code: func expandedShardBounds(c context.Context, cfg *Config, shard uint64) (low, high int64) {
  	totalShards := cfg.TotalShardCount(info.GetNamespace(c))
  	if shard < 0 || uint64(shard) >= totalShards {
  		logging.Warningf(c, "Invalid shard: %d", shard)
  		// return inverted bounds
  		return 0, -1
  	}
  
  	expandedShardsPerShard := int64(math.MaxUint64 / totalShards)
  	low = math.MinInt64 + (int64(shard) * expandedShardsPerShard)
  	if uint64(shard) == totalShards-1 {
  		high = math.MaxInt64
  	} else {
  		high = low + expandedShardsPerShard
  	}
  	return
  }
docs: // expandedShardBounds returns the boundary of the expandedShard order that
  // currently corresponds to this shard number. If Shard is < 0 or > NumShards
  // (the currently configured number of shards), this will return a low > high.
  // Otherwise low < high.