Create DebtFilter module for Bet

-DebtFilter addresses the problem, where some of the created Debts for a
particular Bet would cross-reference each other (PersonA -$50-> PersonB
&& PersonB -$20-> PersonA) or accumulate as separate entries (PerosnA
-$50-> PersonB && PersonA -$20-> PersonB) by filtering them before
saving.
-Added DebtFilter to Bet model. Debts are now filtered before getting
saved.

app/models/bet.rb

@@ -1,4 +1,5 @@
 class Bet < ApplicationRecord
+  include DebtFilter
   has_many :users, through: :user_bets
   has_many :user_bets
   has_many :options, class_name: 'BetOption'
@@ -10,13 +11,13 @@ class Bet < ApplicationRecord
   def resolve(winning_option:)
     winning_option.update!(winner: true)
     debt_relationships = user_bets.winners.to_a.product(user_bets.losers.to_a)
-    debt_relationships.map do |(won_bet, lost_bet)|
-      Debt.create(
+    save_debts(debt_relationships.map do |(won_bet, lost_bet)|
+      Debt.new(
         creditor: won_bet.user,
         debtor: lost_bet.user,
         amount:  amount_owed(lost_bet, won_bet)
       )
-    end
+    end)
   end
 
   def winner_total

app/models/debt_filter.rb

@@ -0,0 +1,55 @@
+module DebtFilter
+  def save_debts(debts)
+    filter_debts(debts).map do |debt|
+      debt.save
+      debt
+    end
+  end
+
+  private
+
+  def filter_debts(debts)
+    reverse_negatives(merge_debts(reverse_duplicates(debts)))
+  end
+
+  def unique_participants(debts)
+    uniques = []
+    debts.each do |debt|
+      next if uniques.include?([debt.creditor, debt.debtor]) ||
+              uniques.include?([debt.debtor, debt.creditor])
+      uniques << [debt.debtor, debt.creditor]
+    end
+    uniques
+  end
+
+  def reverse_duplicates(debts)
+    uniques = unique_participants(debts)
+    debts.map do |debt|
+      if uniques.include?([debt.debtor, debt.creditor])
+        debt
+      else
+        Debt.new(debtor: debt.creditor,
+                 creditor: debt.debtor, amount: -debt.amount)
+      end
+    end
+  end
+
+  def merge_debts(debts)
+    debts.group_by { |debt| [debt.debtor, debt.creditor] }.map do |_, group|
+      Debt.new(debtor: group.first.debtor,
+               creditor: group.first.creditor,
+               amount: group.sum(&:amount))
+    end
+  end
+
+  def reverse_negatives(debts)
+    debts.map do |debt|
+      if debt.amount < 0
+        Debt.new(debtor: debt.creditor,
+                 creditor: debt.debtor, amount: debt.amount.abs)
+      else
+        debt
+      end
+    end
+  end
+end

spec/modules/debt_filter_spec.rb

@@ -0,0 +1,42 @@
+require 'rails_helper'
+
+RSpec.describe DebtFilter, type: :module do
+  let(:debt_filter) { Class.new { extend DebtFilter } }
+  let(:bob) do
+    User.create(username: 'Bob', email: '[email protected]', password: '123')
+  end
+  let(:peter) do
+    User.create(username: 'Peter', email: '[email protected]', password: 'pete1')
+  end
+  let(:jack) do
+    User.create(username: 'Jack', email: '[email protected]', password: 'jeeek')
+  end
+
+  let(:debt1) { Debt.new(debtor: bob, creditor: peter, amount: 20) }
+  let(:debt2) { Debt.new(debtor: peter, creditor: bob, amount: 100) }
+  let(:debt3) { Debt.new(debtor: bob, creditor: peter, amount: 60) }
+  let(:debt4) { Debt.new(debtor: jack, creditor: peter, amount: 55.5) }
+  let(:debts) { [debt1, debt2, debt3, debt4] }
+
+  describe('#save_debts') do
+    subject(:saved_debts) { debt_filter.save_debts(debts) }
+    let(:expected_debt1) do
+      Debt.new(debtor: peter, creditor: bob, amount: 20)
+    end
+    let(:expected_debt2) { debt4 }
+    it 'saves debts after filtering them' do
+      aggregate_failures 'testing debts' do
+        expect(saved_debts.first).to(
+          have_attributes(debtor: expected_debt1.debtor,
+                          creditor: expected_debt1.creditor,
+                          amount: expected_debt1.amount)
+        )
+        expect(saved_debts.second).to(
+          have_attributes(debtor: expected_debt2.debtor,
+                          creditor: expected_debt2.creditor,
+                          amount: expected_debt2.amount)
+        )
+      end
+    end
+  end
+end