AdventOfCode/AoC_2025/Day04.cs

using AdvenOfCode.Contracts;
using AdventOfCode.HelperClasses;

namespace AoC_2025;

public class Day04 : IPuzzleSolver<long>
{
    private string[] ParsePuzzleInput(string path)
    {
        var puzzleInput = File.ReadAllLines(path)
            .Where(line => !string.IsNullOrWhiteSpace(line))
            .Select(line => line.Trim())
            .ToArray();
        return puzzleInput;
    }

    public long SolvePart1(string pathToPuzzleInput)
    {
        var paperrollGrid = ParsePuzzleInput(pathToPuzzleInput);
        var accessiblePaperrolls = GetAccessiblePaperrolls(paperrollGrid);
        var countAccessibleRolls = accessiblePaperrolls.Count();
        return countAccessibleRolls;
    }
    
    public long SolvePart2(string pathToPuzzleInput)
    {
        var paperrollGrid = ParsePuzzleInput(pathToPuzzleInput);
        var rollCoordinates = GetRollCoordinates(paperrollGrid);
        var countAccessibleRolls = RecursiveCountAllRemovableRolls(rollCoordinates);
        return countAccessibleRolls;
    }

    private IEnumerable<Coordinate> GetAccessiblePaperrolls(IEnumerable<string> paperrollGrid)
    {
        var rollCoordinates = GetRollCoordinates(paperrollGrid);
        var rollsWithNeighbours = GetNeighbours(rollCoordinates);
        var accessiblePaperrolls = FilterForLessThan(rollsWithNeighbours, 4);
        return accessiblePaperrolls;
    }
    
    private int RecursiveCountAllRemovableRolls(HashSet<Coordinate> coordinates)
    {
        var rollsWithNeighbours = GetNeighbours(coordinates);
        var accessiblePaperrolls = FilterForLessThan(rollsWithNeighbours, 4).ToArray();
        var countAccessibleRolls = accessiblePaperrolls.Length;
        coordinates.ExceptWith(accessiblePaperrolls);
        return countAccessibleRolls == 0
            ? 0
            : countAccessibleRolls + RecursiveCountAllRemovableRolls(coordinates);
    }

    private HashSet<Coordinate> GetRollCoordinates(IEnumerable<string> paperrollGrid)
    {
        var coordinates = paperrollGrid
            .SelectMany((line, xIndex) => GetRollCoordinates(line, xIndex))
            .ToHashSet();
        return coordinates;
    }

    private IEnumerable<Coordinate> GetRollCoordinates(string gridLine, int xIndex)
    {
        var indexedCharacters = gridLine.Index();
        var validRolls = indexedCharacters.Where(tuple => tuple.Item == '@');
        var rollCoordinates = validRolls.Select(tuple => new Coordinate(xIndex, tuple.Index));
        return rollCoordinates;
    }

    private IEnumerable<(Coordinate coordinate, Coordinate[] neighbours)> GetNeighbours(HashSet<Coordinate> rollCoordinates)
    {
        var coordsWithNeighbourCount = rollCoordinates
            .Select(coord => (coord, GetNeighbours(coord, rollCoordinates)));
        return coordsWithNeighbourCount;
    }

    private Coordinate[] GetNeighbours(Coordinate coordinate, HashSet<Coordinate> coordinates)
    {
        Coordinate[] neighbourDirections =
        [
            new(-1, -1), new(-1, 0), new(-1, 1),
            new(0, -1),              new(0, 1),
            new(1, -1),  new(1, 0),  new(1, 1),
        ];
        var possibleNeighbours = neighbourDirections
            .Select(direction => coordinate + direction);
        var actualNeighbours = possibleNeighbours
            .Where(coord => coordinates.Contains(coord))
            .ToArray();
        return actualNeighbours;
    }

    private IEnumerable<Coordinate> FilterForLessThan(IEnumerable<(Coordinate coordinate, Coordinate[] neighbours)> coordinatesWithNeighbours,
        int lessThan)
    {
        var coordinatesWithLess = coordinatesWithNeighbours
            .Where(roll => roll.neighbours.Length < lessThan)
            .Select(roll => roll.coordinate);
        return coordinatesWithLess;
    }
}