Day05.cs

using AdventOfCode.CSharp.Common;
using System;

namespace AdventOfCode.CSharp.Y2015.Solvers;

public class Day05 : ISolver
{
    public static void Solve(ReadOnlySpan<byte> input, Solution solution)
    {
        int part1 = 0;
        int part2 = 0;

        foreach (ReadOnlySpan<byte> line in input.SplitLines())
        {
            int numVowels = 0;
            bool containsPair = false;
            bool containsForbiddenPair = false;

            // bitset which keeps track of seen characters.
            // this is used to help with finding repeat pairs.
            int seenChars = 0;
            bool containsRepeatPair = false;
            bool containsPalindromicTriple = false;

            byte prev2 = (byte)'\0';
            byte prev = (byte)'\0';
            for (int i = 0; i < line.Length; i++)
            {
                // Part 1 Rules
                byte cur = line[i];
                if (cur is (byte)'a' or (byte)'e' or (byte)'i' or (byte)'o' or (byte)'u')
                {
                    numVowels++;
                }

                if (cur == prev)
                {
                    containsPair = true;
                }

                if ((prev is (byte)'a' or (byte)'c' or (byte)'p' or (byte)'x') && cur == prev + 1)
                {
                    containsForbiddenPair = true;
                }

                // Part 2 Rules

                // check if we have seen the current and previous letter before
                // if we have, scan the line for a repeat pair
                int prevBitMask = 1 << (prev - 'a');
                if ((seenChars & prevBitMask) > 0 && (seenChars & 1 << (cur - 'a')) > 0)
                {
                    for (int j = 0; j < i - 2; j++)
                    {
                        if (line[j] == prev && line[j + 1] == cur)
                        {
                            containsRepeatPair = true;
                        }
                    }
                }

                if (prev2 == cur)
                {
                    containsPalindromicTriple = true;
                }

                // we only add a letter to the seen characters two iterations after it first appears
                // to avoid scanning the line for an overlap.
                seenChars |= prevBitMask;
                prev2 = prev;
                prev = cur;
            }

            if (numVowels >= 3 && containsPair && !containsForbiddenPair)
            {
                part1++;
            }

            if (containsRepeatPair && containsPalindromicTriple)
            {
                part2++;
            }
        }

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