ex_ray_casting

defmodule RayCasting do
  @moduledoc """
  A Ray Casting algorithm for Elixir.
  This module contains the Application that you can use to perform
  solutions for some problems like:

  - To know if a coordinate exists inside a polygon
  - To know if a coordinate exists outside a polygon

  """

  @doc """
  Ray Casting
  """
  def rayCasting(path, point) do
    crossings = 0
    {x, _y} = Enum.map_reduce(path, 0, fn _item, acc ->
      a = Enum.at(path, acc)
      b = Enum.at(path, acc + 1)
      if (a == List.last(path)) do
        b = Enum.at(path, 0)
      end
      if rayCrossesSegment(point, a, b) do
        crossings = crossings + 1
      end
      acc = acc + 1
      {crossings, acc}
    end)
    crossings = List.last(x)
    rem(crossings, 2) == 1

  end

  defp rayCrossesSegment(point, a, b) do
    px = point.lng
    py = point.lat
    ax = a.lng
    ay = a.lat
    bx = b.lng
    by = b.lat

    if (ay > by) do
      ax = b.lng
      ay = b.lat
      bx = a.lng
      by = a.lat
    end
    if (px < 0), do: px = px + 360
    if (ax < 0), do: ax = ax + 360
    if (bx < 0), do: bx = bx + 360
    if (py == ay || py == by), do: py = py + 0.00000001

    if (ax != bx) do
      red = ((by - ay) / (bx - ax))
    else
      red = Infinity
    end
    if (ax != px) do
      blue = ((py - ay) / (px - ax))
    else
      blue = Infinity
    end
    cond do
      ((py > by || py < ay) || (px > max(ax, bx))) -> false
      (px < min(ax, bx)) -> true
      blue >= red -> true
      true -> true
    end
  end

end