defmodule DateTimeNilInfinity do
  @moduledoc false
  # Like `DateTime` except `nil` is considered positive infinity
  def compare(%DateTime{}, nil), do: :lt
  def compare(nil, %DateTime{}), do: :gt
  def compare(%DateTime{} = a, %DateTime{} = b), do: DateTime.compare(a, b)
end

defmodule DateTimeInterval do
  @moduledoc false
  import CompareChain, only: [compare?: 2]

  defmacrop dt?(expr), do: quote(do: compare?(unquote(expr), DateTimeNilInfinity))

  def disjoint?({left1, right1}, {left2, right2}) do
    dt?(right1 < left2 or right2 < left1)
  end

  def overlaps?(a, b) do
    not disjoint?(a, b)
  end

  def within_inclusive?({inside_left, inside_right}, {outside_left, outside_right}) do
    dt?(outside_left <= inside_left and inside_right <= outside_right)
  end
end