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/*
 * Copyright 2021 Google LLC
 *
 * 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
 *
 *     https://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.appengine.api.datastore;

import com.google.common.base.MoreObjects;
import com.google.common.base.Preconditions;
import com.google.common.collect.Iterables;
import com.google.common.collect.Multimap;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

/**
 * Abstract base class with a common {@link CallbackContext} implementation.
 *
 */
abstract class BaseCallbackContext implements CallbackContext {
  private final CurrentTransactionProvider currentTxnProvider;

  /** All elements provided to the operation that triggered the callback. */
  private final List elements;

  /** The index into {@link #elements} of the "current" element. */
  private int currentIndex;

  /**
   * @param currentTxnProvider Provides the current transaction
   * @param elements All elements involved in the operation that triggered the callback.
   */
  BaseCallbackContext(CurrentTransactionProvider currentTxnProvider, List elements) {
    this.currentTxnProvider = Preconditions.checkNotNull(currentTxnProvider);
    this.elements = Collections.unmodifiableList(Preconditions.checkNotNull(elements));
  }

  @Override
  public List getElements() {
    return elements;
  }

  @Override
  public Transaction getCurrentTransaction() {
    return currentTxnProvider.getCurrentTransaction(null);
  }

  @Override
  public int getCurrentIndex() {
    return currentIndex;
  }

  @Override
  public T getCurrentElement() {
    return elements.get(currentIndex);
  }

  /**
   * Executes all appropriate callbacks for the elements in this context.
   *
   * @param callbacksByKind A Multimap containing lists of callbacks, organized by kind.
   * @param noKindCallbacks Callbacks that apply to all elements, independent of kind.
   * @throws IllegalStateException If this method has already been called.
   */
  void executeCallbacks(
      Multimap callbacksByKind,
      Collection noKindCallbacks) {
    Preconditions.checkState(
        currentIndex == 0, "executeCallbacks cannot be called more than once.");
    for (T ele : elements) {
      // We run the callbacks registered with the specific kind and then we run the
      // callbacks registered for all kinds.
      Iterable allCallbacksToRun =
          Iterables.concat(callbacksByKind.get(getKind(ele)), noKindCallbacks);
      for (DatastoreCallbacksImpl.Callback callback : allCallbacksToRun) {
        callback.run(this);
      }
      currentIndex++;
    }
  }

  /** Abstract method that, given an element, knows how to extract its kind. */
  abstract String getKind(T ele);

  @Override
  public String toString() {
    return MoreObjects.toStringHelper(this)
        .add("elements", elements)
        .add("currentIndex", currentIndex)
        .toString();
  }
}