library-fees

elixir/library-fees/lib/library_fees.ex

@@ -0,0 +1,55 @@
+defmodule LibraryFees do
+  def datetime_from_string(string) do
+    case NaiveDateTime.from_iso8601(string) do
+      {:ok, naive_date_time} -> naive_date_time
+      _ -> :error
+    end
+  end
+
+  def before_noon?(datetime) do
+    datetime.hour < 12
+  end
+
+  defp days_to_return(checkout_datetime) do
+    if(before_noon?(checkout_datetime), do: 28, else: 29)
+  end
+
+  def return_date(checkout_datetime) do
+    checkout_datetime
+    |> NaiveDateTime.add(days_to_return(checkout_datetime), :day)
+    |> NaiveDateTime.to_date()
+  end
+
+  def days_late(planned_return_date, actual_return_datetime) do
+    actual_return_datetime
+    |> NaiveDateTime.to_date()
+    |> Date.diff(planned_return_date)
+    |> case do
+      late_in_days when late_in_days > 0 -> late_in_days
+      _late_in_days -> 0
+    end
+  end
+
+  def monday?(datetime) do
+    datetime
+    |> NaiveDateTime.to_date()
+    |> Date.day_of_week(:monday)
+    |> case do
+      1 -> true
+      _ -> false
+    end
+  end
+
+  def calculate_late_fee(checkout, return, rate) do
+    checkout_date_time = datetime_from_string(checkout)
+    actual_return_date_time = datetime_from_string(return)
+    planed_return_date_time = return_date(checkout_date_time)
+
+    fee = days_late(planed_return_date_time, actual_return_date_time) * rate
+
+    case monday?(actual_return_date_time) do
+      true -> floor(fee * 0.5)
+      _ -> fee
+    end
+  end
+end