utils.hpp

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// Copyright (c) 2024 Simons Foundation
//
// 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.txt
//
// 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.
//
// Authors: Thomas Hahn, Olivier Parcollet, Nils Wentzell, chuffa
 
#ifndef _ITERTOOLS_UTILS_HPP
#define _ITERTOOLS_UTILS_HPP
 
#include <cstddef>
#include <iterator>
#include <type_traits>
#include <utility>
#include <vector>
 
namespace itertools {
 
  template <typename Iter1, typename Iter2>
  [[nodiscard]] inline typename std::iterator_traits<Iter1>::difference_type distance(Iter1 first, Iter2 last) {
    // O(1) for random access iterators
    if constexpr (std::is_same_v<typename std::iterator_traits<Iter1>::iterator_category, std::random_access_iterator_tag>) {
      return last - first;
    } else { // O(n) for other iterators
      typename std::iterator_traits<Iter1>::difference_type r(0);
      for (; first != last; ++first) ++r;
      return r;
    }
  }
 
  template <typename R> [[nodiscard]] auto make_vector_from_range(R const &rg) {
    std::vector<std::decay_t<decltype(*(std::begin(rg)))>> vec{};
    // do we really want to reserve memory here? maybe only for random access ranges?
    if constexpr (std::is_same_v<decltype(std::cbegin(rg)), decltype(std::cend(rg))>) {
      auto total_size = distance(std::cbegin(rg), std::cend(rg));
      vec.reserve(total_size);
    }
    for (auto const &x : rg) vec.emplace_back(x);
    return vec;
  }
 
  [[nodiscard]] inline std::pair<std::ptrdiff_t, std::ptrdiff_t> chunk_range(std::ptrdiff_t first, std::ptrdiff_t last, long n_chunks, long rank) {
    auto total_size    = last - first;
    auto chunk_size    = total_size / n_chunks;
    auto n_large_nodes = total_size - n_chunks * chunk_size;
    if (rank < n_large_nodes)
      return {first + rank * (chunk_size + 1), first + (rank + 1) * (chunk_size + 1)};
    else
      return {first + n_large_nodes + rank * chunk_size, first + n_large_nodes + (rank + 1) * chunk_size};
  }
 
} // namespace itertools
 
#endif // _ITERTOOLS_UTILS_HPP