defmodule DateRange do
  @moduledoc """
  A helper library to easily create relative times, like 2 days ago.

  The result of functions is always a `%DateTime{}`.
  """
  
  @opaque time_unit :: :seconds | :minutes | :hours | :days | :weeks
  
  @spec ago(num :: integer, unit :: time_unit) :: %DateTime{}
  @doc """
  Create a past time, yielding a `%DateTime{}`.
  
  Example:
  
  ```
  # Create a DateTime, indicating 2 days ago
  DateRange.ago 2, :days
  ```

  Available units are `:seconds`, `:minutes`, `:hours`, `:days` and `:weeks`.
  """
  def ago(num, unit)
  
  def ago(days, :days) when is_integer(days) do
    ago 86400 * days, :seconds
  end

  def ago(minutes, :minutes) when is_integer(minutes) do
    ago 60 * minutes, :seconds
  end

  def ago(seconds, :seconds) when is_integer(seconds) do
    System.system_time(:seconds)
    |> (fn time -> time - seconds end).()
    |> DateTime.from_unix(:seconds)
    |> (fn {:ok, %DateTime{} = datetime} -> datetime end).()  
  end

  def ago(hours, :hours) when is_integer(hours) do
    ago 3600 * hours, :seconds
  end

  def ago(weeks, :weeks) when is_integer(weeks) do
    ago 604800 * weeks, :seconds
  end
end