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/*
 * Copyright DataStax, Inc.
 *
 * 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.datastax.driver.core.policies;

import static com.datastax.driver.$internal.com.google.common.base.Preconditions.checkArgument;

import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Host;
import com.datastax.driver.core.LatencyTracker;
import com.datastax.driver.core.PercentileTracker;
import com.datastax.driver.core.Statement;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * A policy that triggers speculative executions when the request to the current host is above a
 * given percentile.
 */
public class PercentileSpeculativeExecutionPolicy implements SpeculativeExecutionPolicy {
  private final PercentileTracker percentileTracker;
  private final double percentile;
  private final int maxSpeculativeExecutions;

  /**
   * Builds a new instance.
   *
   * @param percentileTracker the component that will record latencies. It will get {@link
   *     Cluster#register(LatencyTracker) registered} with the cluster when this policy initializes.
   * @param percentile the percentile that a request's latency must fall into to be considered slow
   *     (ex: {@code 99.0}).
   * @param maxSpeculativeExecutions the maximum number of speculative executions that will be
   *     triggered for a given request (this does not include the initial, normal request). Must be
   *     strictly positive.
   */
  public PercentileSpeculativeExecutionPolicy(
      PercentileTracker percentileTracker, double percentile, int maxSpeculativeExecutions) {
    checkArgument(
        maxSpeculativeExecutions > 0,
        "number of speculative executions must be strictly positive (was %d)",
        maxSpeculativeExecutions);
    checkArgument(
        percentile >= 0.0 && percentile < 100, "percentile must be between 0.0 and 100 (was %f)");

    this.percentileTracker = percentileTracker;
    this.percentile = percentile;
    this.maxSpeculativeExecutions = maxSpeculativeExecutions;
  }

  @Override
  public SpeculativeExecutionPlan newPlan(String loggedKeyspace, Statement statement) {
    return new SpeculativeExecutionPlan() {
      private final AtomicInteger remaining = new AtomicInteger(maxSpeculativeExecutions);

      @Override
      public long nextExecution(Host lastQueried) {
        if (remaining.getAndDecrement() > 0)
          return percentileTracker.getLatencyAtPercentile(lastQueried, null, null, percentile);
        else return -1;
      }
    };
  }

  @Override
  public void init(Cluster cluster) {
    cluster.register(percentileTracker);
  }

  @Override
  public void close() {
    // nothing
  }
}