int_pow.hpp

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/*
 * Copyright (C) 2010-2022 The ESPResSo project
 * Copyright (C) 2002,2003,2004,2005,2006,2007,2008,2009,2010
 *   Max-Planck-Institute for Polymer Research, Theory Group
 *
 * This file is part of ESPResSo.
 *
 * ESPResSo is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ESPResSo is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#ifndef UTILS_MATH_INT_POW_HPP
#define UTILS_MATH_INT_POW_HPP
 
#include "utils/device_qualifier.hpp"
 
#include <type_traits>
 
namespace Utils {
namespace detail {
template <class T, unsigned n, class = void> struct int_pow_impl {
  DEVICE_QUALIFIER constexpr T operator()(T x) const {
    return x * int_pow_impl<T, (n - 1) / 2>{}(x * x);
  }
};
 
/* Specialization for n even */
template <class T, unsigned n>
struct int_pow_impl<T, n, std::enable_if_t<n % 2 == 0>> {
  DEVICE_QUALIFIER constexpr T operator()(T x) const {
    return int_pow_impl<T, n / 2>{}(x * x);
  }
};
 
template <class T> struct int_pow_impl<T, 1> {
  DEVICE_QUALIFIER constexpr T operator()(T x) const { return x; }
};
 
template <class T> struct int_pow_impl<T, 0> {
  DEVICE_QUALIFIER constexpr T operator()(T) const { return T{1}; }
};
} // namespace detail
 
/**
 * \brief Calculate integer powers.
 * This functions calculates x^n, where
 * n is a positive integer that is known
 * at compile time. It uses exponentiation by
 * squaring to construct an efficient function.
 */
template <unsigned n, typename T> DEVICE_QUALIFIER constexpr T int_pow(T x) {
  return detail::int_pow_impl<T, n>{}(x);
}
} // namespace Utils
 
#endif