src/main/java/com/google/devtools/build/lib/collect/CollectionUtils.java

// Copyright 2014 The Bazel Authors. All rights reserved.
//
// 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.
package com.google.devtools.build.lib.collect;

import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Maps;
import com.google.devtools.build.lib.collect.compacthashset.CompactHashSet;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import javax.annotation.Nullable;

/** Utilities for collection classes. */
public final class CollectionUtils {

  private CollectionUtils() {}

  /** Returns the set of all elements in the given list that appear more than once. */
  public static <T> Set<T> duplicatedElementsOf(List<T> input) {
    int count = input.size();
    if (count < 2) {
      return ImmutableSet.of();
    }
    Set<T> duplicates = null;
    Set<T> elementSet = CompactHashSet.createWithExpectedSize(count);
    for (T el : input) {
      if (!elementSet.add(el)) {
        if (duplicates == null) {
          duplicates = new HashSet<>();
        }
        duplicates.add(el);
      }
    }
    return duplicates == null ? ImmutableSet.of() : duplicates;
  }

  /** Returns a copy of the Map of Maps parameter. */
  public static <KEY_1, KEY_2, VALUE> Map<KEY_1, Map<KEY_2, VALUE>> copyOf(
      Map<KEY_1, ? extends Map<KEY_2, VALUE>> map) {
    return new HashMap<>(Maps.transformValues(map, HashMap::new));
  }

  /**
   * Checks whether the given collection is either {@code null} or {@linkplain Collection#isEmpty
   * empty}.
   */
  public static boolean isNullOrEmpty(@Nullable Collection<?> collection) {
    return collection == null || collection.isEmpty();
  }
}