gistfile1.ex

def coords({x1, y1}, {x2, y2}) when x1 == x2 or y1 == y2 or abs(x1 - x2) == abs(y1 - y2) do
  do_coords({x1, y1}, {x2, y2}, [])
end

def coords(_, _), do: raise "invalid"

defp do_coords({fromx, fromy}, {tox, toy}, acc) when fromx == tox and fromy == toy, do: tl(acc)

defp do_coords({fromx, fromy}, {tox, toy}, acc) do
  newx = gen_next(fromx, tox)
  newy = gen_next(fromy, toy)
  do_coords({newx, newy}, {tox, toy}, acc ++ [{fromx, fromy}])
end

defp gen_next(x, y) do
  cond do
    x < y -> x + 1
    x > y -> x - 1
    true -> x
  end
end