Grouping and Combining Observables in RxJava - rx-java2

Question

I would like to do the following with RxJava
class Invoice(val dayOfMonth:Int,val amount:Int)
below is the sample monthInvoices:List< Invoice > to process
Invoice(3,100)
Invoice(3,150)
Invoice(3,50)
Invoice(4,350)
Invoice(8,400)
Invoice(8,100)
First, I would like to group it by the day of the month like the following
Invoice(3,300)
Invoice(4,350)
Invoice(8,500)
Then I would like to create a list containing all the days of the month. Say, we are having 30 days for this month, then the output list must contain inserting a empty Invoice object with 0 amount for the days where there is no invoice
Desired output List
Invoice(1,0) //Since day 1 is not in the group summed list
Invoice(2,0) //day 2 is also not there
Invoice(3,300)
Invoice(4,350)
Invoice(5,0)
Invoice(6,0)
Invoice(7,0)
Invoice(8,500)
…..
Invoice(30,0)
Hope I have explained the need clearly. Can anyone please answer me a solution to do it entirely using RxJava?

Answer 1

Try this
fun task(invoices: List<Invoice>) =
Observable.fromIterable(invoices)
.groupBy { it.dayOfMonth }
.flatMapSingle { group -> group.reduce(0) { t1, t2 -> t1 + t2.amount }
.map { group.key to it }}
.toMap({ it.first }, { it.second })
.flatMapObservable { map ->
Observable.range(1, 30)
.map { Invoice(it, map[it] ?: 0) }
}

Answer 2

This can be achieved much more easily using the collection operators inside Kotlin's standard library, but in pure RxJava you can do this by using groupBy and reduce.
val invoices = listOf(
Invoice(3, 100),
Invoice(3, 150),
Invoice(3, 50),
Invoice(4, 350),
Invoice(8, 400),
Invoice(8, 100)
)
Observable.range(1, 30)
.map { Invoice(it, 0) } // Create an Observable of Invoice([day], 0)
.mergeWith(Observable.fromIterable(invoices))
.groupBy { it.dayOfMonth } // Merge the sources and groupBy day
.flatMapMaybe { group ->
group.reduce { t1: Invoice, t2: Invoice ->
Invoice(t1.dayOfMonth, t1.amount + t2.amount) // Reduce each group into a single Invoice
}
}
.subscribe {
// Optionally you can call toList before this if you want to aggregate the emissions into a single list
println(it)
}