✨ Add preprocessor mapping function

Problem:
- It's useful to have `map`/`transform` in the preprocessor.

Solution:
- Add `pp_map.hpp`.

Author: elbeno

CMakeLists.txt

@@ -62,8 +62,8 @@ target_sources(
               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
+              include/stdx/functional.hpp
               include/stdx/intrusive_forward_list.hpp
               include/stdx/intrusive_list.hpp
               include/stdx/iterator.hpp
@@ -72,14 +72,15 @@ target_sources(
               include/stdx/numeric.hpp
               include/stdx/optional.hpp
               include/stdx/panic.hpp
+              include/stdx/pp_map.hpp
               include/stdx/priority.hpp
               include/stdx/ranges.hpp
               include/stdx/rollover.hpp
               include/stdx/span.hpp
               include/stdx/static_assert.hpp
+              include/stdx/tuple.hpp
               include/stdx/tuple_algorithms.hpp
               include/stdx/tuple_destructure.hpp
-              include/stdx/tuple.hpp
               include/stdx/type_traits.hpp
               include/stdx/udls.hpp
               include/stdx/utility.hpp)

include/stdx/pp_map.hpp

@@ -0,0 +1,39 @@
+#pragma once
+
+// NOLINTBEGIN(cppcoreguidelines-macro-usage)
+
+#define STDX_EVAL0(...) __VA_ARGS__
+#define STDX_EVAL1(...) STDX_EVAL0(STDX_EVAL0(STDX_EVAL0(__VA_ARGS__)))
+#define STDX_EVAL2(...) STDX_EVAL1(STDX_EVAL1(STDX_EVAL1(__VA_ARGS__)))
+#define STDX_EVAL3(...) STDX_EVAL2(STDX_EVAL2(STDX_EVAL2(__VA_ARGS__)))
+#define STDX_EVAL4(...) STDX_EVAL3(STDX_EVAL3(STDX_EVAL3(__VA_ARGS__)))
+#define STDX_EVAL5(...) STDX_EVAL4(STDX_EVAL4(STDX_EVAL4(__VA_ARGS__)))
+#define STDX_EVAL(...) STDX_EVAL5(__VA_ARGS__)
+
+#define STDX_MAP_END(...)
+#define STDX_MAP_OUT
+
+#define STDX_EMPTY()
+#define STDX_DEFER(id) id STDX_EMPTY()
+
+#define STDX_MAP_GET_END2() 0, STDX_MAP_END
+#define STDX_MAP_GET_END1(...) STDX_MAP_GET_END2
+#define STDX_MAP_GET_END(...) STDX_MAP_GET_END1
+#define STDX_MAP_NEXT0(test, next, ...) next STDX_MAP_OUT
+#define STDX_MAP_NEXT1(test, next) STDX_DEFER(STDX_MAP_NEXT0)(test, next, 0)
+#define STDX_MAP_NEXT(test, next) STDX_MAP_NEXT1(STDX_MAP_GET_END test, next)
+#define STDX_MAP_INC(X) STDX_MAP_INC_##X
+
+#define STDX_MAP_A(f, x, peek, ...)                                            \
+    , f(x) STDX_DEFER(STDX_MAP_NEXT(peek, STDX_MAP_B))(f, peek, __VA_ARGS__)
+#define STDX_MAP_B(f, x, peek, ...)                                            \
+    , f(x) STDX_DEFER(STDX_MAP_NEXT(peek, STDX_MAP_A))(f, peek, __VA_ARGS__)
+
+#define STDX_DROP0(X, ...) __VA_ARGS__
+#define STDX_DROP1(...) STDX_DROP0(__VA_ARGS__)
+
+#define STDX_MAP(f, ...)                                                       \
+    __VA_OPT__(STDX_DROP1(                                                     \
+        0 STDX_EVAL(STDX_MAP_A(f, __VA_ARGS__, ()()(), ()()(), ()()(), 0))))
+
+// NOLINTEND(cppcoreguidelines-macro-usage)

test/CMakeLists.txt

@@ -54,6 +54,7 @@ add_tests(
     optional
     overload
     panic
+    pp_map
     priority
     ranges
     remove_cvref

test/pp_map.cpp

@@ -0,0 +1,27 @@
+#include <stdx/compiler.hpp>
+#include <stdx/pp_map.hpp>
+
+#include <catch2/catch_test_macros.hpp>
+
+namespace {
+auto count_args = [](auto const &...args) { return sizeof...(args); };
+} // namespace
+
+TEST_CASE("pp_map zero arguments", "[pp_map]") {
+    STDX_PRAGMA(diagnostic push)
+    STDX_PRAGMA(diagnostic ignored "-Wgnu-zero-variadic-macro-arguments")
+    static_assert(count_args(STDX_MAP(int)) == 0);
+    STDX_PRAGMA(diagnostic pop)
+}
+
+TEST_CASE("pp_map one argument", "[pp_map]") {
+    static_assert(count_args(STDX_MAP(double, 1)) == 1.0);
+}
+
+TEST_CASE("pp_map n arguments", "[pp_map]") {
+    static_assert(count_args(STDX_MAP(double, 1, 2, 3)) == 3);
+}
+
+TEST_CASE("pp_map parenthesized arguments", "[pp_map]") {
+    static_assert(count_args(STDX_MAP(double, ((void)1, 2), 3)) == 2);
+}