dft_tools/triqs/arrays/functional/fold.hpp

/*******************************************************************************
 *
 * TRIQS: a Toolbox for Research in Interacting Quantum Systems
 *
 * Copyright (C) 2012 by O. Parcollet
 *
 * TRIQS 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.
 *
 * TRIQS 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
 * TRIQS. If not, see <http://www.gnu.org/licenses/>.
 *
 ******************************************************************************/
#ifndef TRIQS_ARRAYS_EXPRESSION_FOLD_H
#define TRIQS_ARRAYS_EXPRESSION_FOLD_H 
#include "../array.hpp"

namespace triqs {
namespace arrays {

 template <class F> struct fold_worker {
  F f;

  template <class A, class R> struct fold_func_adaptor {
   F const& f;
   A const& a;
   R& r;
   template <typename... Args> void operator()(Args const&... args) { r = f(r, a(args...)); }
  };

  template <class A, class R>
  auto operator()(A const& a,
                  R init) const -> typename std::decay<typename std::result_of<F(typename A::value_type, R)>::type>::type {
   // to take into account that f may be double,double -> double, while one passes 0 (an int...)
   // R = int, R2= double in such case, and the result will be a double, or narrowing will occur
   using R2 = typename std::decay<typename std::result_of<F(typename A::value_type, R)>::type>::type;
   R2 r2 = init;
   foreach(a, fold_func_adaptor<A, R2>{f, a, r2});
   return r2;
  }

  template <class A> typename A::value_type operator()(A const& a) const {
   return (*this)(a, typename A::value_type{});
  }
 };

 template <class F> fold_worker<F> fold(F f) {
  return {std::move(f)};
 }
}
} // namespace triqs::arrays

#endif