I am having issues consolidating data from dictionaries - swift

Question

This was written in swift but its more of a logic question hopefully someone can help with, I've been working on an app that takes expenses and then splits the totals between users based on which users were specified in the "owees" property of the expense. I was able to divvy out the totals by creating a dictionary for all the users with the key being their name and the value being more dictionaries for each of the other users with the key being the other users' name, and the values being the amount owed (postive number if the money is owed to the parent dictionary, negative number for the opposite), my issue is now consolidating all the money so that all the users owed amounts are divvied out amongst each other (e.g.: if User1 owes User2 $10 but User2 owes User3 $5 then User1 now only owes User2 $5 but now also owes User3 $5 and User2's debt is cleared)
I know its seems confusing with the dictionary-inception but here is an example of the dictionary, and a link to my .playground where it shows the idea of what I am trying to and how I set it up, the last for loop is the problem, maybe theres a better way?
User1[
User2: -20
User3: 10
User4: -5
]
User2[
User1: 20
User3: -15
User4: 0
]
gist link:
https://gist.github.com/wilks7/0c5e3ab4f5d95c945579
playground file:
https://www.dropbox.com/s/6h2iuic84un2uh3/expenseSplit.playground.zip?dl=0

Answer 1

What you are trying to model is essentially a Directed Acyclic Graph. However if the queries that you need to make on the relationships are relatively simple (i.e. list the outstanding amounts for each person), then you may be able to get away with some object-oriented modelling rather than a complete DAG.. Constructing classes and embedding logic in those classes will make your code much easier to read compared to trying to accomplish the same thing with arrays and/or dictionaries.
A skeleton example is:
public class Debt {
var owedBy: Person
var amount: Float
init(amount: Float, from: Person) {
self.amount = amount
self.owedBy = from
}
}
public class Person {
var name: String
var loans: [Debt] = []
init(name: String) {
self.name = name
}
public func addLoan(debt: Debt) {
loans.append(debt)
}
public func showLoans() {
print("\(name):")
for debt in loans {
print("Owed \(debt.amount) by \(debt.owedBy.name)")
}
}
}
This would allow you to create loans and show what exists with code like:
let Bill = Person(name: "Bill")
let Tony = Person(name: "Tony")
Bill.addLoan(Debt(amount: 20.0, from: Tony))
Bill.showLoans()
Calling showLoans() for Bill as above would produce:
Bill:
Owed 20.0 by Tony