code: /* LanguageTool, a natural language style checker
   * Copyright (C) 2020 Peter Gromov
   *
   * This library is free software; you can redistribute it and/or
   * modify it under the terms of the GNU Lesser General Public
   * License as published by the Free Software Foundation; either
   * version 2.1 of the License, or (at your option) any later version.
   *
   * This library 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
   * Lesser General Public License for more details.
   *
   * You should have received a copy of the GNU Lesser General Public
   * License along with this library; if not, write to the Free Software
   * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301
   * USA
   */
  package org.languagetool.tools;
  
  import gnu.trove.THashMap;
  import org.jetbrains.annotations.Nullable;
  
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.List;
  import java.util.Map;
  
  /**
   * A map containing multiple values per key, memory-optimized for case when there's only one value.
   */
  public class MostlySingularMultiMap<K, V> {
    private final THashMap<K, Object> map;
  
    public MostlySingularMultiMap(Map<K, List<V>> contents) {
      map = new THashMap<>(contents.size());
      for (Map.Entry<K, List<V>> entry : contents.entrySet()) {
        List<V> value = entry.getValue();
        map.put(entry.getKey(), value.size() == 1 ? value.get(0) : value.toArray());
      }
      map.trimToSize();
    }
  
    @Nullable
    public List<V> getList(K key) {
      Object o = map.get(key);
      //noinspection unchecked
      return o == null ? null :
             o instanceof Object[] ? Arrays.asList((V[]) o) :
             Collections.singletonList((V) o);
    }
  
  
  }
  