✨ Allow using bitset with enum values

Problem:
- A `bitset` cannot be specified with enumeration values, nor have bits
  set/reset using them.

Solution:
- Allow specifying and manipulating a bitset using enumeration values as the
  bits.

Author: elbeno

docs/bitset.adoc

@@ -15,6 +15,7 @@ platform.
 * Stream input and output operators are not implemented.
 * A `std::hash` specialization is not implemented.
 * `to_string`, `to_ulong` and `to_ullong` are not implemented
+* `operator[]` is read-only: it does not return a proxy reference type
 
 A bitset has two template parameters: the size of the bitset and the storage
 element type to use. The storage element type must be unsigned.
@@ -71,3 +72,17 @@ auto j = bs.to_natural();          // 5 (a std::uint16_t)
 
 Bitsets support all the usual bitwise operators (`and`, `or`, `xor` and `not`)
 and also support `operator-` meaning set difference, or `a & ~b`.
+
+A bitset can also be used with an enumeration that represents bits:
+[source,cpp]
+----
+enum struct Bits { ZERO, ONE, TWO, THREE, MAX };
+auto bs = stdx::bitset<Bits::MAX>{stdx::all_bits}; // 4 bits, value 0b1111
+bs.set(Bits::ZERO);
+bs.reset(Bits::ZERO);
+bs.flip(Bits::ZERO);
+auto bit_zero = bs[Bits::ZERO];
+----
+
+NOTE: The enumeration values are the bit positions, not the bits themselves (the
+enumeration values are not fixed to powers-of-2).

include/stdx/bitset.hpp

@@ -156,20 +156,21 @@ template <std::size_t N, typename StorageElem> class bitset {
     }
 
     template <typename T> [[nodiscard]] constexpr auto to() const -> T {
-        static_assert(unsigned_integral<T>,
-                      "Conversion must be to an unsigned integral type!");
-        static_assert(N <= std::numeric_limits<T>::digits,
+        using U = underlying_type_t<T>;
+        static_assert(
+            unsigned_integral<U>,
+            "Conversion must be to an unsigned integral type or enum!");
+        static_assert(N <= std::numeric_limits<U>::digits,
                       "Bitset too big for conversion to T");
-        if constexpr (std::is_same_v<StorageElem, T>) {
-            return storage[0] & lastmask;
+        if constexpr (std::is_same_v<StorageElem, U>) {
+            return static_cast<T>(storage[0] & lastmask);
         } else {
-
-            T result{highbits()};
+            U result{highbits()};
             for (auto i = storage_size - 2u; i < storage_size; --i) {
                 result = static_cast<T>(result << storage_elem_size);
                 result |= storage[i];
             }
-            return result;
+            return static_cast<T>(result);
         }
     }
 
@@ -182,13 +183,16 @@ template <std::size_t N, typename StorageElem> class bitset {
 
     constexpr static std::integral_constant<std::size_t, N> size{};
 
-    [[nodiscard]] constexpr auto operator[](std::size_t pos) const -> bool {
+    template <typename T>
+    [[nodiscard]] constexpr auto operator[](T idx) const -> bool {
+        auto const pos = static_cast<std::size_t>(to_underlying(idx));
         auto const [index, offset] = indices(pos);
         return (storage[index] & (bit << offset)) != 0;
     }
 
-    constexpr auto set(std::size_t pos,
-                       bool value = true) LIFETIMEBOUND -> bitset & {
+    template <typename T>
+    constexpr auto set(T idx, bool value = true) LIFETIMEBOUND -> bitset & {
+        auto const pos = static_cast<std::size_t>(to_underlying(idx));
         auto const [index, offset] = indices(pos);
         if (value) {
             storage[index] |= static_cast<StorageElem>(bit << offset);
@@ -241,7 +245,9 @@ template <std::size_t N, typename StorageElem> class bitset {
         return *this;
     }
 
-    constexpr auto reset(std::size_t pos) LIFETIMEBOUND -> bitset & {
+    template <typename T>
+    constexpr auto reset(T idx) LIFETIMEBOUND -> bitset & {
+        auto const pos = static_cast<std::size_t>(to_underlying(idx));
         auto const [index, offset] = indices(pos);
         storage[index] &= static_cast<StorageElem>(~(bit << offset));
         return *this;
@@ -262,7 +268,8 @@ template <std::size_t N, typename StorageElem> class bitset {
         return set(lsb, len, false);
     }
 
-    constexpr auto flip(std::size_t pos) LIFETIMEBOUND -> bitset & {
+    template <typename T> constexpr auto flip(T idx) LIFETIMEBOUND -> bitset & {
+        auto const pos = static_cast<std::size_t>(to_underlying(idx));
         auto const [index, offset] = indices(pos);
         storage[index] ^= static_cast<StorageElem>(bit << offset);
         return *this;
@@ -406,8 +413,10 @@ constexpr auto for_each(F &&f, bitset<M, S> const &...bs) -> F {
 }
 } // namespace detail
 
-template <std::size_t N, typename StorageElem = void>
-using bitset = detail::bitset<N, decltype(smallest_uint<N, StorageElem>())>;
+template <auto N, typename StorageElem = void>
+using bitset =
+    detail::bitset<to_underlying(N),
+                   decltype(smallest_uint<to_underlying(N), StorageElem>())>;
 
 } // namespace v1
 } // namespace stdx

include/stdx/type_traits.hpp

@@ -13,6 +13,8 @@ template <typename E> constexpr auto to_underlying(E e) noexcept {
         return e;
     }
 }
+template <typename E>
+using underlying_type_t = decltype(to_underlying(std::declval<E>()));
 
 template <typename T> struct remove_cvref {
     using type = std::remove_cv_t<std::remove_reference_t<T>>;

test/bitset.cpp

@@ -422,3 +422,35 @@ TEMPLATE_TEST_CASE("find lowest unset bit (full)", "[bitset]", std::uint8_t,
     constexpr auto bs = stdx::bitset<sz, TestType>{stdx::all_bits};
     static_assert(bs.lowest_unset() == sz);
 }
+
+namespace {
+enum struct Bits : std::uint8_t { ZERO, ONE, TWO, THREE, MAX };
+}
+
+TEST_CASE("use bitset with enum struct (construct)", "[bitset]") {
+    constexpr auto bs = stdx::bitset<Bits::MAX>{};
+    static_assert(bs.size() == stdx::to_underlying(Bits::MAX));
+}
+
+TEST_CASE("use bitset with enum struct (to)", "[bitset]") {
+    constexpr auto bs = stdx::bitset<Bits::MAX>{stdx::all_bits};
+    static_assert(bs.to<Bits>() == static_cast<Bits>(0b1111));
+}
+
+TEST_CASE("use bitset with enum struct (set/flip)", "[bitset]") {
+    auto bs = stdx::bitset<Bits::MAX>{};
+    bs.set(Bits::ZERO);
+    CHECK(bs.to_natural() == 1);
+    bs.reset(Bits::ZERO);
+    CHECK(bs.to_natural() == 0);
+    bs.flip(Bits::ZERO);
+    CHECK(bs.to_natural() == 1);
+}
+
+TEST_CASE("use bitset with enum struct (read index)", "[bitset]") {
+    constexpr auto bs = stdx::bitset<Bits::MAX>{stdx::all_bits};
+    static_assert(bs[Bits::ZERO]);
+    static_assert(bs[Bits::ONE]);
+    static_assert(bs[Bits::TWO]);
+    static_assert(bs[Bits::THREE]);
+}