Here’s what I did. I put the data set into a list of maps using Stream.map. It looks like this:

[%{trip_id: 1, station_code: "A", departure_time: "08:00:00", station_sequence: 1}, 
 %{trip_id: 1, station_code: "B", departure_time: "08:05:00", station_sequence: 2},
 %{...}]

I then output the Stream to a list:

|> Enum.to_list

I used pattern matching to calculate the trip_duration_min values. Here’s my function.

def put_trip_duration_min([], _, _, acc), do: acc
def put_trip_duration_min([%{departure_time: departure_time,
                               station_sequence: station_sequence} = schedule|t],
                               prior_departure_time, trip_duration_min, acc) do
  trip_duration_min =
    case station_sequence do
      1 ->
        0
      _ ->
        duration(departure_time, prior_departure_time, trip_duration_min, :minutes)
    end

  acc = [Map.put(schedule, :trip_duration_min, trip_duration_min)|acc]
  put_trip_duration_min(t, departure_time, trip_duration_min, acc]
end

defp duration(time, prior_time, duration, units) do
  time = Time.from_iso8601!(time) |> Timex.Duration.from_time
  prior_time = Time.from_iso8601!(prior_time) |> Timex.Duration.from_time
  duration + Timex.Duration.diff(time, prior_time, units)
end

I then call the function and reverse the list back to it’s original order at the end!

|> put_trip_duration_min(nil, 0, [])
|> Enum.reverse

I hope this is helpful for someone else down the road