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