defmodule ReverseProxy.LoadBalancer do
  @moduledoc """
  Our Load Balancer. It rotates the servers from which we
  get a response.

  We start the GenServer with the config file filled with hosts
  and upstreams.
  """
  use GenServer

  #Module API
  def start_link(upstreams) do
    GenServer.start_link(__MODULE__, upstreams, name: __MODULE__)
  end

  @doc """
  The public API to select our next server to hit.

  It just takes a hots that we need to hit.
  Exapmle: ReverseProxy.LoadBalancer.select(load.com)
  """
  def select(host) do
    GenServer.call(__MODULE__, {:select, host})
  end

  @doc """
  Initializes the gen server with all the upstreams.
  """
  def init(upstreams) do
    {:ok, upstreams}
  end

  @doc """
  The server callback for the `select` function.
  It rotates the servers every time we hit them with a
  request.
  """
  def handle_call({:select, host}, _from, upstreams) do
    [h | tail] = upstreams[host]
    rotate_servers = tail ++ [h]
    upstreams = %{upstreams | host => rotate_servers}
    {:reply, h, upstreams}
  end
end