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fix: quantity scaling between different prefixes improved
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Resolves #608
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@mpusz
mpusz committed Sep 6, 2024
1 parent 7eb9b76 commit 1570bda
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Showing 2 changed files with 85 additions and 67 deletions.
149 changes: 82 additions & 67 deletions src/core/include/mp-units/bits/sudo_cast.h
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Original file line number Diff line number Diff line change
Expand Up @@ -31,35 +31,25 @@
namespace mp_units::detail {

template<typename T, typename Other>
struct get_common_type : std::common_type<T, Other> {};

template<typename T, typename Other>
using maybe_common_type = MP_UNITS_TYPENAME std::conditional_t<requires { typename std::common_type_t<T, Other>; },
get_common_type<T, Other>, std::type_identity<T>>::type;
using maybe_common_type = std::conditional_t<requires { typename std::common_type_t<T, Other>; },
std::common_type<T, Other>, std::type_identity<T>>::type;

/**
* @brief Details about the conversion from one quantity to another.
* @brief Type-related details about the conversion from one quantity to another
*
* This struct calculates the conversion factor that needs to be applied to a number,
* in order to convert from one quantity to another. In addition to that, it also
* helps to determine what representations to use at which step in the conversion process,
* This trait helps to determine what representations to use at which step in the conversion process,
* in order to avoid overflow and underflow while not causing excessive computations.
*
* @note This is a low-level facility.
*
* @tparam To a target quantity type to cast to
* @tparam From a source quantity type to cast from
* @tparam M common magnitude between the two quantities
* @tparam Rep1 first quantity representation type
* @tparam Rep2 second quantity representation type
*/
template<Quantity To, Quantity From>
requires(castable(From::quantity_spec, To::quantity_spec))
struct magnitude_conversion_traits {
// scale the number
static constexpr Magnitude auto c_mag = get_canonical_unit(From::unit).mag / get_canonical_unit(To::unit).mag;
static constexpr Magnitude auto num = numerator(c_mag);
static constexpr Magnitude auto den = denominator(c_mag);
static constexpr Magnitude auto irr = c_mag * (den / num);
using c_rep_type = maybe_common_type<typename std::remove_reference_t<From>::rep, typename To::rep>;
using c_mag_type = common_magnitude_type<c_mag>;
template<Magnitude auto M, typename Rep1, typename Rep2>
struct conversion_type_traits {
using c_rep_type = maybe_common_type<Rep1, Rep2>;
using c_mag_type = common_magnitude_type<M>;
using multiplier_type = conditional<
treat_as_floating_point<c_rep_type>,
// ensure that the multiplier is also floating-point
Expand All @@ -68,11 +58,28 @@ struct magnitude_conversion_traits {
std::common_type_t<c_mag_type, value_type_t<c_rep_type>>, std::common_type_t<c_mag_type, double>>,
c_mag_type>;
using c_type = maybe_common_type<c_rep_type, multiplier_type>;
static constexpr auto val(Magnitude auto m) { return get_value<multiplier_type>(m); };
static constexpr multiplier_type num_mult = val(num);
static constexpr multiplier_type den_mult = val(den);
static constexpr multiplier_type irr_mult = val(irr);
static constexpr multiplier_type ratio = num_mult / den_mult * irr_mult;
};

/**
* @brief Value-related details about the conversion from one quantity to another
*
* This trait provide ingredients to calculate the conversion factor that needs to be applied
* to a number, in order to convert from one quantity to another.
*
* @note This is a low-level facility.
*
* @tparam M common magnitude between the two quantities
* @tparam T common multiplier representation type
*/
template<Magnitude auto M, typename T>
struct conversion_value_traits {
static constexpr Magnitude auto num = numerator(M);
static constexpr Magnitude auto den = denominator(M);
static constexpr Magnitude auto irr = M * (den / num);
static constexpr T num_mult = get_value<T>(num);
static constexpr T den_mult = get_value<T>(den);
static constexpr T irr_mult = get_value<T>(irr);
static constexpr T ratio = num_mult / den_mult * irr_mult;
};


Expand All @@ -84,35 +91,43 @@ struct magnitude_conversion_traits {
*
* @tparam To a target quantity type to cast to
*/
template<Quantity To, typename From>
requires Quantity<std::remove_cvref_t<From>> &&
(castable(std::remove_reference_t<From>::quantity_spec, To::quantity_spec)) &&
((std::remove_reference_t<From>::unit == To::unit &&
std::constructible_from<typename To::rep, typename std::remove_reference_t<From>::rep>) ||
(std::remove_reference_t<From>::unit != To::unit)) // && scalable_with_<typename To::rep>))
template<Quantity To, typename FwdFrom, typename From = std::remove_cvref_t<FwdFrom>>
requires Quantity<From> && (castable(From::quantity_spec, To::quantity_spec)) &&
((From::unit == To::unit && std::constructible_from<typename To::rep, typename From::rep>) ||
(From::unit != To::unit)) // && scalable_with_<typename To::rep>))
// TODO how to constrain the second part here?
[[nodiscard]] constexpr To sudo_cast(From&& q)
[[nodiscard]] constexpr To sudo_cast(FwdFrom&& q)
{
constexpr auto q_unit = std::remove_reference_t<From>::unit;
constexpr auto q_unit = From::unit;
if constexpr (q_unit == To::unit) {
// no scaling of the number needed
return {static_cast<MP_UNITS_TYPENAME To::rep>(std::forward<From>(q).numerical_value_is_an_implementation_detail_),
return {static_cast<To::rep>(std::forward<FwdFrom>(q).numerical_value_is_an_implementation_detail_),
To::reference}; // this is the only (and recommended) way to do a truncating conversion on a number, so we
// are using static_cast to suppress all the compiler warnings on conversions
} else {
static constexpr Magnitude auto c_mag = get_canonical_unit(From::unit).mag / get_canonical_unit(To::unit).mag;
using type_traits = conversion_type_traits<c_mag, typename From::rep, typename To::rep>;
using multiplier_type = typename type_traits::multiplier_type;
auto scale = [&](std::invocable<typename type_traits::c_type> auto func) {
auto res =
static_cast<To::rep>(func(static_cast<type_traits::c_type>(q.numerical_value_is_an_implementation_detail_)));
return To{res, To::reference};
};

// scale the number
using traits = magnitude_conversion_traits<To, std::remove_reference_t<From>>;
if constexpr (std::is_floating_point_v<typename traits::multiplier_type>) {
// this results in great assembly
auto res = static_cast<MP_UNITS_TYPENAME To::rep>(
static_cast<traits::c_type>(q.numerical_value_is_an_implementation_detail_) * traits::ratio);
return {res, To::reference};
} else {
// this is slower but allows conversions like 2000 m -> 2 km without loosing data
auto res = static_cast<MP_UNITS_TYPENAME To::rep>(
static_cast<traits::c_type>(q.numerical_value_is_an_implementation_detail_) * traits::num_mult /
traits::den_mult * traits::irr_mult);
return {res, To::reference};
if constexpr (is_integral(c_mag))
return scale([&](auto value) { return value * get_value<multiplier_type>(numerator(c_mag)); });
else if constexpr (is_integral(pow<-1>(c_mag)))
return scale([&](auto value) { return value / get_value<multiplier_type>(denominator(c_mag)); });
else {
using value_traits = conversion_value_traits<c_mag, multiplier_type>;
if constexpr (std::is_floating_point_v<multiplier_type>)
// this results in great assembly
return scale([](auto value) { return value * value_traits::ratio; });
else
// this is slower but allows conversions like 2000 m -> 2 km without loosing data
return scale(
[](auto value) { return value * value_traits::num_mult / value_traits::den_mult * value_traits::irr_mult; });
}
}
}
Expand All @@ -126,21 +141,18 @@ template<Quantity To, typename From>
*
* @tparam ToQP a target quantity point type to which to cast to
*/
template<QuantityPoint ToQP, typename FromQP>
requires QuantityPoint<std::remove_cvref_t<FromQP>> &&
(castable(std::remove_reference_t<FromQP>::quantity_spec, ToQP::quantity_spec)) &&
(detail::same_absolute_point_origins(ToQP::point_origin, std::remove_reference_t<FromQP>::point_origin)) &&
((std::remove_reference_t<FromQP>::unit == ToQP::unit &&
std::constructible_from<typename ToQP::rep, typename std::remove_reference_t<FromQP>::rep>) ||
(std::remove_reference_t<FromQP>::unit != ToQP::unit))
[[nodiscard]] constexpr QuantityPoint auto sudo_cast(FromQP&& qp)
template<QuantityPoint ToQP, typename FwdFromQP, typename FromQP = std::remove_cvref_t<FwdFromQP>>
requires QuantityPoint<FromQP> && (castable(FromQP::quantity_spec, ToQP::quantity_spec)) &&
(detail::same_absolute_point_origins(ToQP::point_origin, FromQP::point_origin)) &&
((FromQP::unit == ToQP::unit && std::constructible_from<typename ToQP::rep, typename FromQP::rep>) ||
(FromQP::unit != ToQP::unit))
[[nodiscard]] constexpr QuantityPoint auto sudo_cast(FwdFromQP&& qp)
{
using qp_type = std::remove_reference_t<FromQP>;
if constexpr (is_same_v<std::remove_const_t<decltype(ToQP::point_origin)>,
std::remove_const_t<decltype(qp_type::point_origin)>>) {
std::remove_const_t<decltype(FromQP::point_origin)>>) {
return quantity_point{
sudo_cast<typename ToQP::quantity_type>(std::forward<FromQP>(qp).quantity_from(qp_type::point_origin)),
qp_type::point_origin};
sudo_cast<typename ToQP::quantity_type>(std::forward<FromQP>(qp).quantity_from(FromQP::point_origin)),
FromQP::point_origin};
} else {
// it's unclear how hard we should try to avoid truncation here. For now, the only corner case we cater for,
// is when the range of the quantity type of at most one of QP or ToQP doesn't cover the offset between the
Expand All @@ -152,23 +164,26 @@ template<QuantityPoint ToQP, typename FromQP>
// (c) add/subtract the origin difference
// In the following, we carefully select the order of these three operations: each of (a) and (b) is scheduled
// either before or after (c), such that (c) acts on the largest range possible among all combination of source
// and target unit and represenation.
using traits = magnitude_conversion_traits<typename ToQP::quantity_type, typename qp_type::quantity_type>;
using c_rep_type = typename traits::c_rep_type;
if constexpr (traits::num_mult * traits::irr_mult > traits::den_mult) {
// and target unit and representation.
static constexpr Magnitude auto c_mag = get_canonical_unit(FromQP::unit).mag / get_canonical_unit(ToQP::unit).mag;
using type_traits = conversion_type_traits<c_mag, typename FromQP::rep, typename ToQP::rep>;
using value_traits = conversion_value_traits<c_mag, typename type_traits::multiplier_type>;
using c_rep_type = typename type_traits::c_rep_type;
if constexpr (value_traits::num_mult * value_traits::irr_mult > value_traits::den_mult) {
// original unit had a larger unit magnitude; if we first convert to the common representation but retain the
// unit, we obtain the largest possible range while not causing truncation of fractional values. This is optimal
// for the offset computation.
return sudo_cast<ToQP>(
sudo_cast<quantity_point<qp_type::reference, qp_type::point_origin, c_rep_type>>(std::forward<FromQP>(qp))
sudo_cast<quantity_point<FromQP::reference, FromQP::point_origin, c_rep_type>>(std::forward<FromQP>(qp))
.point_for(ToQP::point_origin));
} else {
// new unit may have a larger unit magnitude; we first need to convert to the new unit (potentially causing
// truncation, but no more than if we did the conversion later), but make sure we keep the larger of the two
// representation types. Then, we can perform the offset computation.
return sudo_cast<ToQP>(sudo_cast<quantity_point<make_reference(qp_type::quantity_spec, ToQP::unit),
qp_type::point_origin, c_rep_type>>(std::forward<FromQP>(qp))
.point_for(ToQP::point_origin));
return sudo_cast<ToQP>(
sudo_cast<quantity_point<make_reference(FromQP::quantity_spec, ToQP::unit), FromQP::point_origin, c_rep_type>>(
std::forward<FromQP>(qp))
.point_for(ToQP::point_origin));
}
}
}
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3 changes: 3 additions & 0 deletions test/static/quantity_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -260,6 +260,9 @@ static_assert(quantity<isq::length[km], int>(2 * km).force_in(km).numerical_valu
static_assert(quantity<isq::length[km], int>(2 * km).force_in(m).numerical_value_in(m) == 2000);
static_assert(quantity<isq::length[m], int>(2000 * m).force_in(km).numerical_value_in(km) == 2);

static_assert((15. * m).in(nm).numerical_value_in(m) == 15.);
static_assert((15'000. * nm).in(m).numerical_value_in(nm) == 15'000.);

template<template<auto, typename> typename Q>
concept invalid_unit_conversion = requires {
requires !requires { Q<isq::length[m], int>(2000 * m).in(km); }; // truncating conversion
Expand Down

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