From ce6db6950fe75e30a3a536cfa3cd938d8dfb436b Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Mon, 18 Dec 2023 16:40:05 +0100
Subject: [PATCH] feat: `quantity_point` support added for `quantity_cast` and
 `value_cast`

---
 .../include/mp-units/bits/quantity_cast.h     | 26 +++++++++++++
 src/core/include/mp-units/bits/value_cast.h   | 38 +++++++++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/src/core/include/mp-units/bits/quantity_cast.h b/src/core/include/mp-units/bits/quantity_cast.h
index b03c7ea3..cf8fbef9 100644
--- a/src/core/include/mp-units/bits/quantity_cast.h
+++ b/src/core/include/mp-units/bits/quantity_cast.h
@@ -23,6 +23,7 @@
 #pragma once
 
 #include <mp-units/bits/quantity_concepts.h>
+#include <mp-units/bits/quantity_point_concepts.h>
 #include <mp-units/reference.h>
 #include <type_traits>
 
@@ -56,4 +57,29 @@ template<QuantitySpec auto ToQS, typename Q>
     return quantity{std::forward<Q>(q).numerical_value_is_an_implementation_detail_, reference<ToQS, Q::unit>{}};
 }
 
+/**
+ * @brief Explicit cast of a quantity point type
+ *
+ * This cast converts only a quantity point type. It might be used to force some quantity point type
+ * conversions that are not implicitly allowed but are allowed explicitly.
+ *
+ * For example:
+ *
+ * @code{.cpp}
+ * auto length = isq::length(quantity_point{42 * m});
+ * auto altitude = quantity_cast<isq::altitude>(length);
+ * @endcode
+ *
+ * @note This cast does not affect the underlying value of a number stored in a quantity point.
+ *
+ * @tparam ToQS a quantity specification to use for a target quantity point
+ */
+template<QuantitySpec auto ToQS, typename QP>
+  requires QuantityPoint<std::remove_cvref_t<QP>> && (castable(QP::quantity_spec, ToQS))
+[[nodiscard]] constexpr QuantityPoint auto quantity_cast(QP&& qp)
+{
+  return QP{quantity_cast<ToQS>(std::forward<QP>(qp).quantity_from_origin_is_an_implementation_detail_),
+            qp.point_origin};
+}
+
 }  // namespace mp_units
diff --git a/src/core/include/mp-units/bits/value_cast.h b/src/core/include/mp-units/bits/value_cast.h
index 7d045163..88316ad8 100644
--- a/src/core/include/mp-units/bits/value_cast.h
+++ b/src/core/include/mp-units/bits/value_cast.h
@@ -23,6 +23,7 @@
 #pragma once
 
 #include <mp-units/bits/quantity_concepts.h>
+#include <mp-units/bits/quantity_point_concepts.h>
 #include <mp-units/bits/representation_concepts.h>
 #include <mp-units/bits/sudo_cast.h>
 #include <mp-units/bits/unit_concepts.h>
@@ -74,4 +75,41 @@ template<Representation ToRep, typename Q>
   return detail::sudo_cast<quantity<std::remove_reference_t<Q>::reference, ToRep>>(std::forward<Q>(q));
 }
 
+/**
+ * @brief Explicit cast of a quantity point's unit
+ *
+ * Implicit conversions between quantity points of different types are allowed only for "safe"
+ * (e.g. non-truncating) conversion. In truncating cases an explicit cast have to be used.
+ *
+ * auto qp = value_cast<si::second>(quantity_point{1234 * ms});
+ *
+ * @tparam ToU a unit to use for a target quantity point
+ */
+template<Unit auto ToU, typename QP>
+[[nodiscard]] constexpr QuantityPoint auto value_cast(QP&& qp)
+  requires QuantityPoint<std::remove_cvref_t<QP>> && (convertible(std::remove_reference_t<QP>::reference, ToU))
+{
+  return QP{value_cast<ToU>(std::forward<QP>(qp).quantity_from_origin_is_an_implementation_detail_), qp.point_origin};
+}
+
+/**
+ * @brief Explicit cast of a quantity point's representation type
+ *
+ * Implicit conversions between quantity points of different types are allowed only for "safe"
+ * (e.g. non-truncating) conversion. In truncating cases an explicit cast have to be used.
+ *
+ * auto qp = value_cast<int>(quantity_point{1.23 * ms});
+ *
+ * @tparam ToRep a representation type to use for a target quantity point
+ */
+template<Representation ToRep, typename QP>
+  requires QuantityPoint<std::remove_cvref_t<QP>> &&
+           RepresentationOf<ToRep, std::remove_reference_t<QP>::quantity_spec.character> &&
+           std::constructible_from<ToRep, typename std::remove_reference_t<QP>::rep>
+[[nodiscard]] constexpr quantity_point<std::remove_reference_t<QP>::reference, QP::point_origin, ToRep> value_cast(
+  QP&& qp)
+{
+  return {value_cast<ToRep>(std::forward<QP>(qp).quantity_from_origin_is_an_implementation_detail_), qp.point_origin};
+}
+
 }  // namespace mp_units