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// ************************************************************************************************
//
// libformfactor: efficient and accurate computation of scattering form factors
//
//! @file ff/Factorial.h
//! @brief Precomputes 1/n!
//!
//! @homepage https://jugit.fz-juelich.de/mlz/libformfactor
//! @license GNU General Public License v3 or higher (see LICENSE)
//! @copyright Forschungszentrum Jülich GmbH 2022
//! @author Joachim Wuttke, Scientific Computing Group at MLZ (see CITATION)
//
// ************************************************************************************************
#ifndef FF_FACTORIAL_H
#define FF_FACTORIAL_H
#include <array>
#include <cstddef>
#include <utility>
namespace {
template <size_t N> struct ReciprocalFactorial {
static constexpr double value = ReciprocalFactorial<N - 1>::value / N;
};
template <> struct ReciprocalFactorial<0> {
static constexpr double value = 1.0;
};
template <template <size_t> class F, size_t... I>
constexpr std::array<double, sizeof...(I)> generateArrayHelper(std::index_sequence<I...>)
{
return {F<I>::value...};
}
} // namespace
namespace ff_aux {
//! Returns a compile-time generated std::array of reciprocal factorials.
template <size_t N, typename Indices = std::make_index_sequence<N>>
constexpr std::array<double, N> generateReciprocalFactorialArray()
{
return generateArrayHelper<ReciprocalFactorial>(Indices{});
}
} // namespace ff_aux
#endif // FF_FACTORIAL_H
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