feat: constant_unit support added

src/core/include/units/unit.h

@@ -220,6 +220,53 @@ template<typename T>
 concept NamedUnit = Unit<T> && requires(T* t) { detail::to_base_specialization_of_named_unit(t); } &&
                     (!detail::is_specialization_of_named_unit<T>);
 
+
+/**
+ * @brief A unit of a physical constant
+ *
+ * Defines a unit of a physical constant together with its value encoded as a unit ratio.
+ *
+ * This allows moving all of the constant-related ratio manipulation to the compile-time domain
+ * (i.e. multiplying and then dividing by the same constant will eliminate the item from the final
+ * type).
+ * As a result we have faster runtime performance and no precision loss due to eager floating-point
+ * operations. Also, if the user prefers integral types for a quantity representation, this will
+ * not force the user to convert to a floating-point type right away. Only when a final quantity
+ * number needs to actually account for the constant value, the floating-point operation (if any)
+ * can be triggered lazily with the `quantity_cast<U>()`.
+ *
+ * For example:
+ *
+ * @code{.cpp}
+ * inline constexpr struct standard_gravity_unit :
+ *   constant_unit<"g", mag<ratio{980'665, 100'000}> * metre / square<second>> {} standard_gravity_unit;
+ * @endcode
+ *
+ * @note A common convention in this library is to assign the same name for a type and an object of this type.
+ *       Besides defining them user never works with the unit types in the source code. All operations
+ *       are done on the objects. Contrarily, the unit types are the only one visible in the compilation
+ *       errors. Having them of the same names improves user experience and somehow blurs those separate domains.
+ *
+ * @tparam Symbol a short text representation of the constant
+ *
+ * @note Constant symbol is printed in the text output encapsulated inside square brackets `[]`
+ *       and before any regular units
+ */
+template<basic_symbol_text Symbol, Unit auto U>
+  requires(!Symbol.empty())
+struct constant_unit : named_unit<'[' + Symbol + ']', U> {};
+
+namespace detail {
+
+template<basic_symbol_text Symbol, Unit auto U>
+void to_base_specialization_of_constant_unit(const volatile constant_unit<Symbol, U>*);
+
+template<typename T>
+inline constexpr bool is_derived_from_specialization_of_constant_unit =
+  requires(T * t) { to_base_specialization_of_constant_unit(t); };
+
+}  // namespace detail
+
 /**
  * @brief Prevents assignment of a prefix to specific units
  *
@@ -444,11 +491,22 @@ template<Unit T, typename... Us>
 
 [[nodiscard]] consteval auto get_canonical_unit(Unit auto u) { return get_canonical_unit_impl(u, u); }
 
+template<Unit Lhs, Unit Rhs>
+[[nodiscard]] consteval bool less(Lhs, Rhs)
+{
+  if ((is_derived_from_specialization_of_constant_unit<Lhs> && is_derived_from_specialization_of_constant_unit<Rhs>) ||
+      (!is_derived_from_specialization_of_constant_unit<Lhs> && !is_derived_from_specialization_of_constant_unit<Rhs>))
+    return type_name<Lhs>() < type_name<Rhs>();
+  else
+    return is_derived_from_specialization_of_constant_unit<Lhs>;
+}
+
+
 // TODO What if the same unit will have different types (i.e. user will inherit its own type from `metre`)?
 // Is there a better way to sort units here? Some of them may not have symbol at all (like all units of
 // dimensionless quantities).
 template<Unit Lhs, Unit Rhs>
-struct unit_less : std::bool_constant<type_name<Lhs>() < type_name<Rhs>()> {};
+struct unit_less : std::bool_constant<less(Lhs{}, Rhs{})> {};
 
 template<typename T1, typename T2>
 using type_list_of_unit_less = expr_less<T1, T2, unit_less>;

test/unit_test/static/unit_test.cpp

@@ -80,6 +80,11 @@ inline constexpr struct mile_ : named_unit<"mi", mag<1760> * yard> {} mile;
 
 inline constexpr struct kilometre_ : decltype(si::kilo<metre>) {} kilometre;
 inline constexpr struct kilojoule_ : decltype(si::kilo<joule>) {} kilojoule;
+
+// physical constant units
+inline constexpr struct standard_gravity_unit_ : constant_unit<"g", mag<ratio{980'665, 100'000}> * metre / square<second>> {} standard_gravity_unit;
+inline constexpr struct speed_of_light_unit_ : constant_unit<"c", mag<299'792'458> * metre / second> {} speed_of_light_unit;
+
 // clang-format on
 
 // concepts verification
@@ -179,6 +184,17 @@ static_assert(joule != newton);
 
 static_assert(is_of_type<nu_second / nu_second, one_>);
 
+// constant_unit
+static_assert(is_of_type<standard_gravity_unit, standard_gravity_unit_>);
+static_assert(
+  is_of_type<get_canonical_unit(standard_gravity_unit).reference_unit, derived_unit<metre_, per<power<second_, 2>>>>);
+static_assert(get_canonical_unit(standard_gravity_unit).mag == mag<ratio{980'665, 100'000}>);
+static_assert(interconvertible(standard_gravity_unit, standard_gravity_unit));
+static_assert(interconvertible(standard_gravity_unit, metre / square<second>));
+static_assert(standard_gravity_unit == standard_gravity_unit);
+static_assert(standard_gravity_unit != metre / square<second>);  // magnitude is different
+static_assert(standard_gravity_unit.symbol == "[g]");
+
 // prefixed_unit
 static_assert(is_of_type<kilometre, kilometre_>);
 static_assert(is_of_type<get_canonical_unit(kilometre).reference_unit, metre_>);
@@ -308,6 +324,16 @@ static_assert(is_of_type<one * metre * square<second> / gram, derived_unit<metre
 static_assert(is_of_type<(metre * square<second> / gram) * one, derived_unit<metre_, power<second_, 2>, per<gram_>>>);
 static_assert(is_of_type<metre * square<second> / gram * one, derived_unit<metre_, power<second_, 2>, per<gram_>>>);
 
+static_assert(is_of_type<standard_gravity_unit * gram, derived_unit<standard_gravity_unit_, gram_>>);
+static_assert(is_of_type<gram * standard_gravity_unit, derived_unit<standard_gravity_unit_, gram_>>);
+static_assert(is_of_type<standard_gravity_unit / gram, derived_unit<standard_gravity_unit_, per<gram_>>>);
+static_assert(is_of_type<gram / standard_gravity_unit, derived_unit<gram_, per<standard_gravity_unit_>>>);
+static_assert(is_of_type<standard_gravity_unit * gram / standard_gravity_unit, gram_>);
+static_assert(is_of_type<speed_of_light_unit * gram * standard_gravity_unit,
+                         derived_unit<speed_of_light_unit_, standard_gravity_unit_, gram_>>);
+static_assert(is_of_type<gram * standard_gravity_unit * speed_of_light_unit,
+                         derived_unit<speed_of_light_unit_, standard_gravity_unit_, gram_>>);
+
 static_assert(std::is_same_v<decltype(1 / second * metre), decltype(metre / second)>);
 static_assert(std::is_same_v<decltype(metre * (1 / second)), decltype(metre / second)>);
 static_assert(std::is_same_v<decltype((metre / second) * (1 / second)), decltype(metre / second / second)>);
@@ -338,6 +364,14 @@ static_assert(
 static_assert(
   is_of_type<get_canonical_unit(1 / pascal).reference_unit, derived_unit<metre_, power<second_, 2>, per<gram_>>>);
 
+static_assert(
+  is_of_type<get_canonical_unit(standard_gravity_unit).reference_unit, derived_unit<metre_, per<power<second_, 2>>>>);
+static_assert(get_canonical_unit(standard_gravity_unit).mag == mag<ratio{980'665, 100'000}>);
+static_assert(is_of_type<get_canonical_unit(standard_gravity_unit* gram).reference_unit,
+                         derived_unit<gram_, metre_, per<power<second_, 2>>>>);
+static_assert(is_of_type<get_canonical_unit(standard_gravity_unit / speed_of_light_unit).reference_unit,
+                         derived_unit<one_, per<second_>>>);
+
 // operations commutativity
 constexpr auto u1 = mag<1000> * kilometre / hour;
 static_assert(is_of_type<u1, scaled_unit<mag<1000>, derived_unit<kilometre_, per<hour_>>>>);
@@ -496,6 +530,7 @@ static_assert(is_of_type<detail::common_unit(metre / second, kilometre / hour),
                          scaled_unit<mag<ratio{1, 18}>, derived_unit<metre_, per<second_>>>>);
 static_assert(is_of_type<detail::common_unit(kilometre, mile), scaled_unit<mag<ratio{8, 125}>, metre_>>);
 static_assert(is_of_type<detail::common_unit(mile, kilometre), scaled_unit<mag<ratio{8, 125}>, metre_>>);
+static_assert(is_of_type<detail::common_unit(speed_of_light_unit, metre / second), derived_unit<metre_, per<second_>>>);
 
 // unit symbols
 #ifdef __cpp_lib_constexpr_string
@@ -571,6 +606,11 @@ static_assert(unit_symbol(pow<1, 2>(metre)) == "m^(1/2)");
 static_assert(unit_symbol(pow<3, 5>(metre)) == "m^(3/5)");
 static_assert(unit_symbol(pow<1, 2>(metre / second)) == "m^(1/2)/s^(1/2)");
 
+// Physical constants
+static_assert(unit_symbol(speed_of_light_unit) == "[c]");
+static_assert(unit_symbol(gram * standard_gravity_unit * speed_of_light_unit) == "[c] [g] g");
+static_assert(unit_symbol(gram / standard_gravity_unit) == "g/[g]");
+
 #endif  // __cpp_lib_constexpr_string
 
 }  // namespace