std::move(value));
+    }
+
+    /*!
+     * Returns an array containing the result of the expression
+     * `fn((*this)[idx])` at position `idx`.
+     * Undefined for `index >= size()`.
+     * It may allocate memory and its complexity is @f$ O(size) @f$.
+     *
+     * @rst
+     *
+     * **Example**
+     *   .. literalinclude:: ../example/array/array.cpp
+     *      :language: c++
+     *      :dedent: 8
+     *      :start-after: update/start
+     *      :end-before:  update/end
+     *
+     * @endrst
+     */
+    template <typename FnT>
+    IMMER_NODISCARD array update(std::size_t index, FnT&& fn) const&
+    {
+        return impl_.update(index, std::forward<FnT>(fn));
+    }
+
+    template <typename FnT>
+    IMMER_NODISCARD decltype(auto) update(size_type index, FnT&& fn) &&
+    {
+        return update_move(move_t{}, index, std::forward<FnT>(fn));
+    }
+
+    /*!
+     * Returns a array containing only the first `min(elems, size())`
+     * elements. It may allocate memory and its complexity is
+     * *effectively* @f$ O(1) @f$.
+     *
+     * @rst
+     *
+     * **Example**
+     *   .. literalinclude:: ../example/array/array.cpp
+     *      :language: c++
+     *      :dedent: 8
+     *      :start-after: take/start
+     *      :end-before:  take/end
+     *
+     * @endrst
+     */
+    IMMER_NODISCARD array take(size_type elems) const&
+    {
+        return impl_.take(elems);
+    }
+
+    IMMER_NODISCARD decltype(auto) take(size_type elems) &&
+    {
+        return take_move(move_t{}, elems);
+    }
+
+    /*!
+     * Returns an @a transient form of this container, an
+     * `immer::array_transient`.
+     */
+    IMMER_NODISCARD transient_type transient() const&
+    {
+        return transient_type{impl_};
+    }
+    IMMER_NODISCARD transient_type transient() &&
+    {
+        return transient_type{std::move(impl_)};
+    }
+
+    // Semi-private
+    const impl_t& impl() const { return impl_; }
+
+private:
+    friend transient_type;
+
+    array(impl_t impl)
+        : impl_(std::move(impl))
+    {}
+
+    array&& push_back_move(std::true_type, value_type value)
+    {
+        impl_.push_back_mut({}, std::move(value));
+        return std::move(*this);
+    }
+    array push_back_move(std::false_type, value_type value)
+    {
+        return impl_.push_back(std::move(value));
+    }
+
+    array&& set_move(std::true_type, size_type index, value_type value)
+    {
+        impl_.assoc_mut({}, index, std::move(value));
+        return std::move(*this);
+    }
+    array set_move(std::false_type, size_type index, value_type value)
+    {
+        return impl_.assoc(index, std::move(value));
+    }
+
+    template <typename Fn>
+    array&& update_move(std::true_type, size_type index, Fn&& fn)
+    {
+        impl_.update_mut({}, index, std::forward<Fn>(fn));
+        return std::move(*this);
+    }
+    template <typename Fn>
+    array update_move(std::false_type, size_type index, Fn&& fn)
+    {
+        return impl_.update(index, std::forward<Fn>(fn));
+    }
+
+    array&& take_move(std::true_type, size_type elems)
+    {
+        impl_.take_mut({}, elems);
+        return std::move(*this);
+    }
+    array take_move(std::false_type, size_type elems)
+    {
+        return impl_.take(elems);
+    }
+
+    impl_t impl_ = impl_t::empty();
+};
+
+} /* namespace immer */