choose.hpp

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#ifndef STAN_MATH_PRIM_FUN_CHOOSE_HPP
#define STAN_MATH_PRIM_FUN_CHOOSE_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/err.hpp>
#include <stan/math/prim/functor/apply_scalar_binary.hpp>
#include <boost/math/special_functions/binomial.hpp>
#include <cmath>
#include <limits>
 
namespace stan {
namespace math {
 
inline int choose(int n, int k) {
  check_nonnegative("choose", "n", n);
  check_nonnegative("choose", "k", k);
  if (k > n) {
    return 0;
  }
  const double choices = boost::math::binomial_coefficient<double>(n, k);
  check_less_or_equal("choose", "n choose k", choices,
                      std::numeric_limits<int>::max());
  return static_cast<int>(std::round(choices));
}
 
template <typename T1, typename T2, require_any_container_t<T1, T2>* = nullptr>
inline auto choose(const T1& a, const T2& b) {
  return apply_scalar_binary(
      a, b, [&](const auto& c, const auto& d) { return choose(c, d); });
}
 
}  // namespace math
}  // namespace stan
#endif