/*
 * Copyright (C) 2012 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.util.concurrent;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static java.lang.Math.max;
import static java.util.concurrent.TimeUnit.MICROSECONDS;
import static java.util.concurrent.TimeUnit.SECONDS;

import com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Stopwatch;
import com.google.common.util.concurrent.SmoothRateLimiter.SmoothBursty;
import com.google.common.util.concurrent.SmoothRateLimiter.SmoothWarmingUp;

import java.util.concurrent.TimeUnit;

import javax.annotation.concurrent.ThreadSafe;

/**
 * A rate limiter. Conceptually, a rate limiter distributes permits at a
 * configurable rate. Each {@link #acquire()} blocks if necessary until a permit is
 * available, and then takes it. Once acquired, permits need not be released.
 *
 * <p>Rate limiters are often used to restrict the rate at which some
 * physical or logical resource is accessed. This is in contrast to {@link
 * java.util.concurrent.Semaphore} which restricts the number of concurrent
 * accesses instead of the rate (note though that concurrency and rate are closely related,
 * e.g. see <a href="http://en.wikipedia.org/wiki/Little's_law">Little's Law</a>).
 *
 * <p>A {@code RateLimiter} is defined primarily by the rate at which permits
 * are issued. Absent additional configuration, permits will be distributed at a
 * fixed rate, defined in terms of permits per second. Permits will be distributed
 * smoothly, with the delay between individual permits being adjusted to ensure
 * that the configured rate is maintained.
 *
 * <p>It is possible to configure a {@code RateLimiter} to have a warmup
 * period during which time the permits issued each second steadily increases until
 * it hits the stable rate.
 *
 * <p>As an example, imagine that we have a list of tasks to execute, but we don't want to
 * submit more than 2 per second:
 *<pre>  {@code
 *  final RateLimiter rateLimiter = RateLimiter.create(2.0); // rate is "2 permits per second"
 *  void submitTasks(List<Runnable> tasks, Executor executor) {
 *    for (Runnable task : tasks) {
 *      rateLimiter.acquire(); // may wait
 *      executor.execute(task);
 *    }
 *  }
 *}</pre>
 *
 * <p>As another example, imagine that we produce a stream of data, and we want to cap it
 * at 5kb per second. This could be accomplished by requiring a permit per byte, and specifying
 * a rate of 5000 permits per second:
 *<pre>  {@code
 *  final RateLimiter rateLimiter = RateLimiter.create(5000.0); // rate = 5000 permits per second
 *  void submitPacket(byte[] packet) {
 *    rateLimiter.acquire(packet.length);
 *    networkService.send(packet);
 *  }
 *}</pre>
 *
 * <p>It is important to note that the number of permits requested <i>never</i>
 * affect the throttling of the request itself (an invocation to {@code acquire(1)}
 * and an invocation to {@code acquire(1000)} will result in exactly the same throttling, if any),
 * but it affects the throttling of the <i>next</i> request. I.e., if an expensive task
 * arrives at an idle RateLimiter, it will be granted immediately, but it is the <i>next</i>
 * request that will experience extra throttling, thus paying for the cost of the expensive
 * task.
 *
 * <p>Note: {@code RateLimiter} does not provide fairness guarantees.
 *
 * @author Dimitris Andreou
 * @since 13.0
 */
// TODO(user): switch to nano precision. A natural unit of cost is "bytes", and a micro precision
//     would mean a maximum rate of "1MB/s", which might be small in some cases.
@ThreadSafe
@Beta
public abstract class RateLimiter {
  /**
   * Creates a {@code RateLimiter} with the specified stable throughput, given as
   * "permits per second" (commonly referred to as <i>QPS</i>, queries per second).
   *
   * <p>The returned {@code RateLimiter} ensures that on average no more than {@code
   * permitsPerSecond} are issued during any given second, with sustained requests
   * being smoothly spread over each second. When the incoming request rate exceeds
   * {@code permitsPerSecond} the rate limiter will release one permit every {@code
   * (1.0 / permitsPerSecond)} seconds. When the rate limiter is unused,
   * bursts of up to {@code permitsPerSecond} permits will be allowed, with subsequent
   * requests being smoothly limited at the stable rate of {@code permitsPerSecond}.
   *
   * @param permitsPerSecond the rate of the returned {@code RateLimiter}, measured in
   *        how many permits become available per second
   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero
   */
  // TODO(user): "This is equivalent to
  //                 {@code createWithCapacity(permitsPerSecond, 1, TimeUnit.SECONDS)}".
  public static RateLimiter create(double permitsPerSecond) {
    /*
     * The default RateLimiter configuration can save the unused permits of up to one second.
     * This is to avoid unnecessary stalls in situations like this: A RateLimiter of 1qps,
     * and 4 threads, all calling acquire() at these moments:
     *
     * T0 at 0 seconds
     * T1 at 1.05 seconds
     * T2 at 2 seconds
     * T3 at 3 seconds
     *
     * Due to the slight delay of T1, T2 would have to sleep till 2.05 seconds,
     * and T3 would also have to sleep till 3.05 seconds.
     */
    return create(SleepingStopwatch.createFromSystemTimer(), permitsPerSecond);
  }

  /*
   * TODO(cpovirk): make SleepingStopwatch the last parameter throughout the class so that the
   * overloads follow the usual convention: Foo(int), Foo(int, SleepingStopwatch)
   */
  @VisibleForTesting
  static RateLimiter create(SleepingStopwatch stopwatch, double permitsPerSecond) {
    RateLimiter rateLimiter = new SmoothBursty(stopwatch, 1.0 /* maxBurstSeconds */);
    rateLimiter.setRate(permitsPerSecond);
    return rateLimiter;
  }

  /**
   * Creates a {@code RateLimiter} with the specified stable throughput, given as
   * "permits per second" (commonly referred to as <i>QPS</i>, queries per second), and a
   * <i>warmup period</i>, during which the {@code RateLimiter} smoothly ramps up its rate,
   * until it reaches its maximum rate at the end of the period (as long as there are enough
   * requests to saturate it). Similarly, if the {@code RateLimiter} is left <i>unused</i> for
   * a duration of {@code warmupPeriod}, it will gradually return to its "cold" state,
   * i.e. it will go through the same warming up process as when it was first created.
   *
   * <p>The returned {@code RateLimiter} is intended for cases where the resource that actually
   * fulfills the requests (e.g., a remote server) needs "warmup" time, rather than
   * being immediately accessed at the stable (maximum) rate.
   *
   * <p>The returned {@code RateLimiter} starts in a "cold" state (i.e. the warmup period
   * will follow), and if it is left unused for long enough, it will return to that state.
   *
   * @param permitsPerSecond the rate of the returned {@code RateLimiter}, measured in
   *        how many permits become available per second
   * @param warmupPeriod the duration of the period where the {@code RateLimiter} ramps up its
   *        rate, before reaching its stable (maximum) rate
   * @param unit the time unit of the warmupPeriod argument
   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero or
   *     {@code warmupPeriod} is negative
   */
  public static RateLimiter create(double permitsPerSecond, long warmupPeriod, TimeUnit unit) {
    checkArgument(warmupPeriod >= 0, "warmupPeriod must not be negative: %s", warmupPeriod);
    return create(SleepingStopwatch.createFromSystemTimer(), permitsPerSecond, warmupPeriod, unit);
  }

  @VisibleForTesting
  static RateLimiter create(
      SleepingStopwatch stopwatch, double permitsPerSecond, long warmupPeriod, TimeUnit unit) {
    RateLimiter rateLimiter = new SmoothWarmingUp(stopwatch, warmupPeriod, unit);
    rateLimiter.setRate(permitsPerSecond);
    return rateLimiter;
  }

  /**
   * The underlying timer; used both to measure elapsed time and sleep as necessary. A separate
   * object to facilitate testing.
   */
  private final SleepingStopwatch stopwatch;

  // Can't be initialized in the constructor because mocks don't call the constructor.
  private volatile Object mutexDoNotUseDirectly;

  private Object mutex() {
    Object mutex = mutexDoNotUseDirectly;
    if (mutex == null) {
      synchronized (this) {
        mutex = mutexDoNotUseDirectly;
        if (mutex == null) {
          mutexDoNotUseDirectly = mutex = new Object();
        }
      }
    }
    return mutex;
  }

  RateLimiter(SleepingStopwatch stopwatch) {
    this.stopwatch = checkNotNull(stopwatch);
  }

  /**
   * Updates the stable rate of this {@code RateLimiter}, that is, the
   * {@code permitsPerSecond} argument provided in the factory method that
   * constructed the {@code RateLimiter}. Currently throttled threads will <b>not</b>
   * be awakened as a result of this invocation, thus they do not observe the new rate;
   * only subsequent requests will.
   *
   * <p>Note though that, since each request repays (by waiting, if necessary) the cost
   * of the <i>previous</i> request, this means that the very next request
   * after an invocation to {@code setRate} will not be affected by the new rate;
   * it will pay the cost of the previous request, which is in terms of the previous rate.
   *
   * <p>The behavior of the {@code RateLimiter} is not modified in any other way,
   * e.g. if the {@code RateLimiter} was configured with a warmup period of 20 seconds,
   * it still has a warmup period of 20 seconds after this method invocation.
   *
   * @param permitsPerSecond the new stable rate of this {@code RateLimiter}
   * @throws IllegalArgumentException if {@code permitsPerSecond} is negative or zero
   */
  public final void setRate(double permitsPerSecond) {
    checkArgument(
        permitsPerSecond > 0.0 && !Double.isNaN(permitsPerSecond), "rate must be positive");
    synchronized (mutex()) {
      doSetRate(permitsPerSecond, stopwatch.readMicros());
    }
  }

  abstract void doSetRate(double permitsPerSecond, long nowMicros);

  /**
   * Returns the stable rate (as {@code permits per seconds}) with which this
   * {@code RateLimiter} is configured with. The initial value of this is the same as
   * the {@code permitsPerSecond} argument passed in the factory method that produced
   * this {@code RateLimiter}, and it is only updated after invocations
   * to {@linkplain #setRate}.
   */
  public final double getRate() {
    synchronized (mutex()) {
      return doGetRate();
    }
  }

  abstract double doGetRate();

  /**
   * Acquires a single permit from this {@code RateLimiter}, blocking until the
   * request can be granted. Tells the amount of time slept, if any.
   *
   * <p>This method is equivalent to {@code acquire(1)}.
   *
   * @return time spent sleeping to enforce rate, in seconds; 0.0 if not rate-limited
   * @since 16.0 (present in 13.0 with {@code void} return type})
   */
  public double acquire() {
    return acquire(1);
  }

  /**
   * Acquires the given number of permits from this {@code RateLimiter}, blocking until the
   * request can be granted. Tells the amount of time slept, if any.
   *
   * @param permits the number of permits to acquire
   * @return time spent sleeping to enforce rate, in seconds; 0.0 if not rate-limited
   * @throws IllegalArgumentException if the requested number of permits is negative or zero
   * @since 16.0 (present in 13.0 with {@code void} return type})
   */
  public double acquire(int permits) {
    long microsToWait = reserve(permits);
    stopwatch.sleepMicrosUninterruptibly(microsToWait);
    return 1.0 * microsToWait / SECONDS.toMicros(1L);
  }

  /**
   * Reserves the given number of permits from this {@code RateLimiter} for future use, returning
   * the number of microseconds until the reservation can be consumed.
   *
   * @return time in microseconds to wait until the resource can be acquired, never negative
   */
  final long reserve(int permits) {
    checkPermits(permits);
    synchronized (mutex()) {
      return reserveAndGetWaitLength(permits, stopwatch.readMicros());
    }
  }

  /**
   * Acquires a permit from this {@code RateLimiter} if it can be obtained
   * without exceeding the specified {@code timeout}, or returns {@code false}
   * immediately (without waiting) if the permit would not have been granted
   * before the timeout expired.
   *
   * <p>This method is equivalent to {@code tryAcquire(1, timeout, unit)}.
   *
   * @param timeout the maximum time to wait for the permit. Negative values are treated as zero.
   * @param unit the time unit of the timeout argument
   * @return {@code true} if the permit was acquired, {@code false} otherwise
   * @throws IllegalArgumentException if the requested number of permits is negative or zero
   */
  public boolean tryAcquire(long timeout, TimeUnit unit) {
    return tryAcquire(1, timeout, unit);
  }

  /**
   * Acquires permits from this {@link RateLimiter} if it can be acquired immediately without delay.
   *
   * <p>
   * This method is equivalent to {@code tryAcquire(permits, 0, anyUnit)}.
   *
   * @param permits the number of permits to acquire
   * @return {@code true} if the permits were acquired, {@code false} otherwise
   * @throws IllegalArgumentException if the requested number of permits is negative or zero
   * @since 14.0
   */
  public boolean tryAcquire(int permits) {
    return tryAcquire(permits, 0, MICROSECONDS);
  }

  /**
   * Acquires a permit from this {@link RateLimiter} if it can be acquired immediately without
   * delay.
   *
   * <p>
   * This method is equivalent to {@code tryAcquire(1)}.
   *
   * @return {@code true} if the permit was acquired, {@code false} otherwise
   * @since 14.0
   */
  public boolean tryAcquire() {
    return tryAcquire(1, 0, MICROSECONDS);
  }

  /**
   * Acquires the given number of permits from this {@code RateLimiter} if it can be obtained
   * without exceeding the specified {@code timeout}, or returns {@code false}
   * immediately (without waiting) if the permits would not have been granted
   * before the timeout expired.
   *
   * @param permits the number of permits to acquire
   * @param timeout the maximum time to wait for the permits. Negative values are treated as zero.
   * @param unit the time unit of the timeout argument
   * @return {@code true} if the permits were acquired, {@code false} otherwise
   * @throws IllegalArgumentException if the requested number of permits is negative or zero
   */
  public boolean tryAcquire(int permits, long timeout, TimeUnit unit) {
    long timeoutMicros = max(unit.toMicros(timeout), 0);
    checkPermits(permits);
    long microsToWait;
    synchronized (mutex()) {
      long nowMicros = stopwatch.readMicros();
      if (!canAcquire(nowMicros, timeoutMicros)) {
        return false;
      } else {
        microsToWait = reserveAndGetWaitLength(permits, nowMicros);
      }
    }
    stopwatch.sleepMicrosUninterruptibly(microsToWait);
    return true;
  }

  private boolean canAcquire(long nowMicros, long timeoutMicros) {
    return queryEarliestAvailable(nowMicros) - timeoutMicros <= nowMicros;
  }

  /**
   * Reserves next ticket and returns the wait time that the caller must wait for.
   *
   * @return the required wait time, never negative
   */
  final long reserveAndGetWaitLength(int permits, long nowMicros) {
    long momentAvailable = reserveEarliestAvailable(permits, nowMicros);
    return max(momentAvailable - nowMicros, 0);
  }

  /**
   * Returns the earliest time that permits are available (with one caveat).
   *
   * @return the time that permits are available, or, if permits are available immediately, an
   *     arbitrary past or present time
   */
  abstract long queryEarliestAvailable(long nowMicros);

    /**
   * Reserves the requested number of permits and returns the time that those permits can be used
   * (with one caveat).
     *
   * @return the time that the permits may be used, or, if the permits may be used immediately, an
   *     arbitrary past or present time
     */
  abstract long reserveEarliestAvailable(int permits, long nowMicros);

  @Override
  public String toString() {
    return String.format("RateLimiter[stableRate=%3.1fqps]", getRate());
  }

  @VisibleForTesting
  abstract static class SleepingStopwatch {
    /*
     * We always hold the mutex when calling this. TODO(cpovirk): Is that important? Perhaps we need
     * to guarantee that each call to reserveEarliestAvailable, etc. sees a value >= the previous?
     * Also, is it OK that we don't hold the mutex when sleeping?
     */
    abstract long readMicros();

    abstract void sleepMicrosUninterruptibly(long micros);

    static final SleepingStopwatch createFromSystemTimer() {
      return new SleepingStopwatch() {
        final Stopwatch stopwatch = Stopwatch.createStarted();

        @Override
        long readMicros() {
          return stopwatch.elapsed(MICROSECONDS);
        }

        @Override
        void sleepMicrosUninterruptibly(long micros) {
          if (micros > 0) {
            Uninterruptibles.sleepUninterruptibly(micros, MICROSECONDS);
          }
        }
      };
    }
  }

  private static int checkPermits(int permits) {
    checkArgument(permits > 0, "Requested permits (%s) must be positive", permits);
    return permits;
  }
}
