AdventOfCode/AdventOfCode.Solutions/2024/Day 05/Day05.cs

using AdventOfCode.Core.Shared.IO;

namespace AdventOfCode.Solutions._2024
{
    public class Day05 : IChallange
    {
        public int Year => 2024;

        public int Day => 5;

        private readonly IInputReader _inputReader;

        public Day05(IInputReader inputReader)
        {
            _inputReader = inputReader;
            _inputReader.SetInput(this);
        }

        // 143
        // 5651
        public async Task<string> GetSolutionPart1()
        {
            int total = 0;
            await foreach ((Rule[] rules, int[] updates) in GetUpdateWithRules(true))
            {
                total += updates[(updates.Length - 1) / 2];
            }

            return total.ToString();
        }

        // 123
        // 4743
        public async Task<string> GetSolutionPart2()
        {
            int total = 0;
            await foreach ((Rule[] rules, int[] updates) in GetUpdateWithRules(false))
            {
                Rule[] requiredRules = rules;
                int A = 0,
                    runs = (updates.Length + 1) / 2;

                for (int run = 0; run < runs; run++)
                {
                    int[] B = requiredRules.Select(r => r.B).Distinct().ToArray();
                    A = requiredRules.Where(r => !B.Contains(r.A)).Select(r => r.A).First();
                    requiredRules = requiredRules.Where(r => r.A != A).ToArray();
                }

                total += A;
            }

            return total.ToString();
        }

        private struct Rule
        {
            public int A { get; set; }
            public int B { get; set; }
        }

        private async IAsyncEnumerable<(Rule[] rules, int[] updates)> GetUpdateWithRules(bool returnValidSet = true)
        {
            List<Rule> rules = [];
            bool inputComplete = false;
            await foreach (string updateString in _inputReader.ReadAsStringLine())
            {
                if (!inputComplete)
                {
                    if (string.IsNullOrEmpty(updateString))
                        inputComplete = true;
                    else
                        rules.Add(ParseRule(updateString));
                    continue;
                }

                int[] updatePages = updateString.Split(',').Select(int.Parse).ToArray();
                Rule[] requiredRules = rules
                    .Where(r => updatePages.Contains(r.A))
                    .Where(r => updatePages.Contains(r.B))
                    .ToArray();

                if (IsValid(requiredRules, updatePages) == returnValidSet)
                {
                    yield return (requiredRules, updatePages);
                }
            }
        }

        private static bool IsValid(Rule[] rules, int[] updates)
        {
            Rule[] requiredRules = rules;
            List<Rule> openRules = [];
            foreach (int update in updates)
            {
                // check if the current update is in the required list as the second value, if so this is an invalid update
                if (requiredRules.Any(r => r.B == update))
                {
                    return false;
                }

                // get new rules
                openRules.AddRange(requiredRules.Where(r => r.A == update));
                requiredRules = requiredRules.Where(r => r.A != update).ToArray();

                // close open rules
                openRules = openRules.Where(r => r.B != update).ToList();
            }

            return true;
        }

        private static Rule ParseRule(string input) 
        {
            int[] numbers = input.Split('|').Select(int.Parse).ToArray();
            return new Rule { A = numbers[0], B = numbers[1] };
        }
    }
}