derived_unit helper added

src/include/units/bits/tools.h

@@ -100,15 +100,15 @@ namespace units {
   // common_ratio
 
   // todo: simplified
-  template<Ratio Ratio1, Ratio Ratio2>
+  template<Ratio R1, Ratio R2>
   struct common_ratio {
-    using gcd_num = static_gcd<Ratio1::num, Ratio2::num>;
+    using gcd_num = static_gcd<R1::num, R2::num>;
-    using gcd_den = static_gcd<Ratio1::den, Ratio2::den>;
+    using gcd_den = static_gcd<R1::den, R2::den>;
-    using type = std::ratio<gcd_num::value, (Ratio1::den / gcd_den::value) * Ratio2::den>;
+    using type = std::ratio<gcd_num::value, (R1::den / gcd_den::value) * R2::den>;
   };
 
-  template<Ratio Ratio1, Ratio Ratio2>
+  template<Ratio R1, Ratio R2>
-  using common_ratio_t = typename common_ratio<Ratio1, Ratio2>::type;
+  using common_ratio_t = typename common_ratio<R1, R2>::type;
 
   // upcasting
 

src/include/units/frequency.h

@@ -39,7 +39,7 @@ namespace units {
   struct millihertz : unit<dimension_frequency, std::milli> {};
   template<> struct upcasting_traits<upcast_from<millihertz>> : upcast_to<millihertz> {};
 
-  struct hertz : unit<dimension_frequency, std::ratio<1>> {};
+  struct hertz : derived_unit<dimension_frequency, second> {};
   template<> struct upcasting_traits<upcast_from<hertz>> : upcast_to<hertz> {};
 
   struct kilohertz : unit<dimension_frequency, std::kilo> {};

src/include/units/unit.h

@@ -50,4 +50,66 @@ namespace units {
       std::is_empty_v<T> &&
       detail::is_unit<upcast_from<T>>;
 
+
+  // derived_unit
+
+  namespace detail {
+
+    template<Dimension D>
+    struct get_unit_base_dim;
+
+    template<Exponent E, Exponent... Rest>
+    struct get_unit_base_dim<dimension<E, Rest...>> {
+      static_assert(sizeof...(Rest) == 0, "Base unit expected");
+      using dimension = typename E::dimension;
+    };
+
+    template<typename BaseDimension, Unit... Us>
+    struct get_ratio {
+      using ratio = std::ratio<1>;
+    };
+
+    template<typename BaseDimension, Unit U, Unit... Rest>
+    struct get_ratio<BaseDimension, U, Rest...> {
+      using unit_base_dim = typename get_unit_base_dim<typename U::dimension::base_type>::dimension;
+      using ratio = std::conditional_t<unit_base_dim::value == BaseDimension::value, typename U::ratio,
+                                       typename get_ratio<BaseDimension, Rest...>::ratio>;
+    };
+
+    template<Ratio Result, int UnitExpValue, Ratio UnitRatio>
+    struct ratio_op;
+
+    template<Ratio Result, Ratio UnitRatio>
+    struct ratio_op<Result, 0, UnitRatio> {
+      using ratio = Result;
+    };
+
+    template<Ratio Result, int UnitExpValue, Ratio UnitRatio>
+    struct ratio_op {
+      using calc_ratio = std::conditional_t<(UnitExpValue > 0), std::ratio_multiply<Result, UnitRatio>,
+                                            std::ratio_divide<Result, UnitRatio>>;
+      static constexpr int value = UnitExpValue > 0 ? UnitExpValue - 1 : UnitExpValue + 1;
+      using ratio = typename ratio_op<calc_ratio, value, UnitRatio>::ratio;
+    };
+
+    template<Dimension D, Unit... Us>
+    struct derived_ratio;
+
+    template<Unit... Us>
+    struct derived_ratio<dimension<>, Us...> {
+      using ratio = std::ratio<1>;
+    };
+
+    template<Exponent E, Exponent... Rest, Unit... Us>
+    struct derived_ratio<dimension<E, Rest...>, Us...> {
+      using rest_ratio = typename derived_ratio<dimension<Rest...>, Us...>::ratio;
+      using e_ratio = typename get_ratio<typename E::dimension, Us...>::ratio;
+      using ratio = typename ratio_op<rest_ratio, E::value, e_ratio>::ratio;
+    };
+
+  }
+
+  template<Dimension D, Unit... Us>
+  using derived_unit = unit<D, typename detail::derived_ratio<typename D::base_type, Us...>::ratio>;
+
 }  // namespace units

src/include/units/velocity.h

@@ -22,7 +22,6 @@
 
 #pragma once
 
-#include <units/base_dimensions.h>
 #include <units/length.h>
 #include <units/time.h>
 
@@ -37,13 +36,13 @@ namespace units {
   template<Unit U = struct meter_per_second, Number Rep = double>
   using velocity = quantity<dimension_velocity, U, Rep>;
 
-  struct meter_per_second : unit<dimension_velocity, std::ratio<1>> {};
+  struct meter_per_second : derived_unit<dimension_velocity, meter, second> {};
   template<> struct upcasting_traits<upcast_from<meter_per_second>> : upcast_to<meter_per_second> {};
 
-  struct kilometer_per_hour : unit<dimension_velocity, std::ratio_divide<kilometer::ratio, hour::ratio>> {};
+  struct kilometer_per_hour : derived_unit<dimension_velocity, kilometer, hour> {};
   template<> struct upcasting_traits<upcast_from<kilometer_per_hour>> : upcast_to<kilometer_per_hour> {};
 
-  struct mile_per_hour : unit<dimension_velocity, std::ratio_divide<mile::ratio, hour::ratio>> {};
+  struct mile_per_hour : derived_unit<dimension_velocity, mile, hour> {};
   template<> struct upcasting_traits<upcast_from<mile_per_hour>> : upcast_to<mile_per_hour> {};
 
   inline namespace literals {

test/test_units.cpp

@@ -38,12 +38,7 @@ namespace {
 
   using namespace units;
 
-  // frequency
+  /* ************** BASE DIMENSIONS **************** */
-
-  static_assert(2 / 1_s == 2_Hz);
-  static_assert(1000 / 1_s == 1_kHz);
-  static_assert(1 / 1_ms == 1_kHz);
-  static_assert(3.2_GHz == 3'200'000'000_Hz);
 
   // time
 
@@ -65,6 +60,17 @@ namespace {
 
 //  static_assert(5_in + 8_cm == 207_mm);
 
+
+
+  /* ************** DERIVED DIMENSIONS **************** */
+
+  // frequency
+
+  static_assert(2 / 1_s == 2_Hz);
+  static_assert(1000 / 1_s == 1_kHz);
+  static_assert(1 / 1_ms == 1_kHz);
+  static_assert(3.2_GHz == 3'200'000'000_Hz);
+
   // velocity
 
   static_assert(std::is_same_v<decltype(1_km / 1_s), velocity<unit<dimension_velocity, std::ratio<1000, 1>>, long long int>>);