/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You 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 org.apache.commons.lang3.concurrent;
  
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.Map;
  import java.util.NoSuchElementException;
  import java.util.Objects;
  import java.util.Set;
  import java.util.concurrent.ExecutorService;
  
  /**
   * A specialized {@link BackgroundInitializer} implementation that can deal with
   * multiple background initialization tasks.
   *
   * <p>
   * This class has a similar purpose as {@link BackgroundInitializer}. However,
   * it is not limited to a single background initialization task. Rather it
   * manages an arbitrary number of {@link BackgroundInitializer} objects,
   * executes them, and waits until they are completely initialized. This is
   * useful for applications that have to perform multiple initialization tasks
   * that can run in parallel (i.e. that do not depend on each other). This class
   * takes care about the management of an {@link ExecutorService} and shares it
   * with the {@link BackgroundInitializer} objects it is responsible for; so the
   * using application need not bother with these details.
   * </p>
   * <p>
   * The typical usage scenario for {@link MultiBackgroundInitializer} is as
   * follows:
   * </p>
   * <ul>
   * <li>Create a new instance of the class. Optionally pass in a pre-configured
   * {@link ExecutorService}. Alternatively {@link MultiBackgroundInitializer} can
   * create a temporary {@link ExecutorService} and delete it after initialization
   * is complete.</li>
   * <li>Create specialized {@link BackgroundInitializer} objects for the
   * initialization tasks to be performed and add them to the {@code
   * MultiBackgroundInitializer} using the
   * {@link #addInitializer(String, BackgroundInitializer)} method.</li>
   * <li>After all initializers have been added, call the {@link #start()} method.
   * </li>
   * <li>When access to the result objects produced by the {@code
   * BackgroundInitializer} objects is needed call the {@link #get()} method. The
   * object returned here provides access to all result objects created during
   * initialization. It also stores information about exceptions that have
   * occurred.</li>
   * </ul>
   * <p>
   * {@link MultiBackgroundInitializer} starts a special controller task that
   * starts all {@link BackgroundInitializer} objects added to the instance.
   * Before the an initializer is started it is checked whether this initializer
   * already has an {@link ExecutorService} set. If this is the case, this {@code
   * ExecutorService} is used for running the background task. Otherwise the
   * current {@link ExecutorService} of this {@link MultiBackgroundInitializer} is
   * shared with the initializer.
   * </p>
   * <p>
   * The easiest way of using this class is to let it deal with the management of
   * an {@link ExecutorService} itself: If no external {@link ExecutorService} is
   * provided, the class creates a temporary {@link ExecutorService} (that is
   * capable of executing all background tasks in parallel) and destroys it at the
   * end of background processing.
   * </p>
   * <p>
   * Alternatively an external {@link ExecutorService} can be provided - either at
   * construction time or later by calling the
   * {@link #setExternalExecutor(ExecutorService)} method. In this case all
   * background tasks are scheduled at this external {@link ExecutorService}.
   * <strong>Important note:</strong> When using an external {@code
   * ExecutorService} be sure that the number of threads managed by the service is
   * large enough. Otherwise a deadlock can happen! This is the case in the
   * following scenario: {@link MultiBackgroundInitializer} starts a task that
   * starts all registered {@link BackgroundInitializer} objects and waits for
   * their completion. If for instance a single threaded {@link ExecutorService}
   * is used, none of the background tasks can be executed, and the task created
   * by {@link MultiBackgroundInitializer} waits forever.
   * </p>
   *
   * @since 3.0
   */
  public class MultiBackgroundInitializer extends BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> {
  
      /**
       * A data class for storing the results of the background initialization
      * performed by {@link MultiBackgroundInitializer}. Objects of this inner
      * class are returned by {@link MultiBackgroundInitializer#initialize()}.
      * They allow access to all result objects produced by the
      * {@link BackgroundInitializer} objects managed by the owning instance. It
      * is also possible to retrieve status information about single
      * {@link BackgroundInitializer}s, i.e. whether they completed normally or
      * caused an exception.
      */
     public static class MultiBackgroundInitializerResults {
 
         /** A map with the child initializers. */
         private final Map<String, BackgroundInitializer<?>> initializers;
 
         /** A map with the result objects. */
         private final Map<String, Object> resultObjects;
 
         /** A map with the exceptions. */
         private final Map<String, ConcurrentException> exceptions;
 
         /**
          * Creates a new instance of {@link MultiBackgroundInitializerResults} and initializes it with maps for the {@link BackgroundInitializer} objects, their
          * result objects and the exceptions thrown by them.
          *
          * @param initializers  The {@link BackgroundInitializer} objects.
          * @param resultObjects The result objects.
          * @param exceptions    The exceptions.
          */
         private MultiBackgroundInitializerResults(final Map<String, BackgroundInitializer<?>> initializers, final Map<String, Object> resultObjects,
                 final Map<String, ConcurrentException> exceptions) {
             this.initializers = initializers;
             this.resultObjects = resultObjects;
             this.exceptions = exceptions;
         }
 
         /**
          * Checks whether an initializer with the given name exists. If not,
          * throws an exception. If it exists, the associated child initializer
          * is returned.
          *
          * @param name The name to check.
          * @return The initializer with this name.
          * @throws NoSuchElementException Thrown if the name is unknown.
          */
         private BackgroundInitializer<?> checkName(final String name) {
             final BackgroundInitializer<?> init = initializers.get(name);
             if (init == null) {
                 throw new NoSuchElementException("No child initializer with name " + name);
             }
             return init;
         }
 
         /**
          * Gets the {@link ConcurrentException} object that was thrown by the
          * {@link BackgroundInitializer} with the given name. If this
          * initializer did not throw an exception, the return value is
          * {@code null}. If the name cannot be resolved, an exception is thrown.
          *
          * @param name The name of the {@link BackgroundInitializer}.
          * @return The exception thrown by this initializer.
          * @throws NoSuchElementException Thrown if the name cannot be resolved.
          */
         public ConcurrentException getException(final String name) {
             checkName(name);
             return exceptions.get(name);
         }
 
         /**
          * Gets the {@link BackgroundInitializer} with the given name. If the
          * name cannot be resolved, an exception is thrown.
          *
          * @param name The name of the {@link BackgroundInitializer}.
          * @return The {@link BackgroundInitializer} with this name.
          * @throws NoSuchElementException Thrown if the name cannot be resolved.
          */
         public BackgroundInitializer<?> getInitializer(final String name) {
             return checkName(name);
         }
 
         /**
          * Gets the result object produced by the {@code
          * BackgroundInitializer} with the given name. This is the object returned by the initializer's {@code initialize()} method. If this
          * {@link BackgroundInitializer} caused an exception, {@code null} is returned. If the name cannot be resolved, an exception is thrown.
          *
          * @param name The name of the {@link BackgroundInitializer}.
          * @return The result object produced by this {@code BackgroundInitializer}.
          * @throws NoSuchElementException Thrown if the name cannot be resolved.
          */
         public Object getResultObject(final String name) {
             checkName(name);
             return resultObjects.get(name);
         }
 
         /**
          * Returns a set with the names of all {@link BackgroundInitializer} objects managed by the {@link MultiBackgroundInitializer}.
          *
          * @return An (unmodifiable) set with the names of the managed {@code BackgroundInitializer} objects.
          */
         public Set<String> initializerNames() {
             return Collections.unmodifiableSet(initializers.keySet());
         }
 
         /**
          * Tests whether the {@link BackgroundInitializer} with the
          * given name caused an exception.
          *
          * @param name The name of the {@link BackgroundInitializer}.
          * @return A flag whether this initializer caused an exception.
          * @throws NoSuchElementException Thrown if the name cannot be resolved.
          */
         public boolean isException(final String name) {
             checkName(name);
             return exceptions.containsKey(name);
         }
 
         /**
          * Tests whether the whole initialization was successful. This
          * is the case if no child initializer has thrown an exception.
          *
          * @return A flag whether the initialization was successful.
          */
         public boolean isSuccessful() {
             return exceptions.isEmpty();
         }
     }
 
     /** A map with the child initializers. */
     private final Map<String, BackgroundInitializer<?>> childInitializers = new HashMap<>();
 
     /**
      * Constructs a new instance of {@link MultiBackgroundInitializer}.
      */
     public MultiBackgroundInitializer() {
     }
 
     /**
      * Constructs a new instance of {@link MultiBackgroundInitializer} and
      * initializes it with the given external {@link ExecutorService}.
      *
      * @param exec The {@link ExecutorService} for executing the background tasks.
      */
     public MultiBackgroundInitializer(final ExecutorService exec) {
         super(exec);
     }
 
     /**
      * Adds a new {@link BackgroundInitializer} to this object. When this {@link MultiBackgroundInitializer} is started, the given initializer will be
      * processed. This method must not be called after {@link #start()} has been invoked.
      *
      * @param name                  The name of the initializer (must not be {@code null}).
      * @param backgroundInitializer The {@link BackgroundInitializer} to add (must not be {@code null}).
      * @throws NullPointerException  Thrown if either {@code name} or {@code backgroundInitializer} is {@code null}.
      * @throws IllegalStateException Thrown if {@code start()} has already been called.
      */
     public void addInitializer(final String name, final BackgroundInitializer<?> backgroundInitializer) {
         Objects.requireNonNull(name, "name");
         Objects.requireNonNull(backgroundInitializer, "backgroundInitializer");
         synchronized (this) {
             if (isStarted()) {
                 throw new IllegalStateException("addInitializer() must not be called after start().");
             }
             childInitializers.put(name, backgroundInitializer);
         }
     }
 
     /**
      * Calls the closer of all child {@code BackgroundInitializer} objects.
      *
      * @throws ConcurrentException Thrown with all other exceptions as suppressed exceptions. ConcurrentException instances thrown by children are unwrapped.
      * @since 3.14.0
      */
     @Override
     public void close() throws ConcurrentException {
         ConcurrentException exception = null;
         for (final BackgroundInitializer<?> child : childInitializers.values()) {
             try {
                 child.close();
             } catch (final Exception e) {
                 if (exception == null) {
                     exception = new ConcurrentException();
                 }
                 if (e instanceof ConcurrentException) {
                     // Because ConcurrentException is only created by classes in this package
                     // we can safely unwrap it.
                     exception.addSuppressed(e.getCause());
                 } else {
                     exception.addSuppressed(e);
                 }
             }
         }
         if (exception != null) {
             throw exception;
         }
     }
 
     /**
      * Gets the number of tasks needed for executing all child {@code
      * BackgroundInitializer} objects in parallel. This implementation sums up
      * the required tasks for all child initializers (which is necessary if one
      * of the child initializers is itself a {@link MultiBackgroundInitializer}
      * ). Then it adds 1 for the control task that waits for the completion of
      * the children.
      *
      * @return The number of tasks required for background processing.
      */
     @Override
     protected int getTaskCount() {
         return 1 + childInitializers.values().stream().mapToInt(BackgroundInitializer::getTaskCount).sum();
     }
 
     /**
      * Creates the results object. This implementation starts all child {@code
      * BackgroundInitializer} objects. Then it collects their results and
      * creates a {@link MultiBackgroundInitializerResults} object with this
      * data. If a child initializer throws a checked exceptions, it is added to
      * the results object. Unchecked exceptions are propagated.
      *
      * @return The results object.
      * @throws Exception Thrown if an error occurs.
      */
     @Override
     protected MultiBackgroundInitializerResults initialize() throws Exception {
         final Map<String, BackgroundInitializer<?>> inits;
         synchronized (this) {
             // create a snapshot to operate on
             inits = new HashMap<>(childInitializers);
         }
         // start the child initializers
         final ExecutorService exec = getActiveExecutor();
         inits.values().forEach(bi -> {
             if (bi.getExternalExecutor() == null) {
                 // share the executor service if necessary
                 bi.setExternalExecutor(exec);
             }
             bi.start();
         });
         // collect the results
         final Map<String, Object> results = new HashMap<>();
         final Map<String, ConcurrentException> excepts = new HashMap<>();
         inits.forEach((k, v) -> {
             try {
                 results.put(k, v.get());
             } catch (final ConcurrentException cex) {
                 excepts.put(k, cex);
             }
         });
         return new MultiBackgroundInitializerResults(inits, results, excepts);
     }
 
     /**
      * Tests whether this all child {@code BackgroundInitializer} objects are initialized. Once initialized, always returns true.
      *
      * @return Whether all child {@code BackgroundInitializer} objects instance are initialized. Once initialized, always returns true. If there are no child
      *         {@code BackgroundInitializer} objects return false.
      * @since 3.14.0
      */
     @Override
     public boolean isInitialized() {
         if (childInitializers.isEmpty()) {
             return false;
         }
         return childInitializers.values().stream().allMatch(BackgroundInitializer::isInitialized);
     }
 }