From 94b72914e7682b41d04323689ac0e8c51baf4cd8 Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Thu, 29 Jan 2026 11:35:27 -0700
Subject: [PATCH] :sparkles: Add type-indexed bitset

Problem:
- Sometimes we have a need to map types to on/off status. The way we do this
  right now is clunky, typically using `bool` variable templates.

Solution:
- Add a `type_bitset` which holds one bit per type, and supports set/reset/read
  on a type basis, as well as some of the usual bitset operations.
---
 include/stdx/bitset.hpp      | 160 ++++++++++++++++++++++++++++++++
 include/stdx/type_traits.hpp |   2 +
 test/CMakeLists.txt          |   1 +
 test/type_bitset.cpp         | 172 +++++++++++++++++++++++++++++++++++
 4 files changed, 335 insertions(+)
 create mode 100644 test/type_bitset.cpp

diff --git a/include/stdx/bitset.hpp b/include/stdx/bitset.hpp
index f69d00b..3d05ac9 100644
--- a/include/stdx/bitset.hpp
+++ b/include/stdx/bitset.hpp
@@ -8,6 +8,8 @@
 #include <stdx/type_traits.hpp>
 #include <stdx/udls.hpp>
 
+#include <boost/mp11/algorithm.hpp>
+
 #include <algorithm>
 #include <array>
 #include <cstddef>
@@ -478,5 +480,163 @@ template <typename T, typename F, typename R, auto M, typename... S>
 #if __cplusplus >= 202002L
 template <std::size_t N> bitset(ct_string<N>) -> bitset<N - 1>;
 #endif
+
+namespace detail {
+template <typename...> constexpr std::size_t index_of = 0;
+
+template <typename T, typename... Us>
+constexpr std::size_t index_of<T, type_list<Us...>> =
+    boost::mp11::mp_find<type_list<Us...>, T>::value;
+} // namespace detail
+
+template <typename... Ts> class type_bitset {
+    using list_t = boost::mp11::mp_unique<type_list<Ts...>>;
+    constexpr static std::size_t N = boost::mp11::mp_size<list_t>::value;
+
+    bitset<N> bs;
+
+    [[nodiscard]] friend constexpr auto operator==(type_bitset const &lhs,
+                                                   type_bitset const &rhs)
+        -> bool {
+        return lhs.bs == rhs.bs;
+    }
+
+    friend constexpr auto operator|(type_bitset lhs, type_bitset const &rhs)
+        -> type_bitset {
+        lhs |= rhs;
+        return lhs;
+    }
+
+    friend constexpr auto operator&(type_bitset lhs, type_bitset const &rhs)
+        -> type_bitset {
+        lhs &= rhs;
+        return lhs;
+    }
+
+    friend constexpr auto operator^(type_bitset lhs, type_bitset const &rhs)
+        -> type_bitset {
+        lhs ^= rhs;
+        return lhs;
+    }
+
+    friend constexpr auto operator-(type_bitset const &lhs, type_bitset rhs)
+        -> type_bitset {
+        rhs.flip();
+        return lhs & rhs;
+    }
+
+    template <typename F, std::size_t... Is>
+    constexpr static auto make_callers(std::index_sequence<Is...>) {
+        using call_t = auto (*)(F &)->void;
+        return std::array<call_t, N>{[](F &f) {
+            f.template operator()<boost::mp11::mp_at_c<list_t, Is>, Is>();
+        }...};
+    }
+
+  public:
+    constexpr type_bitset() = default;
+    constexpr explicit type_bitset(all_bits_t) : bs{all_bits} {}
+    constexpr explicit type_bitset(std::uint64_t value) : bs{value} {}
+
+    template <typename... Us>
+    constexpr explicit type_bitset(type_list<Us...>)
+        : bs{place_bits, detail::index_of<Us, list_t>...} {
+        static_assert((... and (detail::index_of<Us, list_t> < N)),
+                      "Type not found in bitset");
+    }
+
+    template <typename T> [[nodiscard]] constexpr auto to() const -> T {
+        return bs.template to<T>();
+    }
+    [[nodiscard]] constexpr auto to_natural() const { return bs.to_natural(); }
+
+    constexpr static std::integral_constant<std::size_t, N> size{};
+
+    template <typename T>
+    [[nodiscard]] constexpr auto operator[](type_identity<T>) const
+        -> decltype(auto) {
+        constexpr auto idx = detail::index_of<T, list_t>;
+        static_assert(idx < sizeof...(Ts), "Type not found in bitset");
+        return bs[idx];
+    }
+
+    template <typename... Us>
+    constexpr auto set(bool value = true) LIFETIMEBOUND -> type_bitset & {
+        static_assert((... and (detail::index_of<Us, type_list<Ts...>> < N)),
+                      "Type not found in bitset");
+        if constexpr (sizeof...(Us) == 0) {
+            bs.set();
+        } else {
+            (bs.set(detail::index_of<Us, type_list<Ts...>>, value), ...);
+        }
+        return *this;
+    }
+
+    template <typename... Us>
+    constexpr auto reset() LIFETIMEBOUND -> type_bitset & {
+        static_assert((... and (detail::index_of<Us, type_list<Ts...>> < N)),
+                      "Type not found in bitset");
+        if constexpr (sizeof...(Us) == 0) {
+            bs.reset();
+        } else {
+            (bs.reset(detail::index_of<Us, type_list<Ts...>>), ...);
+        }
+        return *this;
+    }
+
+    template <typename... Us>
+    constexpr auto flip() LIFETIMEBOUND -> type_bitset & {
+        static_assert((... and (detail::index_of<Us, type_list<Ts...>> < N)),
+                      "Type not found in bitset");
+        if constexpr (sizeof...(Us) == 0) {
+            bs.flip();
+        } else {
+            (bs.flip(detail::index_of<Us, type_list<Ts...>>), ...);
+        }
+        return *this;
+    }
+
+    [[nodiscard]] constexpr auto count() const -> std::size_t {
+        return bs.count();
+    }
+
+    [[nodiscard]] constexpr auto all() const -> bool {
+        return count() == size();
+    }
+
+    [[nodiscard]] constexpr auto any() const -> bool {
+        return count() != std::size_t{};
+    }
+
+    [[nodiscard]] constexpr auto none() const -> bool { return not any(); }
+
+    [[nodiscard]] constexpr auto operator~() const -> type_bitset {
+        return type_bitset{~bs.template to<std::uint64_t>()};
+    }
+
+    constexpr auto
+    operator|=(type_bitset const &rhs) LIFETIMEBOUND->type_bitset & {
+        bs |= rhs.bs;
+        return *this;
+    }
+
+    constexpr auto
+    operator&=(type_bitset const &rhs) LIFETIMEBOUND->type_bitset & {
+        bs &= rhs.bs;
+        return *this;
+    }
+
+    constexpr auto
+    operator^=(type_bitset const &rhs) LIFETIMEBOUND->type_bitset & {
+        bs ^= rhs.bs;
+        return *this;
+    }
+
+    template <typename F> constexpr auto for_each(F &&f) const -> F {
+        constexpr auto callers = make_callers<F>(std::make_index_sequence<N>{});
+        stdx::for_each([&](auto i) { callers[i](f); }, bs);
+        return f;
+    }
+};
 } // namespace v1
 } // namespace stdx
diff --git a/include/stdx/type_traits.hpp b/include/stdx/type_traits.hpp
index 70e79ad..11d2ce0 100644
--- a/include/stdx/type_traits.hpp
+++ b/include/stdx/type_traits.hpp
@@ -77,6 +77,8 @@ template <typename T> struct type_identity {
     using type = T;
 };
 template <typename T> using type_identity_t = typename type_identity<T>::type;
+template <typename T>
+constexpr static auto type_identity_v = type_identity<T>{};
 
 namespace detail {
 template <typename T, template <typename...> typename U>
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index da67474..1ecbafd 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -63,6 +63,7 @@ add_tests(
     rollover
     span
     to_underlying
+    type_bitset
     type_map
     type_traits
     with_result_of
diff --git a/test/type_bitset.cpp b/test/type_bitset.cpp
new file mode 100644
index 0000000..1b52453
--- /dev/null
+++ b/test/type_bitset.cpp
@@ -0,0 +1,172 @@
+#include <stdx/bitset.hpp>
+#include <stdx/ct_conversions.hpp>
+
+#include <catch2/catch_template_test_macros.hpp>
+#include <catch2/catch_test_macros.hpp>
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <type_traits>
+
+TEST_CASE("bitset size", "[type_bitset]") {
+    STATIC_CHECK(stdx::type_bitset<int>{}.size() == 1u);
+    STATIC_CHECK(stdx::type_bitset<int, float>{}.size() == 2u);
+}
+
+TEST_CASE("index operation", "[type_bitset]") {
+    STATIC_CHECK(not stdx::type_bitset<int>{}[stdx::type_identity_v<int>]);
+}
+
+TEST_CASE("set single bit", "[type_bitset]") {
+    auto bs = stdx::type_bitset<int, float, bool>{};
+    CHECK(not bs[stdx::type_identity_v<int>]);
+    bs.set<int>();
+    CHECK(bs[stdx::type_identity_v<int>]);
+    bs.set<int>(false);
+    CHECK(not bs[stdx::type_identity_v<int>]);
+}
+
+TEST_CASE("reset single bit", "[type_bitset]") {
+    auto bs = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    CHECK(bs[stdx::type_identity_v<int>]);
+    bs.reset<int>();
+    CHECK(not bs[stdx::type_identity_v<int>]);
+}
+
+TEST_CASE("flip single bit", "[type_bitset]") {
+    auto bs = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    CHECK(bs[stdx::type_identity_v<int>]);
+    bs.flip<int>();
+    CHECK(not bs[stdx::type_identity_v<int>]);
+}
+
+TEST_CASE("construct with a value", "[type bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float>{1ul};
+    STATIC_CHECK(bs1[stdx::type_identity_v<int>]);
+
+    constexpr auto bs2 = stdx::type_bitset<int, float>{255ul};
+    STATIC_CHECK(bs2[stdx::type_identity_v<int>]);
+    STATIC_CHECK(bs2[stdx::type_identity_v<float>]);
+}
+
+TEST_CASE("construct with values for bits", "[type_bitset]") {
+    constexpr auto bs =
+        stdx::type_bitset<int, float, bool>{stdx::type_list<int, bool>{}};
+    STATIC_CHECK(bs[stdx::type_identity_v<int>]);
+    STATIC_CHECK(not bs[stdx::type_identity_v<float>]);
+    STATIC_CHECK(bs[stdx::type_identity_v<bool>]);
+}
+
+TEMPLATE_TEST_CASE("convert to unsigned integral type", "[type_bitset]",
+                   std::uint8_t, std::uint16_t, std::uint32_t, std::uint64_t) {
+    constexpr auto bs = stdx::type_bitset<int, float, bool>{255ul};
+    constexpr auto val = bs.template to<TestType>();
+    STATIC_CHECK(std::is_same_v<decltype(val), TestType const>);
+    STATIC_CHECK(val == 7u);
+}
+
+TEST_CASE("convert to type that fits", "[type_bitset]") {
+    constexpr auto bs = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    constexpr auto val = bs.to_natural();
+    STATIC_CHECK(std::is_same_v<decltype(val), std::uint8_t const>);
+    STATIC_CHECK(val == 7u);
+}
+
+TEST_CASE("all", "[type bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    STATIC_CHECK(bs1.all());
+
+    constexpr auto bs2 = stdx::type_bitset<int, float, bool>{};
+    STATIC_CHECK(not bs2.all());
+}
+
+TEST_CASE("any", "[type bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    STATIC_CHECK(bs1.any());
+
+    constexpr auto bs2 = stdx::type_bitset<int, float, bool>{};
+    STATIC_CHECK(not bs2.any());
+}
+
+TEST_CASE("none", "[type bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    STATIC_CHECK(not bs1.none());
+
+    constexpr auto bs2 = stdx::type_bitset<int, float, bool>{};
+    STATIC_CHECK(bs2.none());
+}
+
+TEST_CASE("count", "[type_bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float, bool>{};
+    STATIC_CHECK(bs1.count() == 0u);
+
+    constexpr auto bs2 = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    STATIC_CHECK(bs2.count() == 3u);
+}
+
+TEST_CASE("set all bits", "[type_bitset]") {
+    auto bs = stdx::type_bitset<int, float, bool>{};
+    bs.set();
+    CHECK(bs == stdx::type_bitset<int, float, bool>{stdx::all_bits});
+    bs.set();
+    CHECK(bs == stdx::type_bitset<int, float, bool>{stdx::all_bits});
+}
+
+TEST_CASE("reset all bits", "[type_bitset]") {
+    auto bs = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    bs.reset();
+    CHECK(bs == stdx::type_bitset<int, float, bool>{});
+    bs.reset();
+    CHECK(bs == stdx::type_bitset<int, float, bool>{});
+}
+
+TEST_CASE("flip all bits", "[type_bitset]") {
+    auto bs = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    bs.flip();
+    CHECK(bs == stdx::type_bitset<int, float, bool>{});
+    bs.flip();
+    CHECK(bs == stdx::type_bitset<int, float, bool>{stdx::all_bits});
+}
+
+TEST_CASE("or", "[type_bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float, bool>{0b101ul};
+    constexpr auto bs2 = stdx::type_bitset<int, float, bool>{0b010ul};
+    STATIC_CHECK((bs1 | bs2) ==
+                 stdx::type_bitset<int, float, bool>{stdx::all_bits});
+}
+
+TEST_CASE("and", "[type_bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float, bool>{0b101ul};
+    constexpr auto bs2 = stdx::type_bitset<int, float, bool>{0b100ul};
+    STATIC_CHECK((bs1 & bs2) == stdx::type_bitset<int, float, bool>{0b100ul});
+}
+
+TEST_CASE("xor", "[type_bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float, bool>{0b101ul};
+    constexpr auto bs2 = stdx::type_bitset<int, float, bool>{0b010ul};
+    STATIC_CHECK((bs1 ^ bs2) ==
+                 stdx::type_bitset<int, float, bool>{stdx::all_bits});
+}
+
+TEST_CASE("not", "[type_bitset]") {
+    constexpr auto bs = stdx::type_bitset<int, float, bool>{0b101ul};
+    STATIC_CHECK(~bs == stdx::type_bitset<int, float, bool>{0b10ul});
+}
+
+TEST_CASE("difference", "[type_bitset]") {
+    constexpr auto bs1 = stdx::type_bitset<int, float, bool>{0b101ul};
+    constexpr auto bs2 = stdx::type_bitset<int, float, bool>{0b011ul};
+    STATIC_CHECK((bs1 - bs2) == stdx::type_bitset<int, float, bool>{0b100ul});
+}
+
+#if __cplusplus >= 202002L
+TEST_CASE("for_each", "[type_bitset]") {
+    constexpr auto bs = stdx::type_bitset<int, float, bool>{stdx::all_bits};
+    auto result = std::string{};
+    bs.for_each([&]<typename T, std::size_t I>() -> void {
+        result += std::string{stdx::type_as_string<T>()} + std::to_string(I);
+    });
+    CHECK(result == "int0float1bool2");
+}
+#endif