defmodule Grav do
  use VElixir.GenServer

  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, nil, opts)
  end

  # def g, do: 6.67408e-11

  def init(_) do
    {:ok, _} = VElixir.start_link

    state = %{
      objects: Enum.map(1..100, fn _ ->
        %{vtype: Sphere, pos: V.random(0.85), radius: 0.05, color: Color.random, mass: 5, vel: V.new}
      end),
      # arrow: %{vtype: Arrow, pos: V.new},
      dt: 0, t: 0,
      render: [:objects]
    }

    {:ok, state}
  end

  def handle_cast({:update, dt, t}, state) do
    state = %{state|dt: dt, t: t}

    objects = state.objects
      |> VElixir.Misc.worker_map(10, fn obj -> update(obj, state) end)

    {:noreply, %{state|objects: objects}}#, arrow: Map.put(state.arrow, :dir, hd(objects).pos)}}
  end

  def update(obj, state) do
    obj
      |> calc_force(state.objects)
      |> calc_kinematics(state.dt)
  end

  def calc_force(obj, objects) do
    Map.put(obj, :net_force, Enum.reduce(objects, V.new, fn other_obj, net_force ->
      if obj == other_obj do
        net_force #make sure not to count force from ourselves
      else
        r = other_obj.pos - obj.pos
        net_force + (norm(r) * ((1/length(objects)) * obj.mass * other_obj.mass / :math.pow(mag(r), 1)))
      end
    end))
  end

  def calc_kinematics(obj, dt) do
    accel = obj.net_force / obj.mass
    obj
      |> Map.put(:vel, obj.vel + accel*dt)
      |> Map.put(:pos, obj.pos + obj.vel*dt)
  end
end