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namespace aoc2021;
/// <summary>
/// Day 16: <see href="https://adventofcode.com/2021/day/16"/>
/// </summary>
public sealed class Day16 : Day
{
private readonly Packet _packet;
public Day16() : base(16, "Packet Decoder")
{
var bits = string.Join(string.Empty,
Input.First().Select(c => Convert.ToString(Convert.ToInt32(c.ToString(), 16), 2).PadLeft(4, '0')));
(_packet, _) = Packet.FromBinaryString(bits);
}
private record Packet(int Version, int TypeId, long Value, List<Packet> Packets)
{
public static (Packet packet, int offset) FromBinaryString(string input)
{
var index = 0;
var subPackets = new List<Packet>();
var version = Convert.ToInt32(input[..(index += 3)], 2);
var typeId = Convert.ToInt32(input[index..(index += 3)], 2);
if (typeId == 4)
{
// value packet, gather value bits and return right away
var literalBits = new StringBuilder();
foreach (var chunk in input.Skip(index).Chunk(5))
{
literalBits.Append(chunk[1..5]);
index += 5;
if (chunk[0] == '0') break;
}
return (new(version, typeId, Convert.ToInt64(literalBits.ToString(), 2), new()), index);
}
switch (input[index++])
{
case '0':
var subPacketLength = Convert.ToInt32(input[index..(index += 15)], 2);
while (subPacketLength > 0)
{
var (packet, offset) = FromBinaryString(input[index..(index + subPacketLength)]);
subPackets.Add(packet);
subPacketLength -= offset;
index += offset;
}
break;
case '1':
foreach (var _ in Enumerable.Range(0, Convert.ToInt32(input[index..(index += 11)], 2)))
{
var (packet, offset) = FromBinaryString(input[index..]);
subPackets.Add(packet);
index += offset;
}
break;
}
return (new(version, typeId, 0, subPackets), index);
}
public long VersionTotal => Version + Packets.Sum(p => p.VersionTotal);
public long Eval =>
TypeId switch
{
0 => Packets.Sum(p => p.Eval),
1 => Packets.Aggregate(1L, (p, i) => p * i.Eval),
2 => Packets.Min(p => p.Eval),
3 => Packets.Max(p => p.Eval),
4 => Value,
5 => Packets[0].Eval > Packets[1].Eval ? 1 : 0,
6 => Packets[0].Eval < Packets[1].Eval ? 1 : 0,
7 => Packets[0].Eval == Packets[1].Eval ? 1 : 0,
_ => throw new ArgumentException("invalid packet type", nameof(TypeId))
};
}
public override object Part1() => _packet.VersionTotal;
public override object Part2() => _packet.Eval;
}
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