/*
    * 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.concurrent.ScheduledExecutorService;
  import java.util.concurrent.ScheduledFuture;
  import java.util.concurrent.ScheduledThreadPoolExecutor;
  import java.util.concurrent.TimeUnit;
  import java.util.function.Supplier;
  
  import org.apache.commons.lang3.Validate;
  
  /**
   * A specialized <em>semaphore</em> implementation that provides a number of permits in a given time frame.
   *
   * <p>
   * This class is similar to the {@link java.util.concurrent.Semaphore} class provided by the JDK in that it manages a configurable number of permits. Using the
   * {@link #acquire()} method a permit can be requested by a thread. However, there is an additional timing dimension: there is no {@code
   * release()} method for freeing a permit, but all permits are automatically released at the end of a configurable time frame. If a thread calls
   * {@link #acquire()} and the available permits are already exhausted for this time frame, the thread is blocked. When the time frame ends all permits requested
   * so far are restored, and blocking threads are waked up again, so that they can try to acquire a new permit. This basically means that in the specified time
   * frame only the given number of operations is possible.
   * </p>
   * <p>
   * A use case for this class is to artificially limit the load produced by a process. As an example consider an application that issues database queries on a
   * production system in a background process to gather statistical information. This background processing should not produce so much database load that the
   * functionality and the performance of the production system are impacted. Here a {@link TimedSemaphore} could be installed to guarantee that only a given
   * number of database queries are issued per second.
   * </p>
   * <p>
   * A thread class for performing database queries could look as follows:
   * </p>
   *
   * <pre>
   * public class StatisticsThread extends Thread {
   *     // The semaphore for limiting database load.
   *     private final TimedSemaphore semaphore;
   *     // Create an instance and set the semaphore
   *     public StatisticsThread(TimedSemaphore timedSemaphore) {
   *         semaphore = timedSemaphore;
   *     }
   *     // Gather statistics
   *     public void run() {
   *         try {
   *             while (true) {
   *                 semaphore.acquire();   // limit database load
   *                 performQuery();        // issue a query
   *             }
   *         } catch (InterruptedException) {
   *             // fall through
   *         }
   *     }
   *     ...
   * }
   * </pre>
   *
   * <p>
   * The following code fragment shows how a {@link TimedSemaphore} is created that allows only 10 operations per second and passed to the statistics thread:
   * </p>
   *
   * <pre>
   * TimedSemaphore sem = new TimedSemaphore(1, TimeUnit.SECOND, 10);
   * StatisticsThread thread = new StatisticsThread(sem);
   * thread.start();
   * </pre>
   *
   * <p>
   * When creating an instance the time period for the semaphore must be specified. {@link TimedSemaphore} uses an executor service with a corresponding period to
   * monitor this interval. The {@code
   * ScheduledExecutorService} to be used for this purpose can be provided at construction time. Alternatively the class creates an internal executor service.
   * </p>
   * <p>
   * Client code that uses {@link TimedSemaphore} has to call the {@link #acquire()} method in each processing step. {@link TimedSemaphore} keeps track of the
   * number of invocations of the {@link #acquire()} method and blocks the calling thread if the counter exceeds the limit specified. When the timer signals the
   * end of the time period the counter is reset and all waiting threads are released. Then another cycle can start.
   * </p>
   * <p>
   * An alternative to {@code acquire()} is the {@link #tryAcquire()} method. This method checks whether the semaphore is under the specified limit and increases
   * the internal counter if this is the case. The return value is then <strong>true</strong>, and the calling thread can continue with its action. If the
   * semaphore is already at its limit, {@code tryAcquire()} immediately returns <strong>false</strong> without blocking; the calling thread must then abort its
   * action. This usage scenario prevents blocking of threads.
   * </p>
   * <p>
   * It is possible to modify the limit at any time using the {@link #setLimit(int)} method. This is useful if the load produced by an operation has to be adapted
  * dynamically. In the example scenario with the thread collecting statistics it may make sense to specify a low limit during day time while allowing a higher
  * load in the night time. Reducing the limit takes effect immediately by blocking incoming callers. If the limit is increased, waiting threads are not released
  * immediately, but wake up when the timer runs out. Then, in the next period more processing steps can be performed without blocking. By setting the limit to 0
  * the semaphore can be switched off: in this mode the {@link #acquire()} method never blocks, but lets all callers pass directly.
  * </p>
  * <p>
  * When the {@link TimedSemaphore} is no more needed its {@link #shutdown()} method should be called. This causes the periodic task that monitors the time
  * interval to be canceled. If the {@link ScheduledExecutorService} has been created by the semaphore at construction time, it is also shut down. resources.
  * After that {@link #acquire()} must not be called any more.
  * </p>
  *
  * @since 3.0
  */
 public class TimedSemaphore {
 
     /**
      * Builds new {@link TimedSemaphore}.
      *
      * @since 3.20.0
      */
     public static class Builder implements Supplier<TimedSemaphore> {
 
         private ScheduledExecutorService service;
         private long period;
         private TimeUnit timeUnit;
         private int limit;
 
         /**
          * Constructs a new Builder.
          */
         public Builder() {
             // empty
         }
 
         @Override
         public TimedSemaphore get() {
             return new TimedSemaphore(this);
         }
 
         /**
          * Sets the limit.
          *
          * @param limit The limit.
          * @return {@code this} instance.
          */
         public Builder setLimit(final int limit) {
             this.limit = limit;
             return this;
         }
 
         /**
          * Sets the time period.
          *
          * @param period The time period.
          * @return {@code this} instance.
          */
         public Builder setPeriod(final long period) {
             this.period = period;
             return this;
         }
 
         /**
          * Sets the executor service.
          *
          * @param service The executor service.
          * @return {@code this} instance.
          */
         public Builder setService(final ScheduledExecutorService service) {
             this.service = service;
             return this;
         }
 
         /**
          * Sets the time unit for the period.
          *
          * @param timeUnit The time unit for the period.
          * @return {@code this} instance.
          */
         public Builder setTimeUnit(final TimeUnit timeUnit) {
             this.timeUnit = timeUnit;
             return this;
         }
     }
 
     /**
      * Constant for a value representing no limit. If the limit is set to a value less or equal this constant, the {@link TimedSemaphore} will be effectively
      * switched off.
      */
     public static final int NO_LIMIT = 0;
 
     /** Constant for the thread pool size for the executor. */
     private static final int THREAD_POOL_SIZE = 1;
 
     /**
      * Constructs a new Builder.
      *
      * @return a new Builder.
      * @since 3.20.0
      */
     public static Builder builder() {
         return new Builder();
     }
 
     /** The executor service for managing the timer thread. */
     private final ScheduledExecutorService executorService;
 
     /** The period for this timed semaphore. */
     private final long period;
 
     /** The time unit for the period. */
     private final TimeUnit unit;
 
     /** A flag whether the executor service was created by this object. */
     private final boolean ownExecutor;
 
     /** A future object representing the timer task. */
     private ScheduledFuture<?> task; // @GuardedBy("this")
 
     /** Stores the total number of invocations of the acquire() method. */
     private long totalAcquireCount; // @GuardedBy("this")
 
     /**
      * The counter for the periods. This counter is increased every time a period ends.
      */
     private long periodCount; // @GuardedBy("this")
 
     /** The limit. */
     private int limit; // @GuardedBy("this")
 
     /** The current counter. */
     private int acquireCount; // @GuardedBy("this")
 
     /** The number of invocations of acquire() in the last period. */
     private int lastCallsPerPeriod; // @GuardedBy("this")
 
     /** A flag whether shutdown() was called. */
     private boolean shutdown; // @GuardedBy("this")
 
     private TimedSemaphore(final Builder builder) {
         Validate.inclusiveBetween(1, Long.MAX_VALUE, builder.period, "Time period must be greater than 0.");
         period = builder.period;
         unit = builder.timeUnit;
         if (builder.service != null) {
             executorService = builder.service;
             ownExecutor = false;
         } else {
             final ScheduledThreadPoolExecutor stpe = new ScheduledThreadPoolExecutor(THREAD_POOL_SIZE);
             stpe.setContinueExistingPeriodicTasksAfterShutdownPolicy(false);
             stpe.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
             executorService = stpe;
             ownExecutor = true;
         }
         setLimit(builder.limit);
     }
 
     /**
      * Constructs a new instance of {@link TimedSemaphore} and initializes it with the given time period and the limit.
      *
      * @param timePeriod the time period.
      * @param timeUnit   the unit for the period.
      * @param limit      the limit for the semaphore.
      * @throws IllegalArgumentException if the period is less or equals 0.
      * @deprecated Use {@link #builder()} and {@link Builder}.
      */
     @Deprecated
     public TimedSemaphore(final long timePeriod, final TimeUnit timeUnit, final int limit) {
         this(null, timePeriod, timeUnit, limit);
     }
 
     /**
      * Constructs a new instance of {@link TimedSemaphore} and initializes it with an executor service, the given time period, and the limit. The executor service
      * will be used for creating a periodic task for monitoring the time period. It can be <strong>null</strong>, then a default service will be created.
      *
      * @param service    the executor service.
      * @param timePeriod the time period.
      * @param timeUnit   the unit for the period.
      * @param limit      the limit for the semaphore.
      * @throws IllegalArgumentException if the period is less or equals 0.
      * @deprecated Use {@link #builder()} and {@link Builder}.
      */
     @Deprecated
     public TimedSemaphore(final ScheduledExecutorService service, final long timePeriod, final TimeUnit timeUnit, final int limit) {
         this(builder().setService(service).setPeriod(timePeriod).setTimeUnit(timeUnit).setLimit(limit));
     }
 
     /**
      * Acquires a permit from this semaphore. This method will block if the limit for the current period has already been reached. If {@link #shutdown()} has
      * already been invoked, calling this method will cause an exception. The very first call of this method starts the timer task which monitors the time
      * period set for this {@link TimedSemaphore}. From now on the semaphore is active.
      *
      * @throws InterruptedException  if the thread gets interrupted.
      * @throws IllegalStateException if this semaphore is already shut down.
      */
     public synchronized void acquire() throws InterruptedException {
         prepareAcquire();
         boolean canPass;
         do {
             canPass = acquirePermit();
             if (!canPass) {
                 wait();
             }
         } while (!canPass);
     }
 
     /**
      * Internal helper method for acquiring a permit. This method checks whether currently a permit can be acquired and - if so - increases the internal
      * counter. The return value indicates whether a permit could be acquired. This method must be called with the lock of this object held.
      *
      * @return a flag whether a permit could be acquired.
      */
     private boolean acquirePermit() {
         if (getLimit() <= NO_LIMIT || acquireCount < getLimit()) {
             acquireCount++;
             return true;
         }
         return false;
     }
 
     /**
      * The current time period is finished. This method is called by the timer used internally to monitor the time period. It resets the counter and releases
      * the threads waiting for this barrier.
      */
     synchronized void endOfPeriod() {
         lastCallsPerPeriod = acquireCount;
         totalAcquireCount += acquireCount;
         periodCount++;
         acquireCount = 0;
         notifyAll();
     }
 
     /**
      * Gets the number of invocations of the {@link #acquire()} method for the current period. This may be useful for testing or debugging purposes.
      *
      * @return the current number of {@link #acquire()} invocations.
      */
     public synchronized int getAcquireCount() {
         return acquireCount;
     }
 
     /**
      * Gets the number of calls to the {@link #acquire()} method that can still be performed in the current period without blocking. This method can give an
      * indication whether it is safe to call the {@link #acquire()} method without risking to be suspended. However, there is no guarantee that a subsequent
      * call to {@link #acquire()} actually is not-blocking because in the meantime other threads may have invoked the semaphore.
      *
      * @return the current number of available {@link #acquire()} calls in the current period.
      */
     public synchronized int getAvailablePermits() {
         return getLimit() - getAcquireCount();
     }
 
     /**
      * Gets the average number of successful (i.e. non-blocking) {@link #acquire()} invocations for the entire life-time of this {@code
      * TimedSemaphore}. This method can be used for instance for statistical calculations.
      *
      * @return the average number of {@link #acquire()} invocations per time unit.
      */
     public synchronized double getAverageCallsPerPeriod() {
         return periodCount == 0 ? 0 : (double) totalAcquireCount / (double) periodCount;
     }
 
     /**
      * Gets the executor service used by this instance.
      *
      * @return the executor service.
      */
     protected ScheduledExecutorService getExecutorService() {
         return executorService;
     }
 
     /**
      * Gets the number of (successful) acquire invocations during the last period. This is the number of times the {@link #acquire()} method was called without
      * blocking. This can be useful for testing or debugging purposes or to determine a meaningful threshold value. If a limit is set, the value returned by
      * this method won't be greater than this limit.
      *
      * @return the number of non-blocking invocations of the {@link #acquire()} method.
      */
     public synchronized int getLastAcquiresPerPeriod() {
         return lastCallsPerPeriod;
     }
 
     /**
      * Gets the limit enforced by this semaphore. The limit determines how many invocations of {@link #acquire()} are allowed within the monitored period.
      *
      * @return the limit.
      */
     public final synchronized int getLimit() {
         return limit;
     }
 
     /**
      * Gets the time period. This is the time monitored by this semaphore. Only a given number of invocations of the {@link #acquire()} method is possible in
      * this period.
      *
      * @return the time period.
      */
     public long getPeriod() {
         return period;
     }
 
     /**
      * Gets the time unit. This is the unit used by {@link #getPeriod()}.
      *
      * @return the time unit.
      */
     public TimeUnit getUnit() {
         return unit;
     }
 
     /**
      * Tests whether the {@link #shutdown()} method has been called on this object. If this method returns <strong>true</strong>, this instance cannot be used
      * any longer.
      *
      * @return a flag whether a shutdown has been performed.
      */
     public synchronized boolean isShutdown() {
         return shutdown;
     }
 
     /**
      * Prepares an acquire operation. Checks for the current state and starts the internal timer if necessary. This method must be called with the lock of this
      * object held.
      */
     private void prepareAcquire() {
         if (isShutdown()) {
             throw new IllegalStateException("TimedSemaphore is shut down!");
         }
         if (task == null) {
             task = startTimer();
         }
     }
 
     /**
      * Sets the limit. This is the number of times the {@link #acquire()} method can be called within the time period specified. If this limit is reached,
      * further invocations of {@link #acquire()} will block. Setting the limit to a value &lt;= {@link #NO_LIMIT} will cause the limit to be disabled, i.e. an
      * arbitrary number of{@link #acquire()} invocations is allowed in the time period.
      *
      * @param limit the limit.
      */
     public final synchronized void setLimit(final int limit) {
         this.limit = limit;
     }
 
     /**
      * Initializes a shutdown. After that the object cannot be used anymore. This method can be invoked an arbitrary number of times. All invocations after the
      * first one do not have any effect.
      */
     public synchronized void shutdown() {
         if (!shutdown) {
             if (ownExecutor) {
                 // if the executor was created by this instance, it has
                 // to be shutdown
                 getExecutorService().shutdownNow();
             }
             if (task != null) {
                 task.cancel(false);
             }
             shutdown = true;
         }
     }
 
     /**
      * Starts the timer. This method is called when {@link #acquire()} is called for the first time. It schedules a task to be executed at fixed rate to monitor
      * the time period specified.
      *
      * @return a future object representing the task scheduled.
      */
     protected ScheduledFuture<?> startTimer() {
         return getExecutorService().scheduleAtFixedRate(this::endOfPeriod, getPeriod(), getPeriod(), getUnit());
     }
 
     /**
      * Tries to acquire a permit from this semaphore. If the limit of this semaphore has not yet been reached, a permit is acquired, and this method returns
      * <strong>true</strong>. Otherwise, this method returns immediately with the result <strong>false</strong>.
      *
      * @return <strong>true</strong> if a permit could be acquired; <strong>false</strong> otherwise.
      * @throws IllegalStateException if this semaphore is already shut down.
      * @since 3.5
      */
     public synchronized boolean tryAcquire() {
         prepareAcquire();
         return acquirePermit();
     }
 }