android13/external/dagger2/java/dagger/internal/codegen/extension/DaggerCollectors.java

/*
 * Copyright (C) 2019 The Dagger Authors.
 *
 * 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 dagger.internal.codegen.extension;

import static com.google.common.base.Preconditions.checkNotNull;

import java.util.ArrayList;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Optional;
import java.util.stream.Collector;
import javax.annotation.Nullable;

/**
 * A copy of {@link com.google.common.collect.MoreCollectors} to avoid issues with the '-android'
 * variant of Guava. See b/68008628
 */
public final class DaggerCollectors {

  private static final Collector<Object, ?, Optional<Object>> TO_OPTIONAL =
      Collector.of(
          ToOptionalState::new,
          ToOptionalState::add,
          ToOptionalState::combine,
          ToOptionalState::getOptional,
          Collector.Characteristics.UNORDERED);

  /**
   * A collector that converts a stream of zero or one elements to an {@code Optional}. The returned
   * collector throws an {@code IllegalArgumentException} if the stream consists of two or more
   * elements, and a {@code NullPointerException} if the stream consists of exactly one element,
   * which is null.
   */
  @SuppressWarnings("unchecked")
  public static <T> Collector<T, ?, Optional<T>> toOptional() {
    return (Collector) TO_OPTIONAL;
  }

  private static final Object NULL_PLACEHOLDER = new Object();

  private static final Collector<Object, ?, Object> ONLY_ELEMENT =
      Collector.of(
          ToOptionalState::new,
          (state, o) -> state.add((o == null) ? NULL_PLACEHOLDER : o),
          ToOptionalState::combine,
          state -> {
            Object result = state.getElement();
            return (result == NULL_PLACEHOLDER) ? null : result;
          },
          Collector.Characteristics.UNORDERED);

  /**
   * A collector that takes a stream containing exactly one element and returns that element. The
   * returned collector throws an {@code IllegalArgumentException} if the stream consists of two or
   * more elements, and a {@code NoSuchElementException} if the stream is empty.
   */
  @SuppressWarnings("unchecked")
  public static <T> Collector<T, ?, T> onlyElement() {
    return (Collector) ONLY_ELEMENT;
  }

  private static final class ToOptionalState {
    static final int MAX_EXTRAS = 4;

    @Nullable Object element;
    @Nullable List<Object> extras;

    ToOptionalState() {
      element = null;
      extras = null;
    }

    IllegalArgumentException multiples(boolean overflow) {
      StringBuilder sb =
          new StringBuilder().append("expected one element but was: <").append(element);
      for (Object o : extras) {
        sb.append(", ").append(o);
      }
      if (overflow) {
        sb.append(", ...");
      }
      sb.append('>');
      throw new IllegalArgumentException(sb.toString());
    }

    void add(Object o) {
      checkNotNull(o);
      if (element == null) {
        this.element = o;
      } else if (extras == null) {
        extras = new ArrayList<>(MAX_EXTRAS);
        extras.add(o);
      } else if (extras.size() < MAX_EXTRAS) {
        extras.add(o);
      } else {
        throw multiples(true);
      }
    }

    ToOptionalState combine(ToOptionalState other) {
      if (element == null) {
        return other;
      } else if (other.element == null) {
        return this;
      } else {
        if (extras == null) {
          extras = new ArrayList<>();
        }
        extras.add(other.element);
        if (other.extras != null) {
          this.extras.addAll(other.extras);
        }
        if (extras.size() > MAX_EXTRAS) {
          extras.subList(MAX_EXTRAS, extras.size()).clear();
          throw multiples(true);
        }
        return this;
      }
    }

    Optional<Object> getOptional() {
      if (extras == null) {
        return Optional.ofNullable(element);
      } else {
        throw multiples(false);
      }
    }

    Object getElement() {
      if (element == null) {
        throw new NoSuchElementException();
      } else if (extras == null) {
        return element;
      } else {
        throw multiples(false);
      }
    }
  }

  private DaggerCollectors() {}
}