From da8721b997e5aa037b3e830e9c625e692795a40c Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Sun, 3 Dec 2023 16:15:38 +0100
Subject: [PATCH] feat: users are now allowed to inherit their ow types from
 absolute point origins

---
 .../framework_basics/the_affine_space.md      | 13 ++-
 example/currency.cpp                          |  2 +-
 example/include/geographic.h                  |  6 +-
 example/unmanned_aerial_vehicle.cpp           |  4 +-
 .../mp-units/bits/quantity_point_concepts.h   | 30 ++++---
 src/core/include/mp-units/quantity_point.h    | 42 ++++++++--
 .../mp-units/systems/si/point_origins.h       |  5 +-
 .../usc/include/mp-units/systems/usc/usc.h    |  2 +-
 src/utility/include/mp-units/chrono.h         |  2 +-
 test/unit_test/static/concepts_test.cpp       |  6 +-
 test/unit_test/static/quantity_point_test.cpp | 82 +++++++++++++------
 11 files changed, 133 insertions(+), 61 deletions(-)

diff --git a/docs/users_guide/framework_basics/the_affine_space.md b/docs/users_guide/framework_basics/the_affine_space.md
index 20456d74..608246f2 100644
--- a/docs/users_guide/framework_basics/the_affine_space.md
+++ b/docs/users_guide/framework_basics/the_affine_space.md
@@ -64,9 +64,16 @@ The **absolute point origin** specifies where the "zero" of our measurement's sc
 specify such an origin by deriving from the `absolute_point_origin` class template:
 
 ```cpp
-constexpr struct mean_sea_level : absolute_point_origin<isq::altitude> {} mean_sea_level;
+constexpr struct mean_sea_level : absolute_point_origin<mean_sea_level, isq::altitude> {} mean_sea_level;
 ```
 
+!!! info
+
+    The `absolute_point_origin` class template uses CRTP idiom to enforce the uniqueness of such a type.
+    You should pass the type of a derived class as the first argument of the template instantiation.
+
+*[CRTP]: Curiously Recurring Template Parameter
+
 ### `quantity_point`
 
 The `quantity_point` class template specifies an absolute quantity with respect to an origin:
@@ -185,7 +192,7 @@ the distance we will travel. We have to take a taxi to a local airport, fly to D
 a stopover in FRA, and, in the end, get a cab to the Gaylord Rockies Resort & Convention Center:
 
 ```cpp
-constexpr struct home : absolute_point_origin<isq::distance> {} home;
+constexpr struct home : absolute_point_origin<home, isq::distance> {} home;
 
 quantity_point<isq::distance[km], home> home_airport = home + 15 * km;
 quantity_point<isq::distance[km], home> fra_airport = home_airport + 829 * km;
@@ -246,7 +253,7 @@ two predefined point origins:
 ```cpp
 namespace mp_units::si {
 
-inline constexpr struct absolute_zero : absolute_point_origin<isq::thermodynamic_temperature> {} absolute_zero;
+inline constexpr struct absolute_zero : absolute_point_origin<absolute_zero, isq::thermodynamic_temperature> {} absolute_zero;
 inline constexpr struct ice_point : relative_point_origin<absolute_zero + 273.15 * kelvin> {} ice_point;
 
 }
diff --git a/example/currency.cpp b/example/currency.cpp
index fb1ff9fa..b1bcd871 100644
--- a/example/currency.cpp
+++ b/example/currency.cpp
@@ -33,7 +33,7 @@ inline constexpr struct dim_currency : base_dimension<"$"> {} dim_currency;
 
 QUANTITY_SPEC(currency, dim_currency);
 
-constexpr struct zero : absolute_point_origin<currency> {} zero;
+constexpr struct zero : absolute_point_origin<zero, currency> {} zero;
 
 inline constexpr struct euro : named_unit<"EUR", kind_of<currency>> {} euro;
 inline constexpr struct us_dollar : named_unit<"USD", kind_of<currency>> {} us_dollar;
diff --git a/example/include/geographic.h b/example/include/geographic.h
index d6830896..a647459a 100644
--- a/example/include/geographic.h
+++ b/example/include/geographic.h
@@ -38,7 +38,7 @@
 
 namespace geographic {
 
-inline constexpr struct mean_sea_level : mp_units::absolute_point_origin<mp_units::isq::altitude> {
+inline constexpr struct mean_sea_level : mp_units::absolute_point_origin<mean_sea_level, mp_units::isq::altitude> {
 } mean_sea_level;
 
 using msl_altitude = mp_units::quantity_point<mp_units::isq::altitude[mp_units::si::metre], mean_sea_level>;
@@ -64,9 +64,9 @@ struct MP_UNITS_STD_FMT::formatter<geographic::msl_altitude> : formatter<geograp
 
 namespace geographic {
 
-inline constexpr struct equator : mp_units::absolute_point_origin<mp_units::isq::angular_measure> {
+inline constexpr struct equator : mp_units::absolute_point_origin<equator, mp_units::isq::angular_measure> {
 } equator;
-inline constexpr struct prime_meridian : mp_units::absolute_point_origin<mp_units::isq::angular_measure> {
+inline constexpr struct prime_meridian : mp_units::absolute_point_origin<prime_meridian, mp_units::isq::angular_measure> {
 } prime_meridian;
 
 
diff --git a/example/unmanned_aerial_vehicle.cpp b/example/unmanned_aerial_vehicle.cpp
index f880ba56..228e22f0 100644
--- a/example/unmanned_aerial_vehicle.cpp
+++ b/example/unmanned_aerial_vehicle.cpp
@@ -37,7 +37,7 @@ using namespace geographic;
 enum class earth_gravity_model { egm84_15, egm95_5, egm2008_1 };
 
 template<earth_gravity_model M>
-struct height_above_ellipsoid_t : absolute_point_origin<isq::altitude> {
+struct height_above_ellipsoid_t : absolute_point_origin<height_above_ellipsoid_t<M>, isq::altitude> {
   static constexpr earth_gravity_model egm = M;
 };
 template<earth_gravity_model M>
@@ -107,7 +107,7 @@ hae_altitude<M> to_hae(msl_altitude msl, position<long double> pos)
 // **** HAL ****
 
 // clang-format off
-inline constexpr struct height_above_launch : absolute_point_origin<isq::altitude> {} height_above_launch;
+inline constexpr struct height_above_launch : absolute_point_origin<height_above_launch, isq::altitude> {} height_above_launch;
 // clang-format on
 
 using hal_altitude = quantity_point<isq::altitude[si::metre], height_above_launch>;
diff --git a/src/core/include/mp-units/bits/quantity_point_concepts.h b/src/core/include/mp-units/bits/quantity_point_concepts.h
index 7d2fa735..6488a096 100644
--- a/src/core/include/mp-units/bits/quantity_point_concepts.h
+++ b/src/core/include/mp-units/bits/quantity_point_concepts.h
@@ -30,7 +30,7 @@
 
 namespace mp_units {
 
-template<QuantitySpec auto Q>
+template<typename Derived, QuantitySpec auto QS>
 struct absolute_point_origin;
 
 namespace detail {
@@ -41,11 +41,11 @@ inline constexpr bool is_quantity_point = false;
 template<typename T>
 inline constexpr bool is_specialization_of_absolute_point_origin = false;
 
-template<auto Q>
-inline constexpr bool is_specialization_of_absolute_point_origin<absolute_point_origin<Q>> = true;
+template<typename D, auto Q>
+inline constexpr bool is_specialization_of_absolute_point_origin<absolute_point_origin<D, Q>> = true;
 
-template<auto Q>
-void to_base_specialization_of_absolute_point_origin(const volatile absolute_point_origin<Q>*);
+template<typename D, auto Q>
+void to_base_specialization_of_absolute_point_origin(const volatile absolute_point_origin<D, Q>*);
 
 template<typename T>
 inline constexpr bool is_derived_from_specialization_of_absolute_point_origin =
@@ -123,18 +123,16 @@ template<typename T>
 inline constexpr bool is_quantity_point<T> = true;
 
 template<PointOrigin PO1, PointOrigin PO2>
-[[nodiscard]] consteval bool same_absolute_point_origins(PO1, PO2)
+[[nodiscard]] consteval bool same_absolute_point_origins(PO1 po1, PO2 po2)
 {
-  if constexpr (is_same_v<PO1, PO2>)
-    return true;
-  else if constexpr (is_derived_from_specialization_of_relative_point_origin<PO1> &&
-                     is_derived_from_specialization_of_relative_point_origin<PO2>)
-    return is_same_v<std::remove_const_t<decltype(PO1::absolute_point_origin)>,
-                     std::remove_const_t<decltype(PO2::absolute_point_origin)>>;
-  else if constexpr (is_derived_from_specialization_of_relative_point_origin<PO1>)
-    return is_same_v<std::remove_const_t<decltype(PO1::absolute_point_origin)>, PO2>;
-  else if constexpr (is_derived_from_specialization_of_relative_point_origin<PO2>)
-    return is_same_v<PO1, std::remove_const_t<decltype(PO2::absolute_point_origin)>>;
+  if constexpr (AbsolutePointOrigin<PO1> && AbsolutePointOrigin<PO2>)
+    return po1 == po2;
+  else if constexpr (RelativePointOrigin<PO1> && RelativePointOrigin<PO2>)
+    return po1.absolute_point_origin == po2.absolute_point_origin;
+  else if constexpr (RelativePointOrigin<PO1>)
+    return po1.absolute_point_origin == po2;
+  else if constexpr (RelativePointOrigin<PO2>)
+    return po1 == po2.absolute_point_origin;
   else
     return false;
 }
diff --git a/src/core/include/mp-units/quantity_point.h b/src/core/include/mp-units/quantity_point.h
index a69a29ac..3920437c 100644
--- a/src/core/include/mp-units/quantity_point.h
+++ b/src/core/include/mp-units/quantity_point.h
@@ -29,9 +29,25 @@
 
 namespace mp_units {
 
-template<QuantitySpec auto QS>
+namespace detail {
+
+template<typename T>
+  requires requires {
+    {
+      T::zero()
+    } -> std::equality_comparable_with<T>;
+  }
+[[nodiscard]] constexpr bool is_eq_zero(T v)
+{
+  return v == T::zero();
+}
+
+}  // namespace detail
+
+template<typename Derived, QuantitySpec auto QS>
 struct absolute_point_origin {
   static constexpr QuantitySpec auto quantity_spec = QS;
+  using _type_ = absolute_point_origin;
 };
 
 template<QuantityPoint auto QP>
@@ -47,14 +63,30 @@ struct relative_point_origin {
   static constexpr PointOrigin auto absolute_point_origin = QP.absolute_point_origin;
 };
 
+template<PointOrigin PO1, PointOrigin PO2>
+[[nodiscard]] consteval bool operator==(PO1 po1, PO2 po2)
+{
+  if constexpr (detail::AbsolutePointOrigin<PO1> && detail::AbsolutePointOrigin<PO2>)
+    return is_same_v<typename PO1::_type_, typename PO2::_type_>;
+  else if constexpr (detail::RelativePointOrigin<PO1> && detail::RelativePointOrigin<PO2>)
+    return PO1::quantity_point == PO2::quantity_point;
+  else if constexpr (detail::RelativePointOrigin<PO1>)
+    return detail::same_absolute_point_origins(po1, po2) &&
+           detail::is_eq_zero(PO1::quantity_point.quantity_from(PO1::quantity_point.absolute_point_origin));
+  else if constexpr (detail::RelativePointOrigin<PO2>)
+    return detail::same_absolute_point_origins(po1, po2) &&
+           detail::is_eq_zero(PO2::quantity_point.quantity_from(PO2::quantity_point.absolute_point_origin));
+}
+
 namespace detail {
 
-[[nodiscard]] consteval PointOrigin auto get_absolute_point_origin(PointOrigin auto po)
+template<PointOrigin PO>
+[[nodiscard]] consteval PointOrigin auto get_absolute_point_origin(PO po)
 {
-  if constexpr (requires { po.quantity_point.absolute_point_origin; })
-    return po.quantity_point.absolute_point_origin;
-  else
+  if constexpr (AbsolutePointOrigin<PO>)
     return po;
+  else
+    return po.quantity_point.absolute_point_origin;
 }
 
 }  // namespace detail
diff --git a/src/systems/si/include/mp-units/systems/si/point_origins.h b/src/systems/si/include/mp-units/systems/si/point_origins.h
index d395478e..784b2d12 100644
--- a/src/systems/si/include/mp-units/systems/si/point_origins.h
+++ b/src/systems/si/include/mp-units/systems/si/point_origins.h
@@ -28,8 +28,11 @@
 namespace mp_units::si {
 
 // clang-format off
-inline constexpr struct absolute_zero : absolute_point_origin<isq::thermodynamic_temperature> {} absolute_zero;
+inline constexpr struct absolute_zero : absolute_point_origin<absolute_zero, isq::thermodynamic_temperature> {} absolute_zero;
+inline constexpr struct zeroth_kelvin : decltype(absolute_zero) {} zeroth_kelvin;
+
 inline constexpr struct ice_point : relative_point_origin<absolute_zero + 273.15 * kelvin> {} ice_point;
+inline constexpr struct zeroth_degree_Celsius : decltype(ice_point) {} zeroth_degree_Celsius;
 // clang-format on
 
 }  // namespace mp_units::si
diff --git a/src/systems/usc/include/mp-units/systems/usc/usc.h b/src/systems/usc/include/mp-units/systems/usc/usc.h
index bdfc0365..eb0ac882 100644
--- a/src/systems/usc/include/mp-units/systems/usc/usc.h
+++ b/src/systems/usc/include/mp-units/systems/usc/usc.h
@@ -112,7 +112,7 @@ inline constexpr struct inch_of_mercury : named_unit<"inHg", mag<ratio(3'386'389
 // https://en.wikipedia.org/wiki/United_States_customary_units#Temperature
 inline constexpr struct degree_Fahrenheit : named_unit<basic_symbol_text{"°F", "`F"}, mag<ratio{5, 9}> * si::degree_Celsius> {} degree_Fahrenheit;
 
-inline constexpr struct zero_Fahrenheit : relative_point_origin<si::ice_point - 32 * degree_Fahrenheit> {} zero_Fahrenheit;
+inline constexpr struct zero_Fahrenheit : relative_point_origin<si::zeroth_degree_Celsius - 32 * degree_Fahrenheit> {} zero_Fahrenheit;
 // clang-format on
 
 namespace unit_symbols {
diff --git a/src/utility/include/mp-units/chrono.h b/src/utility/include/mp-units/chrono.h
index d71372e3..5784dabb 100644
--- a/src/utility/include/mp-units/chrono.h
+++ b/src/utility/include/mp-units/chrono.h
@@ -79,7 +79,7 @@ struct quantity_like_traits<std::chrono::duration<Rep, Period>> {
 };
 
 template<typename C>
-struct chrono_point_origin_ : absolute_point_origin<isq::time> {
+struct chrono_point_origin_ : absolute_point_origin<chrono_point_origin_<C>, isq::time> {
   using clock = C;
 };
 template<typename C>
diff --git a/test/unit_test/static/concepts_test.cpp b/test/unit_test/static/concepts_test.cpp
index 978e1356..4312e638 100644
--- a/test/unit_test/static/concepts_test.cpp
+++ b/test/unit_test/static/concepts_test.cpp
@@ -38,7 +38,7 @@ namespace {
 
 using namespace mp_units;
 
-inline constexpr struct my_origin : absolute_point_origin<isq::length> {
+inline constexpr struct my_origin : absolute_point_origin<my_origin, isq::length> {
 } my_origin;
 inline constexpr struct my_relative_origin : relative_point_origin<my_origin + isq::length(42 * si::metre)> {
 } my_relative_origin;
@@ -358,7 +358,7 @@ static_assert(QuantityPoint<quantity_point<isq::length[si::metre], my_relative_o
 static_assert(QuantityPoint<quantity_point<isq::radius[si::metre], my_origin>>);
 static_assert(QuantityPoint<quantity_point<isq::radius[si::metre], my_relative_origin>>);
 static_assert(!QuantityPoint<std::remove_const_t<decltype(isq::length[si::metre])>>);
-static_assert(!QuantityPoint<absolute_point_origin<isq::length>>);
+static_assert(!QuantityPoint<absolute_point_origin<struct my_origin, isq::length>>);
 static_assert(!QuantityPoint<struct my_origin>);
 static_assert(!QuantityPoint<struct my_relative_origin>);
 static_assert(!QuantityPoint<std::chrono::seconds>);
@@ -390,7 +390,7 @@ static_assert(QuantityPointOf<quantity_point<isq::radius[si::metre], my_relative
 // PointOrigin
 static_assert(PointOrigin<struct my_origin>);
 static_assert(PointOrigin<struct my_relative_origin>);
-static_assert(!PointOrigin<absolute_point_origin<isq::length>>);
+static_assert(!PointOrigin<absolute_point_origin<struct my_origin, isq::length>>);
 static_assert(!PointOrigin<relative_point_origin<my_origin + 42 * si::metre>>);
 static_assert(!PointOrigin<quantity_point<si::metre, my_origin>>);
 static_assert(!PointOrigin<quantity_point<isq::length[si::metre], my_origin>>);
diff --git a/test/unit_test/static/quantity_point_test.cpp b/test/unit_test/static/quantity_point_test.cpp
index 54094970..9d4c4754 100644
--- a/test/unit_test/static/quantity_point_test.cpp
+++ b/test/unit_test/static/quantity_point_test.cpp
@@ -36,26 +36,77 @@ using namespace mp_units::si::unit_symbols;
 using namespace std::chrono_literals;
 using sys_seconds = std::chrono::time_point<std::chrono::system_clock, std::chrono::seconds>;
 
-inline constexpr struct mean_sea_level : absolute_point_origin<isq::height> {
+inline constexpr struct mean_sea_level : absolute_point_origin<mean_sea_level, isq::height> {
 } mean_sea_level;
 
+inline constexpr struct my_mean_sea_level : decltype(mean_sea_level) {
+} my_mean_sea_level;
+
+inline constexpr struct same_mean_sea_level : relative_point_origin<mean_sea_level + 0 * isq::height[m]> {
+} same_mean_sea_level;
+
 inline constexpr struct ground_level : relative_point_origin<mean_sea_level + 42 * isq::height[m]> {
 } ground_level;
 
+inline constexpr struct my_ground_level : decltype(ground_level) {
+} my_ground_level;
+
+inline constexpr struct same_ground_level1 : relative_point_origin<mean_sea_level + 42 * isq::height[m]> {
+} same_ground_level1;
+
+inline constexpr struct same_ground_level2 : relative_point_origin<my_mean_sea_level + 42 * isq::height[m]> {
+} same_ground_level2;
+
 inline constexpr struct tower_peak : relative_point_origin<ground_level + 42 * isq::height[m]> {
 } tower_peak;
 
 inline constexpr struct other_ground_level : relative_point_origin<mean_sea_level + 123 * isq::height[m]> {
 } other_ground_level;
 
-inline constexpr struct other_absolute_level : absolute_point_origin<isq::height> {
+inline constexpr struct other_absolute_level : absolute_point_origin<other_absolute_level, isq::height> {
 } other_absolute_level;
 
-inline constexpr struct zero : absolute_point_origin<dimensionless> {
+inline constexpr struct zero : absolute_point_origin<zero, dimensionless> {
 } zero;
 
 QUANTITY_SPEC(special_height, isq::height);
 
+
+//////////////////
+// point origins
+//////////////////
+
+static_assert(si::absolute_zero == si::zeroth_kelvin);
+static_assert(si::ice_point == si::zeroth_degree_Celsius);
+static_assert(si::absolute_zero != si::ice_point);
+static_assert(si::zeroth_kelvin != si::zeroth_degree_Celsius);
+
+static_assert(my_mean_sea_level == mean_sea_level);
+static_assert(my_mean_sea_level == same_mean_sea_level);
+
+static_assert(my_ground_level == ground_level);
+static_assert(same_ground_level1 == ground_level);
+static_assert(same_ground_level2 == my_ground_level);
+
+static_assert(mean_sea_level != other_absolute_level);
+static_assert(my_mean_sea_level != other_absolute_level);
+static_assert(ground_level != other_ground_level);
+
+template<auto QS>
+struct absolute_po_ : absolute_point_origin<absolute_po_<QS>, QS> {};
+template<auto QS>
+inline constexpr absolute_po_<QS> absolute_po;
+
+template<auto QP>
+struct relative_po_ : relative_point_origin<QP> {};
+template<auto QP>
+inline constexpr relative_po_<QP> relative_po;
+
+static_assert(relative_po<absolute_po<isq::length> + isq::height(42 * m)>.quantity_spec == isq::height);
+static_assert(relative_po<absolute_po<kind_of<isq::length>> + isq::height(42 * m)>.quantity_spec == isq::height);
+static_assert(relative_po<absolute_po<isq::height> + 42 * m>.quantity_spec == isq::height);
+
+
 /////////////////////
 // class invariants
 /////////////////////
@@ -1115,7 +1166,7 @@ static_assert(ground_level - other_ground_level == -81 * m);
 static_assert(other_ground_level - tower_peak == 39 * m);
 static_assert(tower_peak - other_ground_level == -39 * m);
 
-inline constexpr struct zero_m_per_s : absolute_point_origin<kind_of<isq::speed>> {
+inline constexpr struct zero_m_per_s : absolute_point_origin<zero_m_per_s, kind_of<isq::speed>> {
 } zero_m_per_s;
 
 // commutativity and associativity
@@ -1159,7 +1210,7 @@ static_assert(
   is_of_type<(zero_m_per_s + 10 * isq::height[m] / (2 * isq::time[s])) + (10 * isq::height[m] / (2 * isq::time[s])),
              quantity_point<(isq::height / isq::time)[m / s], zero_m_per_s, int>>);
 
-inline constexpr struct zero_Hz : absolute_point_origin<kind_of<isq::frequency>> {
+inline constexpr struct zero_Hz : absolute_point_origin<zero_Hz, kind_of<isq::frequency>> {
 } zero_Hz;
 
 static_assert(((zero_Hz + 10 / (2 * isq::period_duration[s])) + 5 * isq::frequency[Hz]).quantity_from(zero_Hz) ==
@@ -1209,7 +1260,7 @@ consteval bool invalid_subtraction(Ts... ts)
   return !requires { (... - ts); };
 }
 
-inline constexpr struct zero_Bq : absolute_point_origin<kind_of<isq::activity>> {
+inline constexpr struct zero_Bq : absolute_point_origin<zero_Bq, kind_of<isq::activity>> {
 } zero_Bq;
 
 static_assert(invalid_addition(zero_Bq + 5 * isq::activity[Bq], 5 * isq::frequency[Hz]));
@@ -1222,23 +1273,4 @@ static_assert(invalid_addition(5 * isq::activity[Bq], zero_Hz + 10 / (2 * isq::t
 static_assert(invalid_addition(5 * isq::activity[Bq], 10 / (2 * isq::time[s]), zero_Hz + 5 * isq::frequency[Hz]));
 static_assert(invalid_subtraction(zero_Bq + 5 * isq::activity[Bq], 10 / (2 * isq::time[s]), 5 * isq::frequency[Hz]));
 
-
-/////////////////////////
-// relative point origin
-/////////////////////////
-
-template<auto QS>
-struct absolute_po_ : absolute_point_origin<QS> {};
-template<auto QS>
-inline constexpr absolute_po_<QS> absolute_po;
-
-template<auto QP>
-struct relative_po_ : relative_point_origin<QP> {};
-template<auto QP>
-inline constexpr relative_po_<QP> relative_po;
-
-static_assert(relative_po<absolute_po<isq::length> + isq::height(42 * m)>.quantity_spec == isq::height);
-static_assert(relative_po<absolute_po<kind_of<isq::length>> + isq::height(42 * m)>.quantity_spec == isq::height);
-static_assert(relative_po<absolute_po<isq::height> + 42 * m>.quantity_spec == isq::height);
-
 }  // namespace