create Core.print_invoices/3 and its tests

lib/telephony/core.ex

@@ -40,6 +40,15 @@ defmodule Telephony.Core do
     |> maybe_print_invoice(year, month)
   end
 
+  def print_invoices(subscribers, year, month) do
+    Enum.reduce(subscribers, [], fn subscriber, acc ->
+      case maybe_print_invoice(subscriber, year, month) do
+        {:error, _message} -> acc
+        invoice -> acc ++ [invoice]
+      end
+    end)
+  end
+
   defp maybe_print_invoice({:error, _message} = err, _year, _month) do
     err
   end

test/telephony/core_test.exs

@@ -348,4 +348,71 @@ defmodule Telephony.CoreTest do
       assert expected == result
     end
   end
+
+  describe "print_invoices/3" do
+    setup do
+      date = NaiveDateTime.utc_now()
+      last_month = NaiveDateTime.add(date, -31, :day)
+      two_months_ago = NaiveDateTime.add(last_month, -31, :day)
+
+      subscribers = [
+        %Subscriber{
+          full_name: "John Doe",
+          phone: "1234567890",
+          type: %Core.Prepaid{
+            credits: 213,
+            recharges: [
+              %Core.Recharge{value: 100, date: date},
+              %Core.Recharge{value: 100, date: last_month},
+              %Core.Recharge{value: 100, date: two_months_ago}
+            ]
+          },
+          calls: [
+            %Core.Call{time_spent: 10, date: date},
+            %Core.Call{time_spent: 20, date: last_month},
+            %Core.Call{time_spent: 30, date: two_months_ago}
+          ]
+        }
+      ]
+
+      {:ok,
+       subscribers: subscribers,
+       date: date,
+       last_month: last_month,
+       two_months_ago: two_months_ago}
+    end
+
+    test "with valid params", %{
+      subscribers: subscribers,
+      date: date,
+      last_month: last_month,
+      two_months_ago: two_months_ago
+    } do
+      expected = [
+        %{
+          subscriber: %Telephony.Core.Subscriber{
+            full_name: "John Doe",
+            phone: "1234567890",
+            type: %Telephony.Core.Prepaid{
+              credits: 213,
+              recharges: [
+                %Telephony.Core.Recharge{value: 100, date: date},
+                %Telephony.Core.Recharge{value: 100, date: last_month},
+                %Telephony.Core.Recharge{value: 100, date: two_months_ago}
+              ]
+            },
+            calls: [
+              %Telephony.Core.Call{time_spent: 10, date: date},
+              %Telephony.Core.Call{time_spent: 20, date: last_month},
+              %Telephony.Core.Call{time_spent: 30, date: two_months_ago}
+            ]
+          },
+          invoice: %{calls: [], credits: 213, recharges: []}
+        }
+      ]
+
+      result = Core.print_invoices(subscribers, 2021, 1)
+      assert expected == result
+    end
+  end
 end