Day07.cs

using System;
using AdventOfCode.CSharp.Common;

namespace AdventOfCode.CSharp.Y2016.Solvers;

public class Day07 : ISolver
{
    public static void Solve(ReadOnlySpan<byte> input, Solution solution)
    {
        int part1 = 0;
        int part2 = 0;

        byte[] abas = new byte[26 * 26];
        byte[] babs = new byte[26 * 26];
        foreach (Range lineRange in input.SplitLines())
        {
            ReadOnlySpan<byte> line = input[lineRange];
            Array.Clear(abas, 0, abas.Length);
            Array.Clear(babs, 0, babs.Length);

            bool hasAbbaOutsideHypernet = false;
            bool hasAbbaInsideHypernet = false;
            bool supportsSSL = false;

            // c1, c2, c3, c4 defines a sliding window of 4 characters across the line
            byte c1 = line[0], c2 = line[1], c3 = line[2];
            bool insideHypernet = false;
            for (int i = 3; i <= line.Length; i++)
            {
                byte c4 = i == line.Length ? (byte)'\0' : line[i];
                if (c1 == '[')
                {
                    insideHypernet = true;
                }
                else if (c1 == ']')
                {
                    insideHypernet = false;
                }
                else
                {
                    if (c1 == c4 && c2 == c3 && c2 != '[' && c1 != c2) // found an ABBA
                    {
                        if (insideHypernet)
                        {
                            hasAbbaInsideHypernet = true;
                        }
                        else
                        {
                            hasAbbaOutsideHypernet = true;
                        }
                    }

                    if (c1 == c3 && c2 != c1 && c2 != '[' && c2 != ']')
                    {
                        if (insideHypernet)
                        {
                            babs[(c1 - 'a') * 26 + (c2 - 'a')] = 1;
                            if (abas[(c2 - 'a') * 26 + (c1 - 'a')] == 1)
                            {
                                supportsSSL = true;
                            }
                        }
                        else
                        {
                            abas[(c1 - 'a') * 26 + (c2 - 'a')] = 1;
                            if (babs[(c2 - 'a') * 26 + (c1 - 'a')] == 1)
                            {
                                supportsSSL = true;
                            }
                        }
                    }
                }

                // slide the window down
                c1 = c2;
                c2 = c3;
                c3 = c4;
            }

            if (hasAbbaOutsideHypernet && !hasAbbaInsideHypernet)
            {
                part1 += 1;
            }

            if (supportsSSL)
            {
                part2 += 1;
            }
        }

        solution.SubmitPart1(part1);
        solution.SubmitPart2(part2);
    }
}