playing with elixir

geoserver.ex

defmodule Geoserver do
  @moduledoc """
  Documentation for Geoserver.
  """

  @doc """
  latlongdistance({ϕ1, λ1}, {ϕ2, λ2}) gives the distance, in kilometers, between
  two points on the globe defined by {ϕ1, λ1}, {ϕ2, λ2}.  Negative ϕ values are
  in the southern hemisphere and negative λ are in the western hemisphere.

  New York to Berlin:

  iex> Geoserver.latlongdistance({40.6, -74.0}, {52.5, 13.1})
  6375.72021777002

  """
  def latlongdistance({ϕd1, λd1}, {ϕd2, λd2}) do
    [ϕ1, λ1, ϕ2, λ2] = [ϕd1, λd1, ϕd2, λd2] |> Enum.map(fn x -> x * (:math.pi() / 180.0) end)
    #calculate (x1,y1,z1), and (x2,y2,z2) and do the dot product.
    dp = :math.cos(ϕ1) * :math.cos(λ1) * :math.cos(ϕ2) * :math.cos(λ2) + :math.cos(ϕ1) * :math.sin(λ1) * :math.cos(ϕ2) * :math.sin(λ2) + :math.sin(ϕ1) * :math.sin(ϕ2)

    :math.acos(dp) * 6371.0
  end
end