From 378b64e5d548848764c005a1ab3778505a1af8a2 Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Fri, 28 Feb 2025 09:49:42 -0700
Subject: [PATCH] :sparkles: Add `env` to hold compile-time values

Problem:
- CIB log env is a generic component that is useful beyond logging and should
  live in stdx.

Solution:
- Add `stdx::env`.

Note:
- For the moment this is dealing with compile-time values only. It may be
  possible in future to unify this with runtime environments used by S&R.
---
 CMakeLists.txt       |  1 +
 include/stdx/env.hpp | 91 ++++++++++++++++++++++++++++++++++++++++++++
 test/CMakeLists.txt  |  9 ++++-
 test/env.cpp         | 44 +++++++++++++++++++++
 4 files changed, 144 insertions(+), 1 deletion(-)
 create mode 100644 include/stdx/env.hpp
 create mode 100644 test/env.cpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e448fa8..a784133 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -60,6 +60,7 @@ target_sources(
               include/stdx/cx_vector.hpp
               include/stdx/detail/bitset_common.hpp
               include/stdx/detail/list_common.hpp
+              include/stdx/env.hpp
               include/stdx/for_each_n_args.hpp
               include/stdx/functional.hpp
               include/stdx/function_traits.hpp
diff --git a/include/stdx/env.hpp b/include/stdx/env.hpp
new file mode 100644
index 0000000..8c7b0a5
--- /dev/null
+++ b/include/stdx/env.hpp
@@ -0,0 +1,91 @@
+#pragma once
+
+#if __cplusplus >= 202002L
+
+#include <stdx/compiler.hpp>
+#include <stdx/ct_string.hpp>
+
+#include <boost/mp11/algorithm.hpp>
+
+namespace stdx {
+inline namespace v1 {
+template <auto Query, auto Value> struct ct_prop {
+    [[nodiscard]] CONSTEVAL static auto query(decltype(Query)) noexcept {
+        return Value;
+    }
+};
+
+namespace _env {
+template <typename Q, typename Env>
+concept valid_query_for = requires(Env const &e) { e.query(Q{}); };
+
+template <typename Q, typename... Envs>
+concept valid_query_over = (... or valid_query_for<Q, Envs>);
+
+template <typename Q> struct has_query {
+    template <typename Env>
+    using fn = std::bool_constant<valid_query_for<Q, Env>>;
+};
+} // namespace _env
+
+template <typename... Envs> struct env {
+    template <_env::valid_query_over<Envs...> Q>
+    CONSTEVAL static auto query(Q) noexcept {
+        using I = boost::mp11::mp_find_if_q<boost::mp11::mp_list<Envs...>,
+                                            _env::has_query<Q>>;
+        using E = boost::mp11::mp_at<boost::mp11::mp_list<Envs...>, I>;
+        return Q{}(E{});
+    }
+};
+
+namespace _env {
+template <typename T> struct autowrap {
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    CONSTEVAL autowrap(T t) : value(t) {}
+    T value;
+};
+
+// NOLINTNEXTLINE(modernize-avoid-c-arrays)
+template <std::size_t N> using str_lit_t = char const (&)[N];
+
+template <std::size_t N> struct autowrap<str_lit_t<N>> {
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    CONSTEVAL autowrap(str_lit_t<N> str) : value(str) {}
+    stdx::ct_string<N> value;
+};
+
+template <typename T> autowrap(T) -> autowrap<T>;
+template <std::size_t N> autowrap(str_lit_t<N>) -> autowrap<str_lit_t<N>>;
+
+template <auto V> struct wrap {
+    constexpr static auto value = V;
+};
+
+template <typename> struct for_each_pair;
+template <std::size_t... Is> struct for_each_pair<std::index_sequence<Is...>> {
+    template <auto... Args>
+    using type =
+        env<ct_prop<boost::mp11::mp_at_c<boost::mp11::mp_list<wrap<Args>...>,
+                                         2 * Is>::value.value,
+                    boost::mp11::mp_at_c<boost::mp11::mp_list<wrap<Args>...>,
+                                         (2 * Is) + 1>::value.value>...>;
+};
+
+template <typename Env = env<>>
+constexpr auto make_env = []<autowrap... Args> {
+    using new_env_t = typename for_each_pair<
+        std::make_index_sequence<sizeof...(Args) / 2>>::template type<Args...>;
+    return boost::mp11::mp_append<new_env_t, Env>{};
+};
+} // namespace _env
+
+template <typename Env, _env::autowrap... Args>
+using extend_env_t =
+    decltype(_env::make_env<Env>.template operator()<Args...>());
+
+template <_env::autowrap... Args>
+using make_env_t = extend_env_t<env<>, Args...>;
+} // namespace v1
+} // namespace stdx
+
+#endif
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 6cf74f2..1f73f6b 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -74,7 +74,14 @@ target_compile_definitions(
     PRIVATE -DATOMIC_CFG="${CMAKE_CURRENT_LIST_DIR}/detail/atomic_cfg.hpp")
 
 if(${CMAKE_CXX_STANDARD} GREATER_EQUAL 20)
-    add_tests(FILES ct_format ct_string indexed_tuple tuple tuple_algorithms)
+    add_tests(
+        FILES
+        ct_format
+        ct_string
+        env
+        indexed_tuple
+        tuple
+        tuple_algorithms)
 endif()
 
 add_subdirectory(fail)
diff --git a/test/env.cpp b/test/env.cpp
new file mode 100644
index 0000000..61700fa
--- /dev/null
+++ b/test/env.cpp
@@ -0,0 +1,44 @@
+#include <stdx/ct_string.hpp>
+#include <stdx/env.hpp>
+
+#include <catch2/catch_test_macros.hpp>
+
+namespace {
+[[maybe_unused]] constexpr inline struct custom_t {
+    template <typename T>
+        requires true // more constrained
+    CONSTEVAL auto operator()(T &&t) const
+        noexcept(noexcept(std::forward<T>(t).query(std::declval<custom_t>())))
+            -> decltype(std::forward<T>(t).query(*this)) {
+        return std::forward<T>(t).query(*this);
+    }
+
+    CONSTEVAL auto operator()(auto &&) const { return 42; }
+} custom;
+} // namespace
+
+TEST_CASE("lookup query with default", "[env]") {
+    static_assert(custom(stdx::env<>{}) == 42);
+}
+
+TEST_CASE("lookup with single-value prop", "[env]") {
+    using E = stdx::ct_prop<custom, 17>;
+    static_assert(custom(E{}) == 17);
+}
+
+TEST_CASE("make an environment", "[env]") {
+    using E = stdx::make_env_t<custom, 17>;
+    static_assert(custom(E{}) == 17);
+}
+
+TEST_CASE("extend an environment", "[env]") {
+    using E1 = stdx::make_env_t<custom, 17>;
+    using E2 = stdx::extend_env_t<E1, custom, 18>;
+    static_assert(custom(E2{}) == 18);
+}
+
+TEST_CASE("environment converts string literals to ct_string", "[env]") {
+    using namespace stdx::literals;
+    using E = stdx::make_env_t<custom, "hello">;
+    static_assert(custom(E{}) == "hello"_cts);
+}