The droids have finished repairing as much of the ship as they can. Their report indicates that this was a Category 6 disaster - not because it was that bad, but because it destroyed the stockpile of Category 6 network cables as well as most of the ship's network infrastructure.

You'll need to rebuild the network from scratch.

Read the full puzzle.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using Packets = System.Collections.Generic.List<(long address, long x, long y)>;

namespace AdventOfCode.Y2019.Day23;

[ProblemName("Category Six")]
class Solution : Solver {

    public object PartOne(string input) => Solve(input, false);
    public object PartTwo(string input) => Solve(input, true);

    long Solve(string input, bool hasNat) {
        var machines = (
            from address in Enumerable.Range(0, 50)
            select Nic(input, address)
        ).ToList();

        var natAddress = 255;

        if (hasNat) {
            machines.Add(Nat(natAddress));
        }

        var packets = new Packets();
        while (!packets.Any(packet => packet.address == natAddress)) {
            foreach (var machine in machines) {
                packets = machine(packets);
            }
        }
        return packets.Single(packet => packet.address == natAddress).y;
    }

    (List<long> data, Packets packets) Receive(Packets packets, int address) {
        var filteredPackets = new Packets();
        var data = new List<long>();
        foreach (var packet in packets) {
            if (packet.address == address) {
                data.Add(packet.x);
                data.Add(packet.y);
            } else {
                filteredPackets.Add(packet);
            }
        }
        return (data, filteredPackets);
    }

    Func<Packets, Packets> Nic(string program, int address) {
        var icm = new IntCodeMachine(program);
        var output = icm.Run(address);
        Debug.Assert(output.Count == 0);

        return (input) => {
            var (data, packets) = Receive(input, address);
            if (!data.Any()) {
                data.Add(-1);
            }
            var output = icm.Run(data.ToArray());
            for (var d = 0; d < output.Count; d += 3) {
                packets.Add((output[d], output[d + 1], output[d + 2]));
            }
            return packets;
        };
    }

    Func<Packets, Packets> Nat(int address) {
        long? yLastSent = null;
        long? x = null;
        long? y = null;
        return (input) => {
            var (data, packets) = Receive(input, address);
            if (data.Any()) {
                (x, y) = (data[^2], data[^1]);
            }
            if (packets.Count == 0) {
                Debug.Assert(x.HasValue && y.HasValue);
                packets.Add((y == yLastSent ? 255 : 0, x.Value, y.Value));
                yLastSent = y;
            }
            return packets;
        };
    }
}

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