func (c *Client) LastError() error {
	c.RLock()
	defer c.RUnlock()
	return fmt.Errorf("[%s] %w", c.lastErrTime.Format(time.RFC3339), c.lastErr)
}

// computes the exponential backoff duration according to
// https://www.awsarchitectureblog.com/2015/03/backoff.html
func exponentialBackoffWait(r *rand.Rand, unit, maxSleep time.Duration) func(uint) time.Duration {
	if unit > time.Hour {
		// Protect against integer overflow

   * Entries appear in the result {@code ImmutableBiMap} in encounter order.
   *
   * <p>If the mapped keys or values contain duplicates (according to {@link
   * Object#equals(Object)}), an {@code IllegalArgumentException} is thrown when the collection
   * operation is performed. (This differs from the {@code Collector} returned by {@link

	// Get start part index and offset.
	partIndex, partOffset, err := fi.ObjectToPartOffset(ctx, startOffset)
	if err != nil {
		return InvalidRange{startOffset, length, fi.Size}
	}

	// Calculate endOffset according to length
	endOffset := startOffset
	if length > 0 {
		endOffset += length - 1
	}

	// Get last part index to read given length.
	lastPartIndex, _, err := fi.ObjectToPartOffset(ctx, endOffset)

   * {@code C}
   *
   * <ul>
   *   <li>All visible static methods are checked such that passing null for any parameter that's
   *       not annotated nullable (according to the rules of {@link NullPointerTester}) should throw
   *       {@link NullPointerException}.
   *   <li>If there is any visible constructor or visible static factory method declared by the

    return ensureInUnitRange(sumOfProductsOfDeltas / Math.sqrt(productOfSumsOfSquaresOfDeltas));
  }

  /**
   * Returns a linear transformation giving the best fit to the data according to <a
   * href="http://mathworld.wolfram.com/LeastSquaresFitting.html">Ordinary Least Squares linear
   * regression</a> of {@code y} as a function of {@code x}. The count must be greater than one, and

      return;
    }
    fail("Should get not equal to equal object error");
  }

  /**
   * Test for an invalid hashCode method, i.e., one that returns different value for objects that
   * are equal according to the equals method
   */
  public void testInvalidHashCode() {
    Object a = new InvalidHashCodeObject(1, 2);
    Object b = new InvalidHashCodeObject(1, 2);
    equalsTester.addEqualityGroup(a, b);
    try {

  }

  /**
   * Searches the specified list for the specified object using the binary search algorithm. The
   * list must be sorted into ascending order according to the specified comparator (as by the
   * {@link Collections#sort(List, Comparator) Collections.sort(List, Comparator)} method), prior to
   * making this call. If it is not sorted, the results are undefined.
   *

   * and values are the result of applying the provided mapping functions to the input elements.
   * Entries appear in the result {@code ImmutableMap} in encounter order.
   *
   * <p>If the mapped keys contain duplicates (according to {@link Object#equals(Object)}, an {@code
   * IllegalArgumentException} is thrown when the collection operation is performed. (This differs

    //                                                                       Validation
    //                                                                           =========
    /**
     * Validates document fields according to index field requirements.
     *
     * @param doc the document to validate
     * @param throwError consumer to handle validation errors
     */

   * elements are the result of applying {@code elementFunction} to the inputs, with counts equal to
   * the result of applying {@code countFunction} to the inputs.
   *
   * <p>If the mapped elements contain duplicates (according to {@link Object#equals}), the first
   * occurrence in encounter order appears in the resulting multiset, with count equal to the sum of
   * the outputs of {@code countFunction.applyAsInt(t)} for each {@code t} mapped to that element.