/*
 * Copyright (C) 2016 The Guava 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 com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.NullnessCasts.uncheckedCastNullableTToT;
import static java.util.Collections.emptyList;

import com.google.common.annotations.GwtCompatible;
import java.util.ArrayList;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Optional;
import java.util.function.Supplier;
import java.util.stream.Collector;
import org.jspecify.annotations.NonNull;
import org.jspecify.annotations.Nullable;

/**
 * Collectors not present in {@code java.util.stream.Collectors} that are not otherwise associated
 * with a {@code com.google.common} type.
 *
 * @author Louis Wasserman
 * @since 21.0
 */
@GwtCompatible
public final class MoreCollectors {

  /*
   * TODO(lowasser): figure out if we can convert this to a concurrent AtomicReference-based
   * collector without breaking j2cl?
   */
  private static final Collector<Object, ?, Optional<Object>> TO_OPTIONAL =
      Collector.of(
          () -> new ToOptionalState<>(null),
          ToOptionalState::add,
          ToOptionalState::combine,
          ToOptionalState::getOptional,
          Collector.Characteristics.UNORDERED);

  /**
   * A collector that converts a stream of zero or one elements to an {@code Optional}.
   *
   * @throws IllegalArgumentException if the stream consists of two or more elements.
   * @throws NullPointerException if any element in the stream is {@code null}.
   * @return {@code Optional.of(onlyElement)} if the stream has exactly one element (must not be
   *     {@code null}) and returns {@code Optional.empty()} if it has none.
   */
  @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<@Nullable Object, ?, @Nullable Object> ONLY_ELEMENT =
      Collector.<@Nullable Object, ToOptionalState<Object>, @Nullable Object>of(
          () -> new ToOptionalState<>(null),
          (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 extends @Nullable Object> Collector<T, ?, T> onlyElement() {
    return (Collector) ONLY_ELEMENT;
  }

  /**
   * This atrocity is here to let us report several of the elements in the stream if there were more
   * than one, not just two.
   */
  @SuppressWarnings("EmptyList") // ImmutableList doesn't support nullable element types
  @IgnoreJRERequirement // see enclosing class (whose annotation Animal Sniffer ignores here...)
  private static final class ToOptionalState<T extends @Nullable Object> {
    static final int MAX_EXTRAS = 4;

    @Nullable T element;
    List<T> extras;
    final @Nullable Supplier<? extends RuntimeException> exceptionSupplier;

    ToOptionalState(@Nullable Supplier<? extends RuntimeException> exceptionSupplier) {
      this.element = null;
      this.extras = emptyList();
      this.exceptionSupplier = exceptionSupplier;
    }

    RuntimeException multiples(boolean overflow) {
      if (exceptionSupplier != null) {
        throw exceptionSupplier.get();
      }
      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(T o) {
      checkNotNull(o);
      if (element == null) {
        this.element = o;
      } else if (extras.isEmpty()) {
        // Replace immutable empty list with mutable list.
        extras = new ArrayList<>(MAX_EXTRAS);
        extras.add(o);
      } else if (extras.size() < MAX_EXTRAS) {
        extras.add(o);
      } else {
        throw multiples(true);
      }
    }

    ToOptionalState<T> combine(ToOptionalState<T> other) {
      if (element == null) {
        return other;
      } else if (other.element == null) {
        return this;
      } else {
        if (extras.isEmpty()) {
          // Replace immutable empty list with mutable list.
          extras = new ArrayList<>();
        }
        extras.add(other.element);
        extras.addAll(other.extras);
        if (extras.size() > MAX_EXTRAS) {
          extras.subList(MAX_EXTRAS, extras.size()).clear();
          throw multiples(true);
        }
        return this;
      }
    }

    Optional<@NonNull T> getOptional() {
      if (extras.isEmpty()) {
        return Optional.ofNullable(element);
      } else {
        throw multiples(false);
      }
    }

    T getElement() {
      if (element == null) {
        if (exceptionSupplier != null) {
          throw exceptionSupplier.get();
        } else {
          throw new NoSuchElementException();
        }
      } else if (extras.isEmpty()) {
        return uncheckedCastNullableTToT(element);
      } else {
        throw multiples(false);
      }
    }
  }

  private MoreCollectors() {}
}