From 13e154a0c294b309fd57ac9b512c61f72522e5a2 Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Wed, 30 Nov 2022 19:16:45 +0900
Subject: [PATCH] 2021 day 16 part 2

---
 2021/day16.exs | 39 ++++++++++++++++++++++++++++-----------
 1 file changed, 28 insertions(+), 11 deletions(-)

diff --git a/2021/day16.exs b/2021/day16.exs
index b0b974c..f457d32 100644
--- a/2021/day16.exs
+++ b/2021/day16.exs
@@ -4,21 +4,17 @@ defmodule Day16 do
     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
+  defp 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
+  defp 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>> ->
@@ -45,21 +41,21 @@ defmodule Day16 do
     {{: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
+  defp 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
+  defp 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
+  defp 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
+  defp 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 =
@@ -78,6 +74,27 @@ defmodule Day16 do
     )
   end
 
+  def part2(input) do
+    {packet, _rest, _count, _version_sum} = decode_packet(input, 0, 0)
+    evaluate(packet)
+  end
+
+  defp evaluate({:literal, literal}), do: literal
+
+  defp evaluate({:operator, op, args}) do
+    args
+    |> Enum.map(&evaluate/1)
+    |> calculate(op)
+  end
+
+  defp calculate(args, 0), do: Enum.sum(args)
+  defp calculate(args, 1), do: Enum.reduce(args, &Kernel.*/2)
+  defp calculate(args, 2), do: Enum.min(args)
+  defp calculate(args, 3), do: Enum.max(args)
+  defp calculate([a, b], 5), do: if(a > b, do: 1, else: 0)
+  defp calculate([a, b], 6), do: if(a < b, do: 1, else: 0)
+  defp calculate([a, b], 7), do: if(a == b, do: 1, else: 0)
+
   def input do
     with [input_filename] <- System.argv(),
          {:ok, input} <- File.read(input_filename) do
@@ -120,4 +137,4 @@ defmodule Day16 do
   end
 end
 
-# Day16.run()
+Day16.run()