binomial_coef.hpp

/*################################################################################
  ##
  ##   Copyright (C) 2016-2024 Keith O'Hara
  ##
  ##   This file is part of the GCE-Math C++ library.
  ##
  ##   Licensed under the Apache License, Version 2.0 (the "License");
  ##   you may not use this file except in compliance with the License.
  ##   You may obtain a copy of the License at
  ##
  ##       http://www.apache.org/licenses/LICENSE-2.0
  ##
  ##   Unless required by applicable law or agreed to in writing, software
  ##   distributed under the License is distributed on an "AS IS" BASIS,
  ##   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  ##   See the License for the specific language governing permissions and
  ##   limitations under the License.
  ##
  ################################################################################*/

#ifndef _gcem_binomial_coef_HPP
#define _gcem_binomial_coef_HPP

namespace internal
{

template<typename T>
constexpr
T
binomial_coef_recur(const T n, const T k)
noexcept
{
    return( // edge cases
                (k == T(0) || n == k) ? T(1) : // deals with 0 choose 0 case
                n == T(0) ? T(0) :
            // else
                binomial_coef_recur(n-1,k-1) + binomial_coef_recur(n-1,k) );
}

template<typename T, typename std::enable_if<std::is_integral<T>::value>::type* = nullptr>
constexpr
T
binomial_coef_check(const T n, const T k)
noexcept
{
    return binomial_coef_recur(n,k);
}

template<typename T, typename std::enable_if<!std::is_integral<T>::value>::type* = nullptr>
constexpr
T
binomial_coef_check(const T n, const T k)
noexcept
{
    return( // NaN check; removed due to MSVC problems; template not being ignored in <int> cases
            // (is_nan(n) || is_nan(k)) ? GCLIM<T>::quiet_NaN() :
            //
            static_cast<T>(binomial_coef_recur(static_cast<ullint_t>(n),static_cast<ullint_t>(k))) );
}

template<typename T1, typename T2, typename TC = common_t<T1,T2>>
constexpr
TC
binomial_coef_type_check(const T1 n, const T2 k)
noexcept
{
    return binomial_coef_check(static_cast<TC>(n),static_cast<TC>(k));
}

}

/**
 * Compile-time binomial coefficient
 *
 * @param n integral-valued input.
 * @param k integral-valued input.
 * @return computes the Binomial coefficient
 * \f[ \binom{n}{k} = \frac{n!}{k!(n-k)!} \f]
 * also known as '\c n choose \c k '.
 */

template<typename T1, typename T2>
constexpr
common_t<T1,T2>
binomial_coef(const T1 n, const T2 k)
noexcept
{
    return internal::binomial_coef_type_check(n,k);
}

#endif