From 3b14b588e9a2883f3be444202f2f204c772971f1 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Tue, 26 Nov 2024 10:09:28 +0100
Subject: [PATCH] refactor: `type_name_less` introduced and used as a default
 predicate for expression templates

---
 .../mp-units/bits/get_associated_quantity.h   |  2 +-
 src/core/include/mp-units/ext/type_name.h     |  3 +++
 .../include/mp-units/framework/dimension.h    | 14 +++----------
 .../mp-units/framework/expression_template.h  | 15 ++++++++------
 .../mp-units/framework/quantity_spec.h        | 20 +++++--------------
 src/core/include/mp-units/framework/unit.h    | 16 +++++----------
 6 files changed, 26 insertions(+), 44 deletions(-)

diff --git a/src/core/include/mp-units/bits/get_associated_quantity.h b/src/core/include/mp-units/bits/get_associated_quantity.h
index 2282ffb2d..dc7b4211b 100644
--- a/src/core/include/mp-units/bits/get_associated_quantity.h
+++ b/src/core/include/mp-units/bits/get_associated_quantity.h
@@ -80,7 +80,7 @@ template<AssociatedUnit U>
   else if constexpr (requires { U::_reference_unit_; })
     return determine_associated_quantity(U::_reference_unit_);
   else if constexpr (requires { typename U::_num_; }) {
-    return expr_map<to_quantity_spec, derived_quantity_spec, struct dimensionless, type_list_of_quantity_spec_less>(u);
+    return expr_map<to_quantity_spec, derived_quantity_spec, struct dimensionless>(u);
   }
 }
 
diff --git a/src/core/include/mp-units/ext/type_name.h b/src/core/include/mp-units/ext/type_name.h
index d368f869c..403ecd65c 100644
--- a/src/core/include/mp-units/ext/type_name.h
+++ b/src/core/include/mp-units/ext/type_name.h
@@ -10,6 +10,7 @@
 import std;
 #else
 #include <string_view>
+#include <type_traits>
 #endif
 #endif
 
@@ -37,6 +38,8 @@ template<typename T>
   return name;
 }
 
+template<typename Lhs, typename Rhs>
+struct type_name_less : std::bool_constant<type_name<Lhs>() < type_name<Rhs>()> {};
 
 // This is typically used to deterministically chose one of the alternatives
 // to guarantee the commutation of the operation (e.g., `a + b` should return
diff --git a/src/core/include/mp-units/framework/dimension.h b/src/core/include/mp-units/framework/dimension.h
index b0ff804f6..21c77929e 100644
--- a/src/core/include/mp-units/framework/dimension.h
+++ b/src/core/include/mp-units/framework/dimension.h
@@ -29,7 +29,6 @@
 #include <mp-units/compat_macros.h>
 #include <mp-units/ext/fixed_string.h>
 #include <mp-units/ext/inplace_vector.h>
-#include <mp-units/ext/type_name.h>
 #include <mp-units/ext/type_traits.h>
 #include <mp-units/framework/dimension_concepts.h>
 #include <mp-units/framework/expression_template.h>
@@ -59,12 +58,6 @@ MP_UNITS_EXPORT struct dimension_one;
 
 namespace detail {
 
-template<typename Lhs, typename Rhs>
-struct base_dimension_less : std::bool_constant<type_name<Lhs>() < type_name<Rhs>()> {};
-
-template<typename T1, typename T2>
-using type_list_of_base_dimension_less = expr_less<T1, T2, base_dimension_less>;
-
 template<typename... Expr>
 struct derived_dimension_impl : expr_fractions<dimension_one, Expr...> {};
 
@@ -72,13 +65,13 @@ struct dimension_interface {
   template<Dimension Lhs, Dimension Rhs>
   [[nodiscard]] friend consteval Dimension auto operator*(Lhs, Rhs)
   {
-    return expr_multiply<derived_dimension, struct dimension_one, type_list_of_base_dimension_less>(Lhs{}, Rhs{});
+    return expr_multiply<derived_dimension, struct dimension_one>(Lhs{}, Rhs{});
   }
 
   template<Dimension Lhs, Dimension Rhs>
   [[nodiscard]] friend consteval Dimension auto operator/(Lhs, Rhs)
   {
-    return expr_divide<derived_dimension, struct dimension_one, type_list_of_base_dimension_less>(Lhs{}, Rhs{});
+    return expr_divide<derived_dimension, struct dimension_one>(Lhs{}, Rhs{});
   }
 
   template<Dimension Lhs, Dimension Rhs>
@@ -197,8 +190,7 @@ template<std::intmax_t Num, std::intmax_t Den = 1, Dimension D>
   requires detail::non_zero<Den>
 [[nodiscard]] consteval Dimension auto pow(D d)
 {
-  return detail::expr_pow<Num, Den, derived_dimension, struct dimension_one, detail::type_list_of_base_dimension_less>(
-    d);
+  return detail::expr_pow<Num, Den, derived_dimension, struct dimension_one>(d);
 }
 
 /**
diff --git a/src/core/include/mp-units/framework/expression_template.h b/src/core/include/mp-units/framework/expression_template.h
index abc897cae..a0e0bc61e 100644
--- a/src/core/include/mp-units/framework/expression_template.h
+++ b/src/core/include/mp-units/framework/expression_template.h
@@ -25,6 +25,7 @@
 #include <mp-units/bits/math_concepts.h>
 #include <mp-units/bits/ratio.h>
 #include <mp-units/bits/type_list.h>
+#include <mp-units/ext/type_name.h>
 #include <mp-units/ext/type_traits.h>
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
@@ -302,6 +303,8 @@ struct expr_less_impl<T, power<T, Ints...>, Pred> : std::true_type {};
 template<typename Lhs, typename Rhs, template<typename, typename, auto...> typename Pred>
 using expr_less = expr_less_impl<Lhs, Rhs, Pred>;
 
+template<typename T1, typename T2>
+using type_list_name_less = expr_less<T1, T2, type_name_less>;
 
 // expr_fractions
 template<typename Num = type_list<>, typename Den = type_list<>>
@@ -387,8 +390,8 @@ template<typename NumList, typename DenList, SymbolicArg OneType, template<typen
  * @tparam Lhs lhs of the operation
  * @tparam Rhs rhs of the operation
  */
-template<template<typename...> typename To, SymbolicArg OneType, template<typename, typename> typename Pred,
-         typename Lhs, typename Rhs>
+template<template<typename...> typename To, SymbolicArg OneType,
+         template<typename, typename> typename Pred = type_list_name_less, typename Lhs, typename Rhs>
 [[nodiscard]] MP_UNITS_CONSTEVAL auto expr_multiply(Lhs, Rhs)
 {
   if constexpr (is_same_v<Lhs, OneType>) {
@@ -422,8 +425,8 @@ template<template<typename...> typename To, SymbolicArg OneType, template<typena
  * @tparam Lhs lhs of the operation
  * @tparam Rhs rhs of the operation
  */
-template<template<typename...> typename To, SymbolicArg OneType, template<typename, typename> typename Pred,
-         typename Lhs, typename Rhs>
+template<template<typename...> typename To, SymbolicArg OneType,
+         template<typename, typename> typename Pred = type_list_name_less, typename Lhs, typename Rhs>
 [[nodiscard]] MP_UNITS_CONSTEVAL auto expr_divide(Lhs lhs, Rhs rhs)
 {
   if constexpr (is_same_v<Lhs, Rhs>) {
@@ -489,7 +492,7 @@ template<std::intmax_t Num, std::intmax_t Den, template<typename...> typename To
  * @tparam T Expression being the base of the operation
  */
 template<std::intmax_t Num, std::intmax_t Den, template<typename...> typename To, SymbolicArg OneType,
-         template<typename, typename> typename Pred, typename T>
+         template<typename, typename> typename Pred = type_list_name_less, typename T>
   requires detail::non_zero<Den>
 [[nodiscard]] consteval auto expr_pow(T v)
 {
@@ -552,7 +555,7 @@ template<template<typename> typename Proj, template<typename...> typename To, Sy
  * @tparam T expression template to map from
  */
 template<template<typename> typename Proj, template<typename...> typename To, SymbolicArg OneType,
-         template<typename, typename> typename Pred, typename T>
+         template<typename, typename> typename Pred = type_list_name_less, typename T>
 [[nodiscard]] consteval auto expr_map(T)
 {
   if constexpr (type_list_size<typename T::_num_> + type_list_size<typename T::_den_> == 0)
diff --git a/src/core/include/mp-units/framework/quantity_spec.h b/src/core/include/mp-units/framework/quantity_spec.h
index cf752f89f..54437067c 100644
--- a/src/core/include/mp-units/framework/quantity_spec.h
+++ b/src/core/include/mp-units/framework/quantity_spec.h
@@ -106,12 +106,6 @@ template<auto... Args>
     return ch;
 }
 
-template<NamedQuantitySpec Lhs, NamedQuantitySpec Rhs>
-struct quantity_spec_less : std::bool_constant<type_name<Lhs>() < type_name<Rhs>()> {};
-
-template<typename T1, typename T2>
-using type_list_of_quantity_spec_less = expr_less<T1, T2, quantity_spec_less>;
-
 template<NamedQuantitySpec Q>
   requires requires { Q::dimension; }
 using to_dimension = MP_UNITS_NONCONST_TYPE(Q::dimension);
@@ -127,16 +121,14 @@ struct quantity_spec_interface_base {
   [[nodiscard]] friend consteval QuantitySpec auto operator*(Lhs lhs, Rhs rhs)
   {
     return clone_kind_of<Lhs{}, Rhs{}>(
-      expr_multiply<derived_quantity_spec, struct dimensionless, type_list_of_quantity_spec_less>(remove_kind(lhs),
-                                                                                                  remove_kind(rhs)));
+      expr_multiply<derived_quantity_spec, struct dimensionless>(remove_kind(lhs), remove_kind(rhs)));
   }
 
   template<QuantitySpec Lhs, QuantitySpec Rhs>
   [[nodiscard]] friend consteval QuantitySpec auto operator/(Lhs lhs, Rhs rhs)
   {
     return clone_kind_of<Lhs{}, Rhs{}>(
-      expr_divide<derived_quantity_spec, struct dimensionless, type_list_of_quantity_spec_less>(remove_kind(lhs),
-                                                                                                remove_kind(rhs)));
+      expr_divide<derived_quantity_spec, struct dimensionless>(remove_kind(lhs), remove_kind(rhs)));
   }
 
   template<QuantitySpec Lhs, QuantitySpec Rhs>
@@ -433,8 +425,7 @@ struct derived_quantity_spec_impl :
   using _base_type_ = derived_quantity_spec_impl;
   using _base_ = expr_fractions<dimensionless, Expr...>;
 
-  static constexpr Dimension auto dimension =
-    expr_map<to_dimension, derived_dimension, struct dimension_one, type_list_of_base_dimension_less>(_base_{});
+  static constexpr Dimension auto dimension = expr_map<to_dimension, derived_dimension, struct dimension_one>(_base_{});
   static constexpr quantity_character character =
     derived_quantity_character(typename _base_::_num_{}, typename _base_::_den_{});
 };
@@ -563,8 +554,7 @@ template<std::intmax_t Num, std::intmax_t Den = 1, QuantitySpec Q>
 [[nodiscard]] consteval QuantitySpec auto pow(Q q)
 {
   return detail::clone_kind_of<Q{}>(
-    detail::expr_pow<Num, Den, derived_quantity_spec, struct dimensionless, detail::type_list_of_quantity_spec_less>(
-      detail::remove_kind(q)));
+    detail::expr_pow<Num, Den, derived_quantity_spec, struct dimensionless>(detail::remove_kind(q)));
 }
 
 
@@ -1526,7 +1516,7 @@ template<QuantitySpec Q>
   } else if constexpr (requires { Q::_parent_; }) {
     return get_kind_tree_root(Q::_parent_);
   } else if constexpr (DerivedQuantitySpec<Q>) {
-    return expr_map<to_kind, derived_quantity_spec, struct dimensionless, type_list_of_quantity_spec_less>(q);
+    return expr_map<to_kind, derived_quantity_spec, struct dimensionless>(q);
   } else {
     // root quantity
     return q;
diff --git a/src/core/include/mp-units/framework/unit.h b/src/core/include/mp-units/framework/unit.h
index ffea57200..132b5fd37 100644
--- a/src/core/include/mp-units/framework/unit.h
+++ b/src/core/include/mp-units/framework/unit.h
@@ -152,12 +152,6 @@ struct derived_unit;
 
 namespace detail {
 
-template<Unit Lhs, Unit Rhs>
-struct unit_less : std::bool_constant<type_name<Lhs>() < type_name<Rhs>()> {};
-
-template<typename T1, typename T2>
-using type_list_of_unit_less = expr_less<T1, T2, unit_less>;
-
 struct unit_interface {
   /**
    * Multiplication by `1` returns the same unit, otherwise `scaled_unit` is being returned.
@@ -200,7 +194,7 @@ struct unit_interface {
   template<Unit Lhs, Unit Rhs>
   [[nodiscard]] friend MP_UNITS_CONSTEVAL Unit auto operator*(Lhs lhs, Rhs rhs)
   {
-    return expr_multiply<derived_unit, struct one, type_list_of_unit_less>(lhs, rhs);
+    return expr_multiply<derived_unit, struct one>(lhs, rhs);
   }
 
   /**
@@ -211,7 +205,7 @@ struct unit_interface {
   template<Unit Lhs, Unit Rhs>
   [[nodiscard]] friend MP_UNITS_CONSTEVAL Unit auto operator/(Lhs lhs, Rhs rhs)
   {
-    return expr_divide<derived_unit, struct one, type_list_of_unit_less>(lhs, rhs);
+    return expr_divide<derived_unit, struct one>(lhs, rhs);
   }
 
   template<Unit Lhs, Unit Rhs>
@@ -621,7 +615,7 @@ template<std::intmax_t Num, std::intmax_t Den = 1, Unit U>
   requires detail::non_zero<Den>
 [[nodiscard]] consteval Unit auto pow(U u)
 {
-  return detail::expr_pow<Num, Den, derived_unit, struct one, detail::type_list_of_unit_less>(u);
+  return detail::expr_pow<Num, Den, derived_unit, struct one>(u);
 }
 
 /**
@@ -696,7 +690,7 @@ template<Unit U1, Unit U2>
     else if constexpr (is_positive_integral_power(canonical_rhs.mag / canonical_lhs.mag))
       return u1;
     else {
-      if constexpr (detail::unit_less<U1, U2>::value)
+      if constexpr (detail::type_name_less<U1, U2>::value)
         return common_unit<U1, U2>{};
       else
         return common_unit<U2, U1>{};
@@ -730,7 +724,7 @@ struct collapse_common_unit_impl<List, NewUnit, true> {
 
 template<Unit NewUnit, Unit... Us>
 using collapse_common_unit = type_list_unique<
-  type_list_sort<typename collapse_common_unit_impl<type_list<>, NewUnit, false, Us...>::type, type_list_of_unit_less>>;
+  type_list_sort<typename collapse_common_unit_impl<type_list<>, NewUnit, false, Us...>::type, type_list_name_less>>;
 
 }  // namespace detail