I am using RethinkDB in my application and I have a lobby that has users.
RethinkDB has the ability to watch for changes to a table and when changes happen it automagically emits the changes so you can do whatever you want with the data, right now I'm trying to make it so when a user leaves the lobby I can send a websocket out to remove the user. The only thing is I'm trying to find out where the difference is in the before / after data which is a slice of members, this is the data:
type change struct {
NewVal *fields `gorethink:"new_val,omitempty"`
OldVal *fields `gorethink:"old_val,omitempty"`
}
type fields struct {
ID string `gorethink:"id"`
Owner string `gorethink:"owner"`
Inqueue bool `gorethink:"inqueue"`
Members []struct {
SteamID string `gorethink:"steamid"`
Username string `gorethink:"username"`
} `gorethink:"members"`
Messages []struct {
Username string `gorethink:"username"`
Message string `gorethink:"message"`
CreatedAt time.Time `gorethink:"createdAt"`
} `gorethink:"messages"`
}
Right now I'm doing
func (l *lobby) watchChanges() {
db := common.DB()
query := gorethink.Table("Lobbys").Get(l.ID).Changes()
res, err := query.Run(db)
if err != nil {
log.Println(err)
}
go func(res *gorethink.Cursor, l *lobby) {
defer res.Close()
changes := new(change)
for res.Next(&changes) {
if changes.NewVal != nil && changes.OldVal != nil {
switch {
case len(changes.NewVal.Members) > len(changes.OldVal.Members):
// Member has joined so announce who it was.
case len(changes.NewVal.Members) < len(changes.OldVal.Members):
// Member has left so announce who it was.
-------->
case len(changes.NewVal.Messages) > len(changes.OldVal.Messages):
// New Message was recieved so announce the message.
}
}
}
}(res, l)
select {
case <-l.KillMe:
res.Close()
break
}
}
The new entries are easy enough I'll just take the end off the slice and send that, but when it comes to the user leaving, how can I compare the changes.NewVal.Members and changes.OldVal.Members to see which index was removed so I can send the right member to remove through the websocket. Hope my question is clear let me know if it isn't.
This is how I'm currently doing it
removedIndex := 0
for i, oldMember := range changes.OldVal.Members {
foundMissing := true
for _, newMember := range changes.NewVal.Members {
if reflect.DeepEqual(oldMember, newMember) {
foundMissing = false
}
}
if foundMissing {
removedIndex = i
break
}
}
but it feels a bit hacky, is there a better way?
Sort the old and new members by a unique and sortable key. It looks like SteamID might be suitable for this purpose. Iterate through both slices checking for added and deleted elements by comparing the keys.
func diff(old []*member, new []*member) {
sort.Sort(bySteamID(old))
sort.Sort(bySteamID(new))
i, j := 0, 0
for i < len(old) && j < len(new) {
switch {
case old[i].SteamID < new[j].SteamID:
fmt.Println(" delete", old[i].SteamID)
i++
case old[i].SteamID > new[j].SteamID:
fmt.Println(" add", new[j].SteamID)
j++
default:
i++
j++
}
}
for i < len(old) {
fmt.Println(" delete", old[i].SteamID)
i++
}
for j < len(new) {
fmt.Println(" add", new[j].SteamID)
j++
}
}