--- /dev/null
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you 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 <utility>
+
+#include "arrow/result.h"
+
+namespace arrow {
+namespace internal {
+
+/// Helper providing single-lookup conditional insertion into std::map or
+/// std::unordered_map. If `key` exists in the container, an iterator to that pair
+/// will be returned. If `key` does not exist in the container, `gen(key)` will be
+/// invoked and its return value inserted.
+template <typename Map, typename Gen>
+auto GetOrInsertGenerated(Map* map, typename Map::key_type key, Gen&& gen)
+ -> decltype(map->begin()->second = gen(map->begin()->first), map->begin()) {
+ decltype(gen(map->begin()->first)) placeholder{};
+
+ auto it_success = map->emplace(std::move(key), std::move(placeholder));
+ if (it_success.second) {
+ // insertion of placeholder succeeded, overwrite it with gen()
+ const auto& inserted_key = it_success.first->first;
+ auto* value = &it_success.first->second;
+ *value = gen(inserted_key);
+ }
+ return it_success.first;
+}
+
+template <typename Map, typename Gen>
+auto GetOrInsertGenerated(Map* map, typename Map::key_type key, Gen&& gen)
+ -> Result<decltype(map->begin()->second = gen(map->begin()->first).ValueOrDie(),
+ map->begin())> {
+ decltype(gen(map->begin()->first).ValueOrDie()) placeholder{};
+
+ auto it_success = map->emplace(std::move(key), std::move(placeholder));
+ if (it_success.second) {
+ // insertion of placeholder succeeded, overwrite it with gen()
+ const auto& inserted_key = it_success.first->first;
+ auto* value = &it_success.first->second;
+ ARROW_ASSIGN_OR_RAISE(*value, gen(inserted_key));
+ }
+ return it_success.first;
+}
+
+} // namespace internal
+} // namespace arrow