diff --git a/2021/day16.exs b/2021/day16.exs
new file mode 100644
index 0000000..b0b974c
--- /dev/null
+++ b/2021/day16.exs
@@ -0,0 +1,123 @@
+defmodule Day16 do
+  def part1(input) do
+    {_type, _rest, _count, version_sum} = decode_packet(input, 0, 0)
+    version_sum
+  end
+
+  def part2(_input) do
+    :ok
+  end
+
+  @literal_type 4
+  @literal_part 1
+  @literal_end 0
+  @total_length_type 0
+  @sub_packets_length_type 1
+
+  def decode_packet(<<version::3, @literal_type::3, rest::bits>>, count, version_sum) do
+    decode_literal(rest, <<>>, count + 6, version_sum + version)
+  end
+
+  def decode_packet(<<version::3, operator_type::3, rest::bits>>, count, version_sum) do
+    {sub_packets, rest, count, version_sum} =
+      case rest do
+        <<@total_length_type::1, length::15, rest::bits>> ->
+          decode_sub_packets_by(
+            :length,
+            length,
+            [],
+            rest,
+            count + 3 + 3 + 1 + 15,
+            version_sum + version
+          )
+
+        <<@sub_packets_length_type::1, num_sub_packets::11, rest::bits>> ->
+          decode_sub_packets_by(
+            :quantity,
+            num_sub_packets,
+            [],
+            rest,
+            count + 3 + 3 + 1 + 11,
+            version_sum + version
+          )
+      end
+
+    {{:operator, operator_type, sub_packets}, rest, count, version_sum}
+  end
+
+  def decode_literal(<<@literal_part::1, group::bits-4, rest::bits>>, acc, count, version_sum) do
+    decode_literal(rest, <<acc::bits, group::bits>>, count + 5, version_sum)
+  end
+
+  def decode_literal(<<@literal_end::1, group::bits-4, rest::bits>>, acc, count, version_sum) do
+    literal_binary = <<acc::bits, group::bits>>
+    <<literal::size(bit_size(literal_binary))>> = literal_binary
+    {{:literal, literal}, rest, count + 5, version_sum}
+  end
+
+  def decode_sub_packets_by(_method, 0, decoded_packets, bits, count, version_sum) do
+    {Enum.reverse(decoded_packets), bits, count, version_sum}
+  end
+
+  def decode_sub_packets_by(method, remaining, decoded_packets, bits, count, version_sum) do
+    {decoded_packet, rest, packet_size, version_sum} = decode_packet(bits, 0, version_sum)
+
+    remaining =
+      case method do
+        :length -> remaining - packet_size
+        :quantity -> remaining - 1
+      end
+
+    decode_sub_packets_by(
+      method,
+      remaining,
+      [decoded_packet | decoded_packets],
+      rest,
+      count + packet_size,
+      version_sum
+    )
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> Base.decode16!()
+    else
+      _ -> :error
+    end
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def(run) do
+    case input() do
+      :error -> print_usage()
+      input -> run_parts_with_timer(input)
+    end
+  end
+
+  defp run_parts_with_timer(input) do
+    run_with_timer(1, fn -> part1(input) end)
+    run_with_timer(2, fn -> part2(input) end)
+  end
+
+  defp run_with_timer(part, fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
+    IO.puts("#{result}\n")
+  end
+
+  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
+  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
+  defp format_time(μsec), do: "#{μsec / 1_000_000}s"
+
+  defp print_usage do
+    IO.puts("Usage: elixir day16.exs input_filename")
+  end
+end
+
+# Day16.run()