Project-AOC-2024/Day10/Program.cs

var rawTopographicMap = File.ReadLines("input.txt");
var topographicMap = CreateTopographicMap(rawTopographicMap);

var availableDirections = new List<(int, int)>()
{
    (-1, 0), // Up
    (1, 0), // Down
    (0, -1), // Left
    (0, 1) // Right
};

Console.WriteLine($"Part1: {Part1(topographicMap, availableDirections)}\nPart2: {Part2(topographicMap, availableDirections)}");


int Part2(int[,] mapInput, List<(int, int)> directions)
{
    var totalRating = 0;
    for (var r = 0; r < mapInput.GetLength(0); r++)
    {
        for (var c = 0; c < mapInput.GetLength(1); c++)
        {
            if (mapInput[r, c] == 0)
            {
                totalRating += FindDistinctTrails(r, c, mapInput, directions);
            }
        }
    }

    return totalRating;
}

// Could reuse the trailHead method with minor edits 
int FindDistinctTrails(int r, int c, int[,] mapInput, List<(int, int)> directions)
{
    var stack = new Stack<List<(int, int)>>();
    stack.Push(new List<(int, int)> { (r, c) });
    var trails = 0;

    while (stack.Count > 0)
    {
        var currentPath = stack.Pop();
        var (cr, cc) = currentPath[^1];
        
        if (mapInput[cr, cc] == 9)
        {
            trails++;
            continue;
        }

        foreach (var (dr, dc) in directions)
        {
            var nr = cr + dr;
            var nc = cc + dc;
            
            if (nr >= 0 && nr < mapInput.GetLength(0) && nc >= 0 && nc < mapInput.GetLength(1) &&
                mapInput[nr, nc] == mapInput[cr, cc] + 1)
            {
                var nextPath = new List<(int, int)>(currentPath);
                nextPath.Add((nr, nc));
                stack.Push(nextPath);
            }
        }
    }

    return trails;
}

int Part1(int[,] map, List<(int, int)> directions)
{
    var totalScore = 0;

    for (var row = 0; row < map.GetLength(0); row++)
    {
        for (var column = 0; column < map.GetLength(1); column++)
        {
            if (map[row, column] == 0)
            {
                totalScore += TrailHead(row, column, map, directions);
            }
        }
    }

    return totalScore;
}

int TrailHead(int startRow, int startColumn, int[,] map, List<(int, int)> directions)
{
    var rows = map.GetLength(0);
    var cols = map.GetLength(1);
    var queue = new Queue<(int, int)>();
    var visited = new HashSet<(int, int)>();
    var reachableNines = new HashSet<(int, int)>();

    queue.Enqueue((startRow, startColumn));

    while (queue.Count > 0)
    {
        var (r, c) = queue.Dequeue();

        if (visited.Contains((r, c)))
            continue;

        visited.Add((r, c));

        if (map[r, c] == 9)
        {
            reachableNines.Add((r, c));
            continue;
        }

        foreach (var dir in directions)
        {
            var nr = r + dir.Item1;
            var nc = c + dir.Item2;

            if (nr >= 0 && nr < rows && nc >= 0 && nc < cols)
            {
                if (map[nr, nc] == map[r, c] + 1)
                {
                    queue.Enqueue((nr, nc));
                }
            }
        }
    }

    return reachableNines.Count;
}

int[,] CreateTopographicMap(IEnumerable<string> lines)
{
    var lineList = lines.Select(line => line.Trim()).ToList();
    var map = new int[lineList.Count, lineList[0].Length];

    for (var r = 0; r < map.GetLength(0); r++)
    {
        for (var c = 0; c < map.GetLength(1); c++)
        {
            map[r, c] = lineList[r][c] - '0';
        }
    }

    return map;
}