fix: add quantity_point::origin, like std::chrono::time_point::clock

.clang-format

@@ -63,7 +63,7 @@ CommentPragmas:  '^ NOLINT'
 # CompactNamespaces: false
 # ConstructorInitializerAllOnOneLineOrOnePerLine: true
 # ConstructorInitializerIndentWidth: 4
-# ContinuationIndentWidth: 4
+ContinuationIndentWidth: 2
 # Cpp11BracedListStyle: true
 DeriveLineEnding: false
 DerivePointerAlignment: false
@@ -109,7 +109,7 @@ IndentRequires:  true
 # KeepEmptyLinesAtTheStartOfBlocks: false
 # MacroBlockBegin: ''
 # MacroBlockEnd:   ''
-# MaxEmptyLinesToKeep: 1
+MaxEmptyLinesToKeep: 2
 # NamespaceIndentation: None
 # ObjCBinPackProtocolList: Never
 # ObjCBlockIndentWidth: 2

docs/framework/basic_concepts.rst

@@ -23,7 +23,7 @@ The most important concepts in the library are `Unit`, `Dimension`,
     ]
 
     [<abstract>QuantityPoint|
-    [quantity_point<Dimension, Unit, Rep>]
+    [quantity_point<PointOrigin, Unit, Rep>]
     ]
 
     [<abstract>QuantityKind|
@@ -39,6 +39,10 @@ The most important concepts in the library are `Unit`, `Dimension`,
     [Unit]<-[Quantity]
     [Quantity]<-[QuantityPoint]
 
+    [<abstract>PointOrigin]<-[QuantityPoint]
+    [Dimension]<-[PointOrigin]
+    [PointOrigin]<-[PointKind]
+
     [<abstract>Kind]<-[QuantityKind]
     [Dimension]<-[Kind]
     [Quantity]<-[QuantityKind]
@@ -60,14 +64,14 @@ derived dimensions. Examples: ``si::dim_time``, ``si::dim_length``, ``si::dim_sp
 specific representation. Examples: ``quantity<si::dim_time, si::second, int>``,
 ``si::length<si::metre, int>``, ``si::speed<si::kilometre_per_hour>``.
 
-`QuantityPoint` is an absolute `Quantity` with respect to some origin.
+`QuantityPoint` is an absolute `Quantity` with respect to an origin.
 Examples: timestamp (as opposed to duration), absolute temperature
 (as opposed to temperature difference).
 
 `QuantityKind` is a `Quantity` with more specific usage. Examples:
 distance (``horizonal_kind``) and height (``vertical_kind``) are different kinds
 of a length quantity.
 
-`QuantityPointKind` is an absolute `QuantityKind` with respect to some origin.
+`QuantityPointKind` is an absolute `QuantityKind` with respect to an origin.
 Examples: altitude is a quantity point of ``vertical_kind`` (as opposed to
 height).

docs/framework/dimensions.rst

@@ -106,8 +106,9 @@ Quantity Points
 +++++++++++++++
 
 Quantity points have a more restricted set of operations.
-Quantity can be added to or subtracted from a quantity point.
-The result will always be a quantity point of the same dimension:
+Quantity can be added to or subtracted
+from a quantity point of the same origin.
+The result will always be a quantity point of the same origin:
 
 .. code-block::
     :emphasize-lines: 3-5
@@ -132,9 +133,10 @@ The result is a relative quantity of the same dimension:
 
     It is not allowed to:
 
-    - add quantity points
-    - subtract a quantity point from a quantity:
-    - multiply nor divide quantity points with anything else.
+    - add quantity points,
+    - subtract a quantity point from a quantity,
+    - multiply nor divide quantity points with anything else, and
+    - mix quantity points with different origins:
 
     .. code-block::
         :emphasize-lines: 3-5
@@ -144,6 +146,8 @@ The result is a relative quantity of the same dimension:
         auto res1 = quantity_point{dist1} + quantity_point{dist2};  // ERROR
         auto res2 = dist1 - quantity_point{dist2};                  // ERROR
         auto res3 = quantity_point{dist1} / (2 * s);                // ERROR
+        auto res4 = quantity_point{std::chrono::utc_second{1s}} +
+                    quantity_point{std::chrono::sys_second{1s}};    // ERROR
 
 Quantity Point Kinds
 ++++++++++++++++++++

docs/framework/quantity_points.rst

@@ -3,9 +3,20 @@
 Quantity Points
 ===============
 
-A quantity point is an absolute quantity with respect to zero
-(which represents some origin) and is represented in the library with a
-`quantity_point` class template.
+A quantity point is an absolute quantity with respect to an origin
+and is represented in the library with a `quantity_point` class template.
+
+Point Origins
+-------------
+
+We need a `point_origin` to represent the origin of a quantity point::
+
+    struct mean_sea_level : point_origin<si::dim_length> {};
+
+Quantities points with this origin represent a point from the mean sea level.
+
+The library offers a `dynamic_origin<Dimension>`
+for quantity points whose origin is not specified in the type system.
 
 
 Construction
@@ -14,7 +25,7 @@ Construction
 To create the quantity point object from a `quantity` we just have to pass
 the value to the `quantity_point` class template explicit constructor::
 
-    quantity_point<si::dim_length, si::kilometre, double> d(123 * km);
+    quantity_point<dynamic_origin<si::dim_length>, si::kilometre, double> d(123 * km);
 
 .. note::
 
@@ -25,7 +36,7 @@ the value to the `quantity_point` class template explicit constructor::
     `copy initialization <https://en.cppreference.com/w/cpp/language/copy_initialization>`_
     **does not compile**::
 
-        quantity_point<si::dim_length, si::kilometre, double> d = 123 * km;  // ERROR
+        quantity_point<dynamic_origin<si::dim_length>, si::kilometre, double> d = 123 * km;  // ERROR
 
 
 Differences To Quantity

docs/use_cases/interoperability.rst

@@ -56,11 +56,12 @@ such an explicit conversion::
 
 For external quantity point-like types, `quantity_point_like_traits` is also provided.
 It works just like `quantity_like_traits`, except that
-``number(T)`` is replaced with ``relative(T)`` that returns the `QuantityLike` value.
+``number(T)`` is replaced with ``relative(T)`` that returns the `QuantityLike` value
+and ``dimension`` is replaced with ``origin``.
 
 Similar to `quantity` and `quantity_kind`, `quantity_point` and `quantity_kind_point`
 provide a deduction guide from `QuantityPointLike`::
 
     using namespace std::chrono_literals;
 
-    static_assert(quantity_point{std::chrono::sys_seconds{1s}} + 1 * s == quantity_point{2s});
+    static_assert((quantity_point{std::chrono::sys_seconds{1s}} + 1 * s).relative() == 2s);

example/glide_computer/include/glide_computer.h

@@ -30,6 +30,7 @@
 #include <units/quantity_point_kind.h>
 // IWYU pragma: end_exports
 
+#include <units/chrono.h>
 #include <units/format.h>
 #include <units/math.h> // IWYU pragma: keep
 #include <algorithm>
@@ -88,7 +89,7 @@ using altitude = units::quantity_point_kind<vertical_point_kind, units::isq::si:
 
 // time
 using duration = units::isq::si::time<units::isq::si::second>;
-using timestamp = units::quantity_point<units::isq::si::dim_time, units::isq::si::second>;
+using timestamp = units::quantity_point<units::clock_origin<std::chrono::system_clock>, units::isq::si::second>;
 
 // speed
 using velocity = units::quantity_kind<velocity_kind, units::isq::si::kilometre_per_hour>;

src/core/include/units/bits/basic_concepts.h

@@ -205,6 +205,52 @@ concept UnitOf =
   Dimension<D> &&
   std::same_as<typename U::reference, typename dimension_unit<D>::reference>;
 
+// PointOrigin
+
+template<Dimension D>
+struct point_origin;
+
+/**
+ * @brief A concept matching a point origin
+ *
+ * Satisfied by types derived from an specialization of @c point_origin.
+ */
+template<typename T>
+concept PointOrigin = is_derived_from_specialization_of<T, point_origin> &&
+  requires {
+    typename T::dimension;
+    requires Dimension<typename T::dimension>;
+    typename T::point_origin;
+    requires std::same_as<typename T::point_origin, point_origin<typename T::dimension>>;
+    requires !std::same_as<T, point_origin<typename T::dimension>>;
+  };
+
+// RebindablePointOriginFor
+
+namespace detail {
+
+template<typename O, typename D>
+struct rebind_point_origin_dimension_impl {
+  using type = typename O::template rebind<D>;
+};
+
+}  // namespace detail
+
+template<PointOrigin O, Dimension D>
+using rebind_point_origin_dimension = typename conditional<is_same_v<typename O::dimension, D>, std::type_identity<O>,
+                                                           detail::rebind_point_origin_dimension_impl<O, D>>::type;
+
+/**
+ * @brief A concept predicating the possibility of changing an origin's dimension
+ *
+ * Satisfied by point origins whose dimension can be made to be `D`.
+ */
+template<typename T, typename D>
+concept RebindablePointOriginFor =
+  requires { typename rebind_point_origin_dimension<T, D>; } &&
+  PointOrigin<rebind_point_origin_dimension<T, D>> &&
+  std::same_as<D, typename rebind_point_origin_dimension<T, D>::dimension>;
+
 // Kind
 namespace detail {
 
@@ -216,7 +262,7 @@ struct _kind_base;
 template<typename T, template<typename...> typename Base>
 concept kind_impl_ =
   is_derived_from_specialization_of<T, Base> &&
-  requires(T* t) {
+  requires {
     typename T::base_kind;
     typename T::dimension;
     requires Dimension<typename T::dimension>;
@@ -236,7 +282,7 @@ concept Kind =
 // PointKind
 namespace detail {
 
-template<Kind>
+template<Kind, PointOrigin>
 struct _point_kind_base;
 
 }  // namespace detail
@@ -247,7 +293,12 @@ struct _point_kind_base;
  * Satisfied by all point kind types derived from an specialization of @c point_kind.
  */
 template<typename T>
-concept PointKind = kind_impl_<T, detail::_point_kind_base>;
+concept PointKind =
+  kind_impl_<T, detail::_point_kind_base> &&
+  requires { typename T::origin; } &&
+  PointOrigin<typename T::origin> &&
+  std::same_as<typename T::dimension, typename T::base_kind::dimension> &&
+  std::same_as<typename T::dimension, typename T::origin::dimension>;
 
 // Reference
 namespace detail {

src/core/include/units/bits/common_quantity.h

@@ -31,7 +31,7 @@ namespace units {
 template<Dimension D, UnitOf<D> U, Representation Rep>
 class quantity;
 
-template<Dimension D, UnitOf<D> U, Representation Rep>
+template<PointOrigin O, UnitOf<typename O::dimension> U, Representation Rep>
 class quantity_point;
 
 template<Kind K, UnitOf<typename K::dimension> U, Representation Rep>
@@ -99,7 +99,8 @@ template<units::QuantityPoint QP1, units::QuantityPointEquivalentTo<QP1> QP2>
   requires requires { typename common_type_t<typename QP1::rep, typename QP2::rep>; }
 struct common_type<QP1, QP2> {
   using type = units::quantity_point<
-    typename common_type_t<typename QP1::quantity_type, typename QP2::quantity_type>::dimension,
+    units::rebind_point_origin_dimension<typename QP1::origin,
+      typename common_type_t<typename QP1::quantity_type, typename QP2::quantity_type>::dimension>,
     typename common_type_t<typename QP1::quantity_type, typename QP2::quantity_type>::unit,
     typename common_type_t<typename QP1::quantity_type, typename QP2::quantity_type>::rep>;
 };

src/core/include/units/bits/equivalent.h

@@ -81,29 +81,48 @@ template<Unit U1, Dimension D1, Unit U2, Dimension D2>
 struct equivalent_unit : std::disjunction<equivalent_impl<U1, U2>,
                                           std::bool_constant<U1::ratio / dimension_unit<D1>::ratio == U2::ratio / dimension_unit<D2>::ratio>> {};
 
+// point origins
+
+template<PointOrigin T, PointOrigin U>
+struct equivalent_impl<T, U> : std::bool_constant<requires {  // TODO: Simplify when Clang catches up.
+  requires RebindablePointOriginFor<T, typename U::dimension> && RebindablePointOriginFor<U, typename T::dimension> &&
+    std::same_as<T, rebind_point_origin_dimension<U, typename T::dimension>> &&
+    std::same_as<U, rebind_point_origin_dimension<T, typename U::dimension>>;
+  } && equivalent_impl<typename T::dimension, typename U::dimension>::value> {};
+
 
 // (point) kinds
 
-template<typename T, typename U>
-  requires (Kind<T> && Kind<U>) || (PointKind<T> && PointKind<U>)
+template<Kind T, Kind U>
 struct equivalent_impl<T, U> :
   std::conjunction<std::is_same<typename T::base_kind, typename U::base_kind>,
                    equivalent_impl<typename T::dimension, typename U::dimension>> {};
 
+template<PointKind T, PointKind U>
+struct equivalent_impl<T, U> :
+  std::conjunction<equivalent_impl<typename T::base_kind, typename U::base_kind>,
+                   equivalent_impl<typename T::origin, typename U::origin>> {};
+
 
 // quantities, quantity points, quantity (point) kinds
 
-template<typename Q1, typename Q2>
-  requires (Quantity<Q1> && Quantity<Q2>) || (QuantityPoint<Q1> && QuantityPoint<Q2>)
+template<Quantity Q1, Quantity Q2>
 struct equivalent_impl<Q1, Q2> : std::conjunction<equivalent_impl<typename Q1::dimension, typename Q2::dimension>,
                                                                   equivalent_unit<typename Q1::unit, typename Q1::dimension,
                                                                                   typename Q2::unit, typename Q2::dimension>> {};
 
-template<typename QK1, typename QK2>
-  requires (QuantityKind<QK1> && QuantityKind<QK2>) || (QuantityPointKind<QK1> && QuantityPointKind<QK2>)
+template<QuantityPoint QP1, QuantityPoint QP2>
+struct equivalent_impl<QP1, QP2> : std::conjunction<equivalent_impl<typename QP1::quantity_type, typename QP2::quantity_type>,
+                                                    equivalent_impl<typename QP1::origin, typename QP2::origin>> {};
+
+template<QuantityKind QK1, QuantityKind QK2>
 struct equivalent_impl<QK1, QK2> : std::conjunction<equivalent_impl<typename QK1::kind_type, typename QK2::kind_type>,
                                                     equivalent_impl<typename QK1::quantity_type, typename QK2::quantity_type>> {};
 
+template<QuantityPointKind QPK1, QuantityPointKind QPK2>
+struct equivalent_impl<QPK1, QPK2> : std::conjunction<equivalent_impl<typename QPK1::quantity_kind_type, typename QPK2::quantity_kind_type>,
+                                                      equivalent_impl<typename QPK1::origin, typename QPK2::origin>> {};
+
 }  // namespace detail
 
 template<typename T, typename U>

src/core/include/units/bits/quantity_of.h

@@ -85,21 +85,20 @@ template<typename Q1, typename Q2>
 concept QuantityEquivalentTo = Quantity<Q1> && QuantityOf<Q2, typename Q1::dimension>;
 
 /**
- * @brief A concept matching all quantity points with provided dimension
+ * @brief A concept matching all quantity points of the provided origin
  *
- * Satisfied by all quantity points with a dimension being the instantiation derived from
- * the provided dimension type.
+ * Satisfied by all quantity points with an origin equivalent to the provided one.
  */
-template<typename QP, typename Dim>
-concept QuantityPointOf = QuantityPoint<QP> && Dimension<Dim> && equivalent<typename QP::dimension, Dim>;
+template<typename QP, typename Orig>
+concept QuantityPointOf = QuantityPoint<QP> && PointOrigin<Orig> && equivalent<typename QP::origin, Orig>;
 
 /**
  * @brief A concept matching two equivalent quantity points
  *
- * Satisfied by quantity points having equivalent dimensions.
+ * Satisfied by quantity points having equivalent origins.
  */
 template<typename QP1, typename QP2>
-concept QuantityPointEquivalentTo = QuantityPoint<QP1> && QuantityPointOf<QP2, typename QP1::dimension>;
+concept QuantityPointEquivalentTo = QuantityPoint<QP1> && QuantityPointOf<QP2, typename QP1::origin>;
 
 /**
  * @brief A concept matching only quantity kinds of a specific kind.

src/core/include/units/chrono.h

@@ -25,6 +25,7 @@
 #include <units/customization_points.h>
 // IWYU pragma: begin_exports
 #include <units/isq/si/time.h>
+#include <units/point_origin.h>
 #include <chrono>
 // IWYU pragma: end_exports
 
@@ -38,15 +39,25 @@ struct quantity_like_traits<std::chrono::duration<Rep, Period>> {
   [[nodiscard]] static constexpr rep number(const std::chrono::duration<Rep, Period>& q) { return q.count(); }
 };
 
+template<typename C>
+struct clock_origin : point_origin<isq::si::dim_time> { };
+
 template<typename C, typename Rep, typename Period>
 struct quantity_point_like_traits<std::chrono::time_point<C, std::chrono::duration<Rep, Period>>> {
-  using dimension = isq::si::dim_time;
-  using unit = downcast_unit<dimension, ratio(Period::num, Period::den)>;
+  using origin = clock_origin<C>;
+  using unit = downcast_unit<typename origin::dimension, ratio(Period::num, Period::den)>;
   using rep = Rep;
   [[nodiscard]] static constexpr auto relative(
     const std::chrono::time_point<C, std::chrono::duration<Rep, Period>>& qp) {
     return qp.time_since_epoch();
   }
 };
 
+namespace detail {
+
+template<typename C, typename Rep, typename Period>
+inline constexpr bool is_quantity_point_like<std::chrono::time_point<C, std::chrono::duration<Rep, Period>>> = true;
+
+}  // namespace detail
+
 } // namespace units

src/core/include/units/customization_points.h

@@ -88,7 +88,7 @@ struct quantity_like_traits;
 /**
  * @brief Provides support for external quantity point-like types
  * 
- * The type trait should provide the following nested type aliases: @c dimension, @c unit, @c rep,
+ * The type trait should provide the following nested type aliases: @c origin, @c unit, @c rep,
  * and a static member function @c relative(T) that will return the quantity-like value of the quantity point.
  * 
  * Usage example can be found in @c units/chrono.h header file.