type sequenceIDGenerator struct {
	idMgrClient IDManagerClient // client for getting IDPrefix
	preGenerateIDs chan string // pre-generated ids
}

// Pre-generate id and send to channel.
// This method should be called once and only once when init generator
func (g *sequenceIDGenerator) preGenIDs() {
	var idBase, id string
	var seq uint64

	for {
		if idBase == "" || seq == 0 { // first call or over max uint 64
			idBase = g.idMgrClient.GetNewIDPrefix("ServiceA")
		}
		id = fmt.Sprintf("%s-%d", idBase, strconv.FormatUint(seq, 32))
		g.preGenerateIDs <- id
		seq++
	}
}

// NewSequenceIDGenerator returns new id generator
func NewSequenceIDGenerator(imc IDManagerClient) *sequenceIDGenerator {
	g := &sequenceIDGenerator{
		idMgrClient: imc,
		preGenerateIDs: make(chan string, 10000),
	}
	go g.preGenIDs()
	return g
}

// NextID returns next id.
func (g *sequenceIDGenerator) NextID() string {
	return <-g.preGenerateIDs
}