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package edu.byu.hbll.box.internal.util;

import edu.byu.hbll.box.BoxQuery;
import java.time.Duration;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.Collection;
import java.util.Collections;
import java.util.LinkedHashSet;
import java.util.Random;
import java.util.Set;
import java.util.stream.Collectors;

/**
 * Random utilities used by Box.
 *
 * @author Charles Draper
 */
public class BoxUtils {

  private static final Random r = new Random();

  /**
   * Calculates a retry date/time based on a base date/time (eg. first attempt), a retry delay, and
   * retry jitter.
   *
   * The minimum (ie, the first) delay corresponds to the retryDelay. Subsequent
   * delays are calculated by taking the difference between NOW and the base. This
   * essentially doubles the delay each time. A random amount of jitter +/- is added to the
   * calculated delay. The bounds of the jitter is based on the retryJitter parameter
   * which is a percentage of the calculated delay. This random variation can be critical if
   * failures occur during certain windows of time or as part of batches with other conflicting
   * items.
   *
   * 
Examples:
   *
   * 

   *   Given a base of 1 second ago, retryDelay of 1 minute, 
   *       retryJitter of 0.5, the retry date/time will be anywhere from 30 seconds to 90
   *       seconds from NOW (minimum of 1 minute +/- 1 minute * 0.5).
   *   
Given a base of 8 minutes ago, retryDelay of 1 minute, 
   *       retryJitter of 0.5, the retry date/time will be anywhere from 4 minutes to 12
   *       minutes from NOW (8 minutes +/- 8 minutes * 0.5)..
   * 
   *
   * @param base the base date/time the retry date/time is calculated from
   * @param retryDelay the initial or minimum delay before processing again
   * @param retryJitter a percentage of the calculated delay added or subtracted
   * @return the retry date/time
   */
  public static Instant retryDateTime(Instant base, Duration retryDelay, double retryJitter) {
    long delay = Math.max(retryDelay.toMillis(), ChronoUnit.MILLIS.between(base, Instant.now()));
    double jitterCoefficient = r.nextDouble() * (retryJitter * 2) + (1 - retryJitter);
    long delayWithJitter = (long) (delay * jitterCoefficient);
    return Instant.now().plusMillis(delayWithJitter);
  }

  /**
   * Converts non _at_doc fields into _at_doc fields.
   *
   * @param fields dot-notated fields denoting what should be projected
   * @return the new fields
   */
  public static Set canonicalizeFields(Collection fields) {
    return Collections.unmodifiableSet(
        (Set)
            fields.stream()
                .map(
                    field -> {
                      if (!field.equals(BoxQuery.DOCUMENT_FIELD)
                          && !field.startsWith("@doc.")
                          && !field.equals(BoxQuery.METADATA_FIELD)
                          && !field.startsWith("@box.")) {
                        return "@doc." + field;
                      } else {
                        return field;
                      }
                    })
                .collect(Collectors.toCollection(LinkedHashSet::new)));
  }
}