AdventOfCode/AdventOfCode.Solutions/2023/Day 10/Day10.cs

using AdventOfCode.Core;
using AdventOfCode.Core.Shared.Grid;
using System.Linq.Expressions;

namespace AdventOfCode.Solutions._2023
{
    public class Day10(InputReader reader) : IChallange
    {
        private InputReader _inputReader = reader;

        public async Task<string> GetSolutionPart1()
        {
            Grid<Pipe> grid = await _inputReader.ReadToGrid<Pipe>();
            return GetLoop(grid).Max(p => p.Steps).ToString();
        }

        public async Task<string> GetSolutionPart2()
        {
            Grid<Pipe> grid = await _inputReader.ReadToGrid<Pipe>();
            _ = GetLoop(grid); // marks all the pipes from the main loop

            int inside = 0;

            for (int rowIndex = 0; rowIndex < grid.Rows; rowIndex++)
            {
                char prevSegment = '.';
                int pipesCrossed = 0;
                for (int columnIndex = 0; columnIndex < grid.Columns; columnIndex++)
                {
                    // filter the special cases that need checking
                    Pipe value = grid.GetNode(columnIndex, rowIndex);

                    if (value.Steps >= 0) // this is in the main loop
                    {
                        if (value.Value == '-') // never crosses, also to not update prev
                            continue;

                        if ((prevSegment == 'L' && value.Value == '7') // this is L7 meaning that there is no crossing
                            || prevSegment == 'F' && value.Value == 'J') // this is FJ meaning that there is no crossing
                        {
                            prevSegment = value.Value;
                            continue;
                        }

                        prevSegment = value.Value;
                        pipesCrossed++;
                        continue;
                    }

                    if (pipesCrossed % 2 == 1) // inside the main loop
                        inside++;
                }
            }

            return inside.ToString();
        }

        private List<Pipe> GetLoop(Grid<Pipe> grid)
        {
            Pipe startPipe = grid.FindWithValue('S').First();
            startPipe.Steps = 0;

            Pipe[] lineToWalk = grid.GetNeighbors(startPipe, false)
                .Where(n => n.Value != '.')
                .Where(n => n.GetNeighbors().Any(np => np.X == startPipe.X && np.Y == startPipe.Y))
                .ToArray();

            for (int i = 0; i < lineToWalk.Length; i++)
                lineToWalk[i].From = startPipe;

            List<Pipe> mainLoop = [startPipe];

            int steps = 1;
            while (true)
            {
                
                List<Pipe> nextLoop = [];
                foreach (Pipe line in lineToWalk)
                {
                    line.Steps = steps;

                    // update the pipe in the grid with the steps
                    grid.UpdateNode(line);
                    mainLoop.Add(line);
                    // get the pipe location
                    Pipe next = line.GetNextPipe();

                    // get the actual pipe from the grid
                    next = grid.GetNode(next.X, next.Y);
                    if (next.Steps != -1)
                    {
                        return mainLoop;
                    }
                    next.From = line;

                    nextLoop.Add(next);
                }
                lineToWalk = [.. nextLoop];
                steps++;
            }
        }

        public class Pipe : Point
        {
            public Pipe From { get; set; } = null;

            public int Steps { get; set; } = -1;

            public Pipe GetNextPipe() 
            {
                if (From == null)
                    throw new ArgumentException("From not set");
                var neighbors = GetNeighbors();
                return neighbors.First(n => n.X != From.X || n.Y != From.Y);
            }

            public IEnumerable<Pipe> GetNeighbors()
            {
                switch (Value)
                {
                    case '|':
                        yield return GetNorth();
                        yield return GetSouth();
                        yield break;
                    case '-':
                        yield return GetWest();
                        yield return GetEast();
                        yield break;

                    case 'L':
                        yield return GetNorth();
                        yield return GetEast();
                        yield break;
                    case 'J':
                        yield return GetNorth();
                        yield return GetWest();
                        yield break;
                    case '7':
                        yield return GetSouth();
                        yield return GetWest();
                        yield break;
                    case 'F':
                        yield return GetSouth();
                        yield return GetEast();
                        yield break;
                }
            }

            private Pipe GetNorth() => new() { X = this.X, Y = this.Y - 1 };
            private Pipe GetSouth() => new() { X = this.X, Y = this.Y + 1 };
            private Pipe GetWest() => new() { X = this.X - 1, Y = this.Y };
            private Pipe GetEast() => new() { X = this.X + 1, Y = this.Y };
        }
    }
}