/*
    * ====================================================================
    * 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
   *
   *   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.
   * ====================================================================
   *
   * This software consists of voluntary contributions made by many
   * individuals on behalf of the Apache Software Foundation.  For more
   * information on the Apache Software Foundation, please see
   * <http://www.apache.org/>.
   *
   */
  package org.apache.hc.client5.http.impl.schedule;
  
  import org.apache.hc.client5.http.schedule.SchedulingStrategy;
  import org.apache.hc.core5.annotation.Contract;
  import org.apache.hc.core5.annotation.ThreadingBehavior;
  import org.apache.hc.core5.util.Args;
  import org.apache.hc.core5.util.TimeValue;
  
  /**
   * An implementation that backs off exponentially based on the number of
   * consecutive failed attempts. It uses the following defaults:
   * <pre>
   *      no delay in case it was never tried or didn't fail so far
   *     6 s delay for one failed attempt (= {@link #getInitialExpiry()})
   *    60 s delay for two failed attempts
   *  10 min delay for three failed attempts
   * 100 min delay for four failed attempts
   *   ~16 h delay for five failed attempts
   *    24 h delay for six or more failed attempts (= {@link #getMaxExpiry()})
   * </pre>
   *
   * The following equation is used to calculate the delay for a specific pending operation:
   * <pre>
   *     delay = {@link #getInitialExpiry()} * Math.pow({@link #getBackOffRate()},
   *     {@code consecutiveFailedAttempts} - 1))
   * </pre>
   * The resulting delay won't exceed {@link #getMaxExpiry()}.
   *
   * @since 5.0
   */
  @Contract(threading = ThreadingBehavior.STATELESS)
  public class ExponentialBackOffSchedulingStrategy implements SchedulingStrategy {
  
      public static final long DEFAULT_BACK_OFF_RATE = 10;
      public static final TimeValue DEFAULT_INITIAL_EXPIRY = TimeValue.ofSeconds(6);
      public static final TimeValue DEFAULT_MAX_EXPIRY = TimeValue.ofSeconds(86400);
  
      private static final ExponentialBackOffSchedulingStrategy INSTANCE = new ExponentialBackOffSchedulingStrategy(
              DEFAULT_BACK_OFF_RATE, DEFAULT_INITIAL_EXPIRY, DEFAULT_MAX_EXPIRY);
  
      private final long backOffRate;
      private final TimeValue initialExpiry;
      private final TimeValue maxExpiry;
  
      public ExponentialBackOffSchedulingStrategy(
              final long backOffRate,
              final TimeValue initialExpiry,
              final TimeValue maxExpiry) {
          this.backOffRate = Args.notNegative(backOffRate, "BackOff rate");
          this.initialExpiry = Args.notNull(initialExpiry, "Initial expiry");
          this.maxExpiry = Args.notNull(maxExpiry, "Max expiry");
      }
  
      public ExponentialBackOffSchedulingStrategy(final long backOffRate, final TimeValue initialExpiry) {
          this(backOffRate, initialExpiry, DEFAULT_MAX_EXPIRY);
      }
  
      public ExponentialBackOffSchedulingStrategy(final long backOffRate) {
          this(backOffRate, DEFAULT_INITIAL_EXPIRY, DEFAULT_MAX_EXPIRY);
      }
  
      public ExponentialBackOffSchedulingStrategy() {
          this(DEFAULT_BACK_OFF_RATE, DEFAULT_INITIAL_EXPIRY, DEFAULT_MAX_EXPIRY);
      }
  
      @Override
      public TimeValue schedule(final int attemptNumber) {
          return calculateDelay(attemptNumber);
      }
  
      public long getBackOffRate() {
          return backOffRate;
      }
 
     public TimeValue getInitialExpiry() {
         return initialExpiry;
     }
 
     public TimeValue getMaxExpiry() {
         return maxExpiry;
     }
 
     protected TimeValue calculateDelay(final int consecutiveFailedAttempts) {
         if (consecutiveFailedAttempts > 0) {
             final long delay = (long) (initialExpiry.toMilliseconds() * Math.pow(backOffRate, consecutiveFailedAttempts - 1));
             return TimeValue.ofMilliseconds(Math.min(delay, maxExpiry.toMilliseconds()));
         }
         return TimeValue.ZERO_MILLISECONDS;
     }
 
 }