add overlap functionality to Timex.Interval

* determine the overlap duration between
  two intervals.
* Can specify a valid unit for Durations.
* Will return a Duration struct or integer
  depending on the unit provided (default
  to :seconds)
* Will return 0 (or eqivalent Duration
  struct) for intervals that do not overlap.

Author: aaronglasenapp

lib/interval/interval.ex

@@ -382,6 +382,47 @@ defmodule Timex.Interval do
   def max(%__MODULE__{until: until, right_open: false}), do: until
   def max(%__MODULE__{until: until}), do: Timex.shift(until, microseconds: -1)
 
+  @doc """
+  Returns the duration of the overlap between two intervals. defaults to seconds.
+  Like the `duration` function, if unit is :duration, returns a Timex.Duration.t(),
+  otherwise returns an integer in "unit".
+  """
+  @spec overlap(__MODULE__.t(), __MODULE__.t(), atom()) :: Duration.t() | Integer.t()
+  def overlap(%__MODULE__{} = a, %__MODULE__{} = b, unit \\ :seconds) do
+    case {overlaps?(a, b), unit} do
+      {false, :duration} ->
+        Duration.from_seconds(0)
+
+      {false, _} ->
+        0
+
+      {true, unit} ->
+        new(from: start_of_overlap(a, b), until: end_of_overlap(a, b))
+        |> duration(unit)
+    end
+  end
+
+  @doc "Take the later start time of the two overlapping intervals."
+  defp start_of_overlap(%__MODULE__{} = a, %__MODULE__{} = b) do
+    case Timex.before?(min(a), min(b)) do
+      true -> min(b)
+      false -> min(a)
+    end
+  end
+
+  @doc """
+  Take the earlier end time of the 2 overlapping intervals.
+  Force `right_open: false` because we don't want
+  the microsecond offset that results from
+  `right_open: true` when calculating overlap.
+  """
+  defp end_of_overlap(%__MODULE__{} = a, %__MODULE__{} = b) do
+    case Timex.before?(max(a), max(b)) do
+      true -> max(Map.merge(a, %{right_open: false}))
+      false -> max(Map.merge(b, %{right_open: false}))
+    end
+  end
+
   defimpl Enumerable do
     alias Timex.Interval
 

test/interval_test.exs

@@ -196,6 +196,40 @@ defmodule IntervalTests do
     end
   end
 
+  describe "overlap" do
+    test "non-overlapping intervals" do
+      a = Interval.new(from: ~N[2017-01-02 15:00:00], until: ~N[2017-01-02 15:15:00])
+      b = Interval.new(from: ~N[2017-01-02 15:30:00], until: ~N[2017-01-02 15:45:00])
+
+      assert Interval.overlap(a, b) == 0
+    end
+
+    test "overlapping at single instant" do
+      a = Interval.new(from: ~N[2017-01-02 15:00:00], until: ~N[2017-01-02 15:15:00])
+      b = Interval.new(from: ~N[2017-01-02 15:15:00], until: ~N[2017-01-02 15:30:00])
+
+      assert Interval.overlap(a, b) == 0
+    end
+
+    test "partially overlapping" do
+      a = Interval.new(from: ~N[2017-01-02 15:00:00], until: ~N[2017-01-02 15:15:00])
+      b = Interval.new(from: ~N[2017-01-02 15:10:00], until: ~N[2017-01-02 15:30:00])
+
+      assert Interval.overlap(a, b) == 300
+      assert Interval.overlap(a, b, :minutes) == 5
+
+    end
+
+    test "first subset of second" do
+      a = Interval.new(from: ~N[2017-01-02 15:00:00], until: ~N[2017-01-02 15:45:00])
+      b = Interval.new(from: ~N[2017-01-02 15:20:00], until: ~N[2017-01-02 15:30:00])
+
+      assert Interval.overlap(a, b, :minutes) == 10
+      %Duration{seconds: seconds} = Interval.overlap(a, b, :duration)
+      assert seconds == 600
+    end
+  end
+
   describe "contains?/2" do
     test "non-overlapping" do
       earlier = Interval.new(from: ~D[2018-01-01], until: ~D[2018-01-04])