example: Add simple router example (#12)

examples/simple_router/lib/simple_router/config.ex

@@ -0,0 +1,55 @@
+defmodule SimpleRouter.Config do
+  @moduledoc false
+
+  alias Tres.IPv4Address
+
+  @spec interfaces() :: %{String.t() => map()}
+  def interfaces do
+    :interfaces
+    |> get_env(%{})
+    |> Enum.reduce(%{}, &interfaces_to_erl/2)
+  end
+
+  @spec routes() :: [tuple()]
+  def routes do
+    :routes
+    |> get_env(%{})
+    |> Enum.reduce([], &routes_to_erl/2)
+  end
+
+  # private functions
+
+  defp interfaces_to_erl({ifname, %{mac_address: mac, ip_address: ip}}, acc) do
+    {ipaddr, mask} = IPv4Address.parse(ip)
+
+    entry =
+      %{
+        mac_address: String.replace(mac, ~r/:/, ""),
+        ip_address: ipaddr,
+        subnet_mask: mask,
+        network_address: IPv4Address.to_network({ipaddr, mask}),
+        prefix_length: get_cidr(ip)
+    }
+
+    Map.merge(acc, %{ifname => entry})
+  end
+
+  defp routes_to_erl({dst, nexthop}, acc) do
+    {dstaddr, mask} = IPv4Address.parse(dst)
+    {nexthop, _netmask} = IPv4Address.parse(nexthop)
+    entry = %{dst: dstaddr, mask: mask, prefix_length: get_cidr(dst), nexthop: nexthop}
+    [entry | acc]
+  end
+
+  defp get_cidr(ip) do
+    case String.split(ip, ~r/\//) do
+      [^ip] ->
+        32
+      [_addr, cidr] ->
+        String.to_integer(cidr)
+    end
+  end
+
+  defp get_env(key, default),
+    do: Application.get_env(:simple_router, key, default)
+end