text: /*
   * Copyright 2012-present Facebook, Inc.
   *
   * 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.
   */
  
  #pragma once
  
  #include <folly/Conv.h>
  #include <folly/Optional.h>
  #include <folly/functional/Invoke.h>
  #include <tuple>
  
  namespace folly {
  
  /**
   * Given a map and a key, return the value corresponding to the key in the map,
   * or a given default value if the key doesn't exist in the map.
   */
  template <typename Map, typename Key>
  typename Map::mapped_type get_default(const Map& map, const Key& key) {
    auto pos = map.find(key);
    return (pos != map.end()) ? (pos->second) : (typename Map::mapped_type{});
  }
  template <
      class Map,
      typename Key = typename Map::key_type,
      typename Value = typename Map::mapped_type,
      typename std::enable_if<!is_invocable<Value>::value>::type* = nullptr>
  typename Map::mapped_type
  get_default(const Map& map, const Key& key, Value&& dflt) {
    using M = typename Map::mapped_type;
    auto pos = map.find(key);
    return (pos != map.end()) ? (pos->second) : M(std::forward<Value>(dflt));
  }
  
  /**
   * Give a map and a key, return the value corresponding to the key in the map,
   * or a given default value if the key doesn't exist in the map.
   */
  template <
      class Map,
      typename Key = typename Map::key_type,
      typename Func,
      typename = typename std::enable_if<
          is_invocable_r<typename Map::mapped_type, Func>::value>::type>
  typename Map::mapped_type
  get_default(const Map& map, const Key& key, Func&& dflt) {
    auto pos = map.find(key);
    return pos != map.end() ? pos->second : dflt();
  }
  
  /**
   * Given a map and a key, return the value corresponding to the key in the map,
   * or throw an exception of the specified type.
   */
  template <
      class E = std::out_of_range,
      class Map,
      typename Key = typename Map::key_type>
  const typename Map::mapped_type& get_or_throw(
      const Map& map,
      const Key& key,
      const std::string& exceptionStrPrefix = std::string()) {
    auto pos = map.find(key);
    if (pos != map.end()) {
      return pos->second;
    }
    throw E(folly::to<std::string>(exceptionStrPrefix, key));
  }
  
  template <
      class E = std::out_of_range,
      class Map,
      typename Key = typename Map::key_type>
  typename Map::mapped_type& get_or_throw(
      Map& map,
      const Key& key,
      const std::string& exceptionStrPrefix = std::string()) {
    auto pos = map.find(key);
    if (pos != map.end()) {
      return pos->second;
    }
    throw E(folly::to<std::string>(exceptionStrPrefix, key));
  }
  
  /**
   * Given a map and a key, return a Optional<V> if the key exists and None if the
   * key does not exist in the map.
   */
  template <class Map, typename Key = typename Map::key_type>
  folly::Optional<typename Map::mapped_type> get_optional(
      const Map& map,
      const Key& key) {
    auto pos = map.find(key);
    if (pos != map.end()) {
      return folly::Optional<typename Map::mapped_type>(pos->second);
    } else {
      return folly::none;
    }
  }
  
  /**
   * Given a map and a key, return a reference to the value corresponding to the
   * key in the map, or the given default reference if the key doesn't exist in
   * the map.
   */
  template <class Map, typename Key = typename Map::key_type>
  const typename Map::mapped_type& get_ref_default(
      const Map& map,
      const Key& key,
      const typename Map::mapped_type& dflt) {
    auto pos = map.find(key);
    return (pos != map.end() ? pos->second : dflt);
  }
  
  /**
   * Passing a temporary default value returns a dangling reference when it is
   * returned. Lifetime extension is broken by the indirection.
   * The caller must ensure that the default value outlives the reference returned
   * by get_ref_default().
   */
  template <class Map, typename Key = typename Map::key_type>
  const typename Map::mapped_type& get_ref_default(
      const Map& map,
      const Key& key,
      typename Map::mapped_type&& dflt) = delete;
  
  template <class Map, typename Key = typename Map::key_type>
  const typename Map::mapped_type& get_ref_default(
      const Map& map,
      const Key& key,
      const typename Map::mapped_type&& dflt) = delete;
  
  /**
   * Given a map and a key, return a reference to the value corresponding to the
   * key in the map, or the given default reference if the key doesn't exist in
   * the map.
   */
  template <
      class Map,
      typename Key = typename Map::key_type,
      typename Func,
      typename = typename std::enable_if<
          is_invocable_r<const typename Map::mapped_type&, Func>::value>::type,
      typename = typename std::enable_if<
          std::is_reference<invoke_result_t<Func>>::value>::type>
  const typename Map::mapped_type&
  get_ref_default(const Map& map, const Key& key, Func&& dflt) {
    auto pos = map.find(key);
    return (pos != map.end() ? pos->second : dflt());
  }
  
  /**
   * Given a map and a key, return a pointer to the value corresponding to the
   * key in the map, or nullptr if the key doesn't exist in the map.
   */
  template <class Map, typename Key = typename Map::key_type>
  const typename Map::mapped_type* get_ptr(const Map& map, const Key& key) {
    auto pos = map.find(key);
    return (pos != map.end() ? &pos->second : nullptr);
  }
  
  /**
   * Non-const overload of the above.
   */
  template <class Map, typename Key = typename Map::key_type>
  typename Map::mapped_type* get_ptr(Map& map, const Key& key) {
    auto pos = map.find(key);
    return (pos != map.end() ? &pos->second : nullptr);
  }
  
  // TODO: Remove the return type computations when clang 3.5 and gcc 5.1 are
  // the minimum supported versions.
  namespace detail {
  template <
      class T,
      size_t pathLength,
      class = typename std::enable_if<(pathLength > 0)>::type>
  struct NestedMapType {
    using type = typename NestedMapType<T, pathLength - 1>::type::mapped_type;
  };
  
  template <class T>
  struct NestedMapType<T, 1> {
    using type = typename T::mapped_type;
  };
  
  template <typename... KeysDefault>
  struct DefaultType;
  
  template <typename Default>
  struct DefaultType<Default> {
    using type = Default;
  };
  
  template <typename Key, typename... KeysDefault>
  struct DefaultType<Key, KeysDefault...> {
    using type = typename DefaultType<KeysDefault...>::type;
  };
  
  template <class... KeysDefault>
  auto extract_default(const KeysDefault&... keysDefault) ->
      typename DefaultType<KeysDefault...>::type const& {
    return std::get<sizeof...(KeysDefault) - 1>(std::tie(keysDefault...));
  }
  } // namespace detail
  
  /**
   * Given a map of maps and a path of keys, return a pointer to the nested value,
   * or nullptr if the key doesn't exist in the map.
   */
  template <class Map, class Key1, class Key2, class... Keys>
  auto get_ptr(
      const Map& map,
      const Key1& key1,
      const Key2& key2,
      const Keys&... keys) ->
      typename detail::NestedMapType<Map, 2 + sizeof...(Keys)>::type const* {
    auto pos = map.find(key1);
    return pos != map.end() ? get_ptr(pos->second, key2, keys...) : nullptr;
  }
  
  template <class Map, class Key1, class Key2, class... Keys>
  auto get_ptr(Map& map, const Key1& key1, const Key2& key2, const Keys&... keys)
      -> typename detail::NestedMapType<Map, 2 + sizeof...(Keys)>::type* {
    auto pos = map.find(key1);
    return pos != map.end() ? get_ptr(pos->second, key2, keys...) : nullptr;
  }
  
  /**
   * Given a map and a path of keys, return the value corresponding to the nested
   * value, or a given default value if the path doesn't exist in the map.
   * The default value is the last parameter, and is copied when returned.
   */
  template <
      class Map,
      class Key1,
      class Key2,
      class... KeysDefault,
      typename = typename std::enable_if<sizeof...(KeysDefault) != 0>::type>
  auto get_default(
      const Map& map,
      const Key1& key1,
      const Key2& key2,
      const KeysDefault&... keysDefault) ->
      typename detail::NestedMapType<Map, 1 + sizeof...(KeysDefault)>::type {
    if (const auto* ptr = get_ptr(map, key1)) {
      return get_default(*ptr, key2, keysDefault...);
    }
    return detail::extract_default(keysDefault...);
  }
  
  /**
   * Given a map and a path of keys, return a reference to the value corresponding
   * to the nested value, or the given default reference if the path doesn't exist
   * in the map.
   * The default value is the last parameter, and must be a lvalue reference.
   */
  template <
      class Map,
      class Key1,
      class Key2,
      class... KeysDefault,
      typename = typename std::enable_if<sizeof...(KeysDefault) != 0>::type,
      typename = typename std::enable_if<std::is_lvalue_reference<
          typename detail::DefaultType<KeysDefault...>::type>::value>::type>
  auto get_ref_default(
      const Map& map,
      const Key1& key1,
      const Key2& key2,
      KeysDefault&&... keysDefault) ->
      typename detail::NestedMapType<Map, 1 + sizeof...(KeysDefault)>::type
      const& {
    if (const auto* ptr = get_ptr(map, key1)) {
      return get_ref_default(*ptr, key2, keysDefault...);
    }
    return detail::extract_default(keysDefault...);
  }
  } // namespace folly
  