✨ Add intrusive_list

A simple doubly-linked intrusive list. Notably it only provides forward
iteration, to avoid complexity in handling reverse iterators. However it does
allow pushing and popping at both ends.

docs/index.adoc

@@ -45,6 +45,7 @@ The following headers are available:
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/ct_string.hpp[`ct_string.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/function_traits.hpp[`function_traits.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/functional.hpp[`functional.hpp`]
+* https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/intrusive_list.hpp[`instrusive_list.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/priority.hpp[`priority.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/type_traits.hpp[`type_traits.hpp`]
 * https://github.com/intel/cpp-std-extensions/blob/main/include/stdx/utility.hpp[`utility.hpp`]
@@ -239,6 +240,36 @@ v.emplace(0, stdx::with_result_of{make_S}); // this constructs S in-place thanks
 `with_result_of` can help to achieve in-place construction, effectively by deferring
 evaluation of function arguments.
 
+== `intrusive_list.hpp`
+
+`intrusive_list` is a doubly-linked list designed for use at compile-time or
+with static objects. It supports pushing and popping at the front or back, and
+removal from the middle.
+
+[source,cpp]
+----
+// A node in an intrusive_list must have prev and next pointers
+struct node {
+  node *prev{};
+  node *next{};
+};
+
+stdx::intrusive_list<node> l;
+
+node n1{};
+l.push_front(&n1);
+
+node n2{};
+l.push_back(&n2);
+
+node n3{};
+l.push_back(&n3);
+
+l.remove(&n2); // removal from the middle is constant-time
+l.pop_front();
+l.pop_back();
+----
+
 == `priority.hpp`
 
 `priority_t<N>` is a class that can be used for easily selecting complex

include/stdx/intrusive_list.hpp

@@ -0,0 +1,183 @@
+#pragma once
+
+#include <stdx/concepts.hpp>
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+
+namespace stdx {
+inline namespace v1 {
+
+#if __cpp_concepts >= 201907L
+namespace detail {
+template <typename T>
+concept base_single_linkable = requires(T node) {
+    { node->next } -> same_as<T &>;
+};
+} // namespace detail
+
+template <typename T>
+concept double_linkable = requires(T *node) {
+    requires detail::base_single_linkable<
+        std::remove_cvref_t<decltype(node->next)>>;
+    requires detail::base_single_linkable<
+        std::remove_cvref_t<decltype(node->prev)>>;
+};
+
+#define STDX_DOUBLE_LINKABLE double_linkable
+#else
+#define STDX_DOUBLE_LINKABLE typename
+#endif
+
+template <STDX_DOUBLE_LINKABLE NodeType> class intrusive_list {
+    template <typename N> struct iterator_t {
+        using difference_type = std::ptrdiff_t;
+        using value_type = N;
+        using pointer = value_type *;
+        using reference = value_type &;
+        using iterator_category = std::forward_iterator_tag;
+
+        constexpr iterator_t() = default;
+        constexpr explicit iterator_t(pointer n) : node{n} {}
+
+        constexpr auto operator*() -> reference { return *node; }
+        constexpr auto operator*() const -> reference { return *node; }
+        constexpr auto operator->() -> pointer { return node; }
+        constexpr auto operator->() const -> pointer { return node; }
+
+        constexpr auto operator++() -> iterator_t & {
+            node = node->next;
+            return *this;
+        }
+        constexpr auto operator++(int) -> iterator_t {
+            auto tmp = *this;
+            ++(*this);
+            return tmp;
+        }
+
+      private:
+        pointer node{};
+
+#if __cpp_impl_three_way_comparison < 201907L
+        friend constexpr auto operator==(iterator_t lhs, iterator_t rhs)
+            -> bool {
+            return lhs.node == rhs.node;
+        }
+        friend constexpr auto operator!=(iterator_t lhs, iterator_t rhs)
+            -> bool {
+            return not(lhs == rhs);
+        }
+#else
+        friend constexpr auto operator==(iterator_t, iterator_t)
+            -> bool = default;
+#endif
+    };
+
+  public:
+    using value_type = NodeType;
+    using size_type = std::size_t;
+    using difference_type = std::ptrdiff_t;
+    using reference = value_type &;
+    using const_reference = value_type const &;
+    using pointer = value_type *;
+    using const_pointer = value_type const *;
+    using iterator = iterator_t<value_type>;
+    using const_iterator = iterator_t<value_type const>;
+
+  private:
+    pointer head{};
+    pointer tail{};
+
+  public:
+    constexpr auto begin() -> iterator { return iterator{head}; }
+    constexpr auto begin() const -> const_iterator {
+        return const_iterator{head};
+    }
+    constexpr auto cbegin() const -> const_iterator {
+        return const_iterator{head};
+    }
+    constexpr auto end() -> iterator { return {}; }
+    constexpr auto end() const -> const_iterator { return {}; }
+    constexpr auto cend() -> const_iterator { return {}; }
+
+    constexpr auto push_front(pointer n) -> void {
+        if (head != nullptr) {
+            head->prev = n;
+        }
+        n->next = head;
+        head = n;
+        n->prev = nullptr;
+        if (tail == nullptr) {
+            tail = n;
+        }
+    }
+
+    constexpr auto push_back(pointer n) -> void {
+        if (tail != nullptr) {
+            tail->next = n;
+        }
+        n->prev = tail;
+        tail = n;
+        n->next = nullptr;
+        if (head == nullptr) {
+            head = n;
+        }
+    }
+
+    constexpr auto pop_front() -> pointer {
+        pointer poppedNode = head;
+        head = head->next;
+
+        if (head == nullptr) {
+            tail = nullptr;
+        } else {
+            head->prev = nullptr;
+        }
+
+        return poppedNode;
+    }
+
+    constexpr auto pop_back() -> pointer {
+        pointer poppedNode = tail;
+        tail = tail->prev;
+
+        if (tail == nullptr) {
+            head = nullptr;
+        } else {
+            tail->next = nullptr;
+        }
+
+        return poppedNode;
+    }
+
+    [[nodiscard]] constexpr auto empty() const -> bool {
+        return head == nullptr;
+    }
+
+    constexpr auto clear() -> void {
+        head = nullptr;
+        tail = nullptr;
+    }
+
+    constexpr auto remove(pointer n) -> void {
+        pointer nextNode = n->next;
+        pointer prevNode = n->prev;
+
+        if (prevNode == nullptr) {
+            head = nextNode;
+        } else {
+            prevNode->next = nextNode;
+        }
+
+        if (nextNode == nullptr) {
+            tail = prevNode;
+        } else {
+            nextNode->prev = prevNode;
+        }
+    }
+};
+
+#undef STDX_DOUBLE_LINKABLE
+} // namespace v1
+} // namespace stdx

test/CMakeLists.txt

@@ -10,6 +10,7 @@ add_unit_test(
     conditional.cpp
     ct_string.cpp
     function_traits.cpp
+    intrusive_list.cpp
     overload.cpp
     priority.cpp
     remove_cvref.cpp