Base dimensions refactored

-

README.md

@@ -61,6 +61,8 @@ NOTE: This library as of now compiles correctly only with gcc-9.1 and newer.
   - Added `pow()` and `sqrt()` operations on quantities
   - `units` removed from a `std::experimental` namespace
   - Downcasting facility refactored so the user does not have to write the boilerplate code anymore
+  - From now on base dimensions should inherit from `base_dimension` class template
+  - Added unit symbols definitions to `base_dimension` and `derived_unit`
 
 - 0.3.1 Sep 18, 2019
   - cmcstl2 dependency changed to range-v3 0.9.1

doc/DESIGN.md

@@ -119,27 +119,39 @@ struct exp {
 
 Both a base dimension and a derived dimension can be provided to `units::exp` class template.
 
-`units::BaseDimension` represents a base dimension and should be implemented as a type with
-a unique compile-time text describing the dimension name:
+`units::base_dimension` represents a base dimension and has assigned a unique compile-time text
+describing the dimension name:
+
+```cpp
+template<typename Name, typename Symbol>
+struct base_dimension {
+  using name = Name;
+  using symbol = Symbol;
+};
+```
+
+`units::BaseDimension` is a concept to match all types derived from `base_dimension` instantiations:
 
 ```cpp
 template<typename T>
 concept BaseDimension = std::is_empty_v<T> &&
     requires {
-    { T::value } -> std::same_as<const char*>;
-  };
+      typename T::name;
+      typename T::symbol;
+    } &&
+    std::derived_from<T, base_dimension<typename T::name, typename T::symbol>>;
 ```
 
 For example here is a list of SI base dimensions:
 
 ```cpp
-struct base_dim_length { static constexpr const char* value = "length"; };
-struct base_dim_mass { static constexpr const char* value = "mass"; };
-struct base_dim_time { static constexpr const char* value = "time"; };
-struct base_dim_current { static constexpr const char* value = "current"; };
-struct base_dim_temperature { static constexpr const char* value = "temperature"; };
-struct base_dim_substance { static constexpr const char* value = "substance"; };
-struct base_dim_luminous_intensity { static constexpr const char* value = "luminous intensity"; };
+struct base_dim_length : base_dimension<"length", "m"> {};
+struct base_dim_mass : base_dimension<"mass", "kg"> {};
+struct base_dim_time : base_dimension<"time", "s"> {};
+struct base_dim_current : base_dimension<"current", "A"> {};
+struct base_dim_temperature : base_dimension<"temperature", "K"> {};
+struct base_dim_substance : base_dimension<"substance", "mol"> {};
+struct base_dim_luminous_intensity : base_dimension<"luminous intensity", "cd"> {};
 ```
 
 #### `derived_dimension`
@@ -624,7 +636,7 @@ In order to extend the library with custom dimensions the user has to:
 1. Create a new base dimension if the predefined ones are not enough to form a new derived dimension:
 
     ```cpp
-    inline constexpr units::base_dimension base_dim_digital_information{"digital information"};
+    struct base_dim_digital_information : units::base_dimension<"digital information", "b"> {};
     ```
 
 2. Create a new dimension type with the recipe of how to construct it from base dimensions and

src/include/units/bits/fixed_string.h

@@ -24,31 +24,64 @@
 
 namespace units {
 
-//   TODO gcc:92101
-//   Gated by the following gcc bug
-//   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92101
-//
-//   template<typename CharT, std::size_t N>
-//   struct basic_fixed_string {
-//     CharT data_[N+1] = {};
+  template<typename CharT, std::size_t N>
+  struct basic_fixed_string {
+    CharT data_[N+1] = {};
 
-//     constexpr basic_fixed_string(const CharT (&txt)[N+1]) noexcept
-//     {
-//       for(std::size_t i = 0; i <= N; ++i)
-//         data_[i] = txt[i];
-//     }
-//     // auto operator==(const basic_fixed_string &) = default;
-//     [[nodiscard]] constexpr const CharT* c_str() const noexcept { return data_; }
-//   };
+    constexpr basic_fixed_string(const CharT (&txt)[N+1]) noexcept
+    {
+      for(std::size_t i = 0; i <= N; ++i)
+        data_[i] = txt[i];
+    }
 
-//   template<typename CharT, std::size_t N>
-//   basic_fixed_string(const CharT (&str)[N]) -> basic_fixed_string<CharT, N-1>;
+    [[nodiscard]] constexpr bool size() const noexcept { return N; }
+    [[nodiscard]] constexpr const CharT* c_str() const noexcept { return data_; }
 
-//   template<std::size_t N>
-//   using fixed_string = basic_fixed_string<char, N>;
+    // auto operator==(const basic_fixed_string &) = default;
+
+    [[nodiscard]] constexpr friend bool operator==(const basic_fixed_string& lhs, const basic_fixed_string& rhs) noexcept
+    {
+      for(size_t i = 0; i != size(lhs.data_); ++i)
+        if(lhs.name_[i] != rhs.data_[i])
+          return false;
+      return true;
+    }
+
+    template<typename CharT2, std::size_t N2>
+    [[nodiscard]] constexpr friend bool operator==(const basic_fixed_string&, const basic_fixed_string<CharT2, N2>&) noexcept
+    {
+      return false;
+    }
+
+    template<typename CharT2, std::size_t N2>
+    [[nodiscard]] constexpr friend bool operator<(const basic_fixed_string& lhs, const basic_fixed_string<CharT2, N2>& rhs) noexcept
+    {
+      using std::begin, std::end;
+      auto first1 = begin(lhs.data_);
+      auto first2 = begin(rhs.data_);
+      const auto last1 = std::prev(end(lhs.data_));  // do not waste time for '\0'
+      const auto last2 = std::prev(end(rhs.data_));
+
+      for(; (first1 != last1) && (first2 != last2); ++first1, (void)++first2 ) {
+        if(*first1 < *first2) return true;
+        if(*first2 < *first1) return false;
+      }
+      return first1 == last1 && first2 != last2;
+    }
+  };
+
+  template<typename CharT, std::size_t N>
+  basic_fixed_string(const CharT (&str)[N]) -> basic_fixed_string<CharT, N-1>;
+
+  template<std::size_t N>
+  using fixed_string = basic_fixed_string<char, N>;
+
+  // TODO gcc:92101
+  // hacked version to work with derived_unit
+  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92101
 
   template<typename CharT, CharT... Chars>
-  struct basic_fixed_string {
+  struct basic_fixed_string_hack {
     static constexpr CharT txt[] = { Chars..., '\0' };
 
     static constexpr const CharT* c_str() noexcept
@@ -60,7 +93,7 @@ namespace units {
   inline namespace hacks {
 
     template<typename T, T... chars>
-    constexpr basic_fixed_string<T, chars...> operator""_fs() { return {}; }
+    constexpr basic_fixed_string_hack<T, chars...> operator""_fs() { return {}; }
 
   }
 

src/include/units/dimension.h

@@ -24,37 +24,30 @@
 
 #include <units/bits/type_list.h>
 #include <units/bits/downcasting.h>
+#include <units/bits/fixed_string.h>
 #include <units/ratio.h>
 #include <ratio>
 
 namespace units {
 
+  template<basic_fixed_string Name, basic_fixed_string Symbol>
+  struct base_dimension {
+    static constexpr auto name = Name;
+    static constexpr auto symbol = Symbol;
+  };
+
   template<typename T>
   concept BaseDimension = std::is_empty_v<T> &&
       requires {
-        { T::value } -> std::same_as<const char*>;
-      };
-
-  namespace detail {
-
-    template<BaseDimension D1, BaseDimension D2>
-    constexpr bool less()
-    {
-      const char* p1 = D1::value;
-      const char* p2 = D2::value;
-      for(; (*p1 != '\0') && (*p2 != '\0'); ++p1, (void) ++p2) {
-        if(*p1 < *p2) return true;
-        if(*p2 < *p1) return false;
-      }
-      return (*p1 == '\0') && (*p2 != '\0');
-    }
-
-  }
+        T::name;
+        T::symbol;
+      };// &&  // TODO file a bug for this gcc issue
+//      std::derived_from<T, base_dimension<T::name, T::symbol>>;
 
   // base_dimension_less
 
   template<BaseDimension D1, BaseDimension D2>
-  struct base_dimension_less : std::bool_constant<detail::less<D1, D2>()> {
+  struct base_dimension_less : std::bool_constant<D1::name < D2::name> {
   };
 
   // is_exp

src/include/units/dimensions/base_dimensions.h

@@ -26,12 +26,12 @@
 
 namespace units {
 
-  struct base_dim_length { static constexpr const char* value = "length"; };
-  struct base_dim_mass { static constexpr const char* value = "mass"; };
-  struct base_dim_time { static constexpr const char* value = "time"; };
-  struct base_dim_current { static constexpr const char* value = "current"; };
-  struct base_dim_temperature { static constexpr const char* value = "temperature"; };
-  struct base_dim_substance { static constexpr const char* value = "substance"; };
-  struct base_dim_luminous_intensity { static constexpr const char* value = "luminous intensity"; };
+  struct base_dim_length : base_dimension<"length", "m"> {};
+  struct base_dim_mass : base_dimension<"mass", "kg"> {};
+  struct base_dim_time : base_dimension<"time", "s"> {};
+  struct base_dim_current : base_dimension<"current", "A"> {};
+  struct base_dim_temperature : base_dimension<"temperature", "K"> {};
+  struct base_dim_substance : base_dimension<"substance", "mol"> {};
+  struct base_dim_luminous_intensity : base_dimension<"luminous intensity", "cd"> {};
 
 }  // namespace units

src/include/units/unit.h

@@ -24,7 +24,6 @@
 
 #include <units/dimension.h>
 #include <units/ratio.h>
-#include <units/bits/fixed_string.h>
 #include <ratio>
 
 namespace units {

test/unit_test/static/custom_unit_test.cpp

@@ -28,7 +28,7 @@
 
 namespace {
 
-  struct base_dim_digital_information { static constexpr const char* value = "digital information"; };
+  struct base_dim_digital_information : units::base_dimension<"digital information", "b"> {};
 
   struct digital_information : units::derived_dimension<digital_information, units::exp<base_dim_digital_information, 1>> {};
 
@@ -36,6 +36,7 @@ namespace {
   concept DigitalInformation = units::QuantityOf<T, digital_information>;
 
   using namespace units::hacks;
+
   struct bit : units::derived_unit<bit, decltype("b"_fs), digital_information> {};
   struct byte : units::derived_unit<byte, decltype("B"_fs), digital_information, units::ratio<8>> {};
 

test/unit_test/static/dimension_test.cpp

@@ -27,10 +27,10 @@ using namespace units;
 
 namespace {
 
-  struct d0 { static constexpr const char* value = "d0"; };
-  struct d1 { static constexpr const char* value = "d1"; };
-  struct d2 { static constexpr const char* value = "d2"; };
-  struct d3 { static constexpr const char* value = "d3"; };
+  struct d0 : base_dimension<"d0", ""> {};
+  struct d1 : base_dimension<"d1", ""> {};
+  struct d2 : base_dimension<"d2", ""> {};
+  struct d3 : base_dimension<"d3", ""> {};
 
   // exp_invert
 

test/unit_test/static/type_list_test.cpp

@@ -83,8 +83,8 @@ namespace {
       std::is_same_v<type_list_split_half<type_list<int, long, double, float>>::second_list, type_list<double, float>>);
 
   // type_list_merge_sorted
-  struct d0 { static constexpr const char* value = "d0"; };
-  struct d1 { static constexpr const char* value = "d1"; };
+  struct d0 : base_dimension<"d0", ""> {};
+  struct d1 : base_dimension<"d1", ""> {};
 
   static_assert(std::is_same_v<type_list_merge_sorted<type_list<exp<d0, 1>>, type_list<exp<d1, 1>>, exp_less>,
                                type_list<exp<d0, 1>, exp<d1, 1>>>);