Initial upload

elixir/library-fees/lib/library_fees.ex

@@ -0,0 +1,39 @@
+defmodule LibraryFees do
+  def datetime_from_string(string) do
+    {:ok, dt} = NaiveDateTime.from_iso8601(string)
+    dt
+  end
+
+  def before_noon?(datetime) do
+    datetime.hour < 12
+  end
+
+  def return_date(checkout_datetime) do
+    dt = NaiveDateTime.to_date(checkout_datetime)
+    case before_noon?(checkout_datetime) do
+      true -> Date.add(dt, 28)
+      _ -> Date.add(dt, 29)
+    end
+  end
+
+  def days_late(planned_return_date, actual_return_datetime) do
+    actual_return_datetime
+    |> Date.diff(planned_return_date)
+    |> max(0)
+  end
+
+  def monday?(datetime) do
+    Date.day_of_week(NaiveDateTime.to_date(datetime)) == 1
+  end
+
+  def calculate_late_fee(checkout, return, rate) do
+    checkout_datetime = datetime_from_string(checkout)
+    return_datetime = datetime_from_string(return)
+    raw_fee = days_late(return_date(checkout_datetime), return_datetime) * rate
+    if monday?(return_datetime) do
+      floor(raw_fee * 0.5)
+    else
+      raw_fee
+    end
+  end
+end