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func build(im typeutil.ImmutablemapString2string) *Builder {
	builder := &Builder{
		reference: im,
		mutable:   nil,
	}
	return builder
}

type Builder struct {
	reference typeutil.ImmutablemapString2string
	mutable   map[string]string
}

func (bdr *Builder) maybeClone() {
	if !bdr.reference.IsEmpty() {
		bdr.mutable = make(map[string]string)
		for k, v := range bdr.reference.GetCopy() {
			bdr.mutable[k] = v
		}

		bdr.reference = typeutil.ImmutablemapString2string{}
	} else {
		bdr.mutable = make(map[string]string)
	}
}

func (bdr *Builder) getnamefromalias(key string) (string, bool) {
	bdr.maybeClone()
	rstr, rbool := bdr.mutable[key]
	return rstr, rbool
}

func (bdr *Builder) putalias2name(key string, val string) (string, bool) {
	bdr.maybeClone()
	var pre string
	v, ok := bdr.mutable[key]
	if ok {
		pre = v
	} else {
		pre = ""
	}
	bdr.mutable[key] = val
	return pre, ok
}

func (bdr *Builder) removealias2name(key string) (string, bool) {
	bdr.maybeClone()
	var pre string
	v, ok := bdr.mutable[key]
	if ok {
		pre = v
		delete(bdr.mutable, key)
	} else {
		pre = ""
	}
	return pre, ok
}

func (bdr *Builder) Build() typeutil.ImmutablemapString2string {
	if !bdr.reference.IsEmpty() {
		reference := bdr.reference
		bdr.reference = typeutil.ImmutablemapString2string{}
		return reference
	}

	mutable := bdr.mutable
	bdr.mutable = nil
	res := typeutil.NewImmutablemapString2string(mutable)
	return res

}


func (mt * metatable) addAlias(collectionAlias string, collectionName string) (error){
    mt.ddLock.Lock()
    defer mt.ddLock.Unlock()
    ...
    ts = getTimestamp()
    tspre = mt.newestAliasTs
    Bdr = build(mt.ts2alias2name[tspre])
    _,ok := Bdr.putalias2name(collectionAlias, collectionName)
    if ok{
        return fmt.Errorf("alias already exist when add alias") 
    }
    mt.ts2alias2name[ts] = Bdr.build()
    mt.newestAliasTs = ts
    ...
    return nil
}

func (mt * metatable) dropAlias(collectionAlias string) (error){
    mt.ddLock.Lock()
    defer mt.ddLock.Unlock()
    ...
    ts = getTimestamp()
    tspre = mt.newestAliasTs
    Bdr = build(mt.ts2alias2name[tspre])
    _, ok := Bdr.removealias2id(collectionAlias)
    if !ok{
        return fmt.Errorf("alias not exist when drop alias") 
    }
    mt.ts2alias2name[ts] = Bdr.build()
    mt.newestAliasTs = ts
    ...
    return nil
}

func (mt * metatable) alterAlias(collectionAlias string, collectionName string) (string, error){
    mt.ddLock.Lock()
    defer mt.ddLock.Unlock()
    ...
    ts = getTimestamp()
    tspre = mt.newestAliasTs
    Bdr = build(mt.ts2alias2name[tspre])
    pre_,ok := Bdr.putalias2name(collectionAlias, collectionName)
    if !ok{
        return nil, fmt.Errorf("alias not exist when alter alias") 
    }
    mt.ts2alias2name[ts] = Bdr.build()
    mt.newestAliasTs = ts
    ...
    return pre, nil
}