total := serverPools.TotalAvailable()
	if total == 0 {
		return -1
	}
	// choose when we reach this many
	choose := rand.Uint64() % total
	atTotal := uint64(0)
	for _, pool := range serverPools {
		atTotal += pool.Available
		if atTotal > choose && pool.Available > 0 {
			return pool.Index
		}
	}
	// Should not happen, but print values just in case.

   * without contention. Because assignment of entries to these partitions is not necessarily
   * uniform, the actual concurrency observed may vary. Ideally, you should choose a value to
   * accommodate as many threads as will ever concurrently modify the table. Using a significantly
   * higher value than you need can waste space and time, and a significantly lower value can lead

   * cartesian set is constructed, the input sets are merely copied. Only as the resulting set is
   * iterated are the individual lists created, and these are not retained after iteration.
   *
   * @param sets the sets to choose elements from, in the order that the elements chosen from those
   *     sets should appear in the resulting lists
   * @param <B> any common base class shared by all axes (often just {@link Object})

   * without contention. Because assignment of entries to these partitions is not necessarily
   * uniform, the actual concurrency observed may vary. Ideally, you should choose a value to
   * accommodate as many threads as will ever concurrently modify the table. Using a significantly
   * higher value than you need can waste space and time, and a significantly lower value can lead

   * cartesian set is constructed, the input sets are merely copied. Only as the resulting set is
   * iterated are the individual lists created, and these are not retained after iteration.
   *
   * @param sets the sets to choose elements from, in the order that the elements chosen from those
   *     sets should appear in the resulting lists
   * @param <B> any common base class shared by all axes (often just {@link Object})

   * ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
   *     fetchCounterFuture, FetchException.class, x -> immediateFuture(0), directExecutor());
   * }
   *
   * <p>The fallback can also choose to propagate the original exception when desired:
   *
   * {@snippet :
   * ListenableFuture<Integer> fetchCounterFuture = ...;
   *
   * // Falling back to a zero counter only in case the exception was a

   * returned to the caller, although this is neither guaranteed to occur nor required to be
   * consistent. For example, this method <i>might</i> choose to pass through recognized
   * implementations of {@code PeekingIterator} when the behavior of the implementation is known to
   * meet the contract guaranteed by this method.
   *

   * returned to the caller, although this is neither guaranteed to occur nor required to be
   * consistent. For example, this method <i>might</i> choose to pass through recognized
   * implementations of {@code PeekingIterator} when the behavior of the implementation is known to
   * meet the contract guaranteed by this method.
   *

   */
  // TODO(fry): empirically optimize this
  static final int DRAIN_MAX = 16;

  // Fields

  /**
   * Mask value for indexing into segments. The upper bits of a key's hash code are used to choose
   * the segment.
   */
  final transient int segmentMask;

  /**
   * Shift value for indexing within segments. Helps prevent entries that end up in the same segment