debtors.rb

class Balance
  attr_accessor :person, :amount

  def initialize(person, amount)
    @person, @amount = person, amount
  end

  def to_s
    "#{ person }: #{ amount }"
  end
end

class Ledger
  attr_accessor :people, :balances

  def initialize(people)
    @people = people
    @balances = people.each_with_object({}) do |person, bal|
      bal[person] = Balance.new(person, 0)
    end
  end

  def expense(cost:, payer:, players:)
    balances[payer].amount -= cost
    cost_per_player = cost / players.count
    players.each { |player| balances[player].amount += cost_per_player }
  end

  def compute_debts
    result = {}
    creditors, debtors = balances.values.partition { |balance| balance.amount < 0 }
    credit_index, debt_index = creditors.count - 1, debtors.count - 1
    loop do
      return result if credit_index < 0 || debt_index < 0
      credit, debt = creditors[credit_index], debtors[debt_index]
      if -credit.amount == debt.amount
        (result[debt.person] ||= []).push([credit.person, debt.amount])
        credit.amount = debt.amount = 0
        credit_index -= 1
        debt_index -= 1
      elsif -credit.amount < debt.amount
        (result[debt.person] ||= []).push([credit.person, -credit.amount])
        debt.amount += credit.amount
        credit.amount = 0;
        credit_index -= 1
      else
        (result[debt.person] ||= []).push([credit.person, debt.amount])
        credit.amount += debt.amount
        debt.amount = 0
        debt_index -= 1
      end
    end
  end
end

nick, maksim, alice, bob = 'Nick', 'Maksim', 'Alice', 'Bob'

ledger = Ledger.new([nick, maksim, alice, bob])
ledger.expense(cost: 200, payer: nick, players: [nick, maksim, alice, bob])
ledger.expense(cost: 100, payer: maksim, players: [alice, bob])

ledger.compute_debts.each do |(debtor, debts)|
  puts "\n#{ debtor } owes:"
  debts.each do |debt|
    creditor, amount = debt
    puts "...#{ amount } to #{ creditor }"
  end
end