AdventOfCode/AoC_2025/Day07.cs

using System.Collections.Generic;
using System.Data.Common;
using System.IO;
using System.Linq;
using AdvenOfCode.Contracts;
using Coordinate = (int x, int y);

namespace AoC_2025;

public class Day07 : IPuzzleSolver<long>
{
    private string[] ParsePuzzleInput(string path)
    {
        var puzzleInput = File.ReadAllLines(path)
            .Where(line => !string.IsNullOrWhiteSpace(line))
            .ToArray();
        return puzzleInput;
    }

    public long SolvePart1(string pathToPuzzleInput)
    {
        var grid = ParsePuzzleInput(pathToPuzzleInput);
        var start = GetStart(grid);
        var count = TraverseGridAndCountSplitters(grid, start);
        return count;
    }
    
    public long SolvePart2(string pathToPuzzleInput)
    {
        var grid = ParsePuzzleInput(pathToPuzzleInput);
        var start = GetStart(grid);
        var nodesWithCount = GetTimelinesCountRecursive(grid, start, []);
        return nodesWithCount;
    }
    
    public long SolvePart2Again(string pathToPuzzleInput)
    {
        var grid = ParsePuzzleInput(pathToPuzzleInput);
        var nodesWithCount = GetTimelinesCount(grid);
        return nodesWithCount.Sum();
    }

    private Coordinate GetStart(string[] grid)
    {
        return (0, grid[0].IndexOf('S'));
    }

    private long TraverseGridAndCountSplitters(string[] grid, Coordinate start)
    {
        HashSet<Coordinate> visited = [start];
        var seen = new Queue<Coordinate>();
        seen.Enqueue(start);
        var splittersHit = 0L;
        while (seen.TryDequeue(out var current))
        {
            if (current.x >= grid.Length) continue;
            if (current.y >= grid[0].Length || current.y < 0) continue;

            if (grid[current.x][current.y] == '^')
            {
                splittersHit++;
                CheckAndEnqueueIfNotVisited((current.x, current.y - 1));
                CheckAndEnqueueIfNotVisited((current.x, current.y + 1));
            }
            else
            {
                CheckAndEnqueueIfNotVisited((current.x + 1, current.y));
            }
        }

        return splittersHit;
        
        void CheckAndEnqueueIfNotVisited(Coordinate coordinate)
        {
            if (visited.Add(coordinate)) seen.Enqueue(coordinate);
        }
    }

    // Dictionary only for memoization
    private long GetTimelinesCountRecursive(string[] grid, Coordinate current, Dictionary<Coordinate, long> memory)
    {
        if (current.x >= grid.Length)
        {
            return 1;
        }
        if(memory.TryGetValue(current, out var count)) return count;

        if (grid[current.x][current.y] == '^')
        {
            var resLeft = GetTimelinesCountRecursive(grid, (current.x, current.y - 1), memory);
            var resRight = GetTimelinesCountRecursive(grid, (current.x, current.y + 1), memory);
            memory[current] = resLeft + resRight;
            return resLeft + resRight;
        }
        
        var res = GetTimelinesCountRecursive(grid, (current.x + 1, current.y), memory);
        memory[current] = res;
        return res;
    }

    private long[] GetTimelinesCount(string[] grid)
    {
        var numbersGrid = grid
            .Select(line => line.Select(c => 0L).ToArray())
            .ToArray();

        for (var x = 1; x < numbersGrid.Length; x++)
        {
            for (var y = 0; y < numbersGrid[x].Length; y++)
            {
                var gridValue = grid[x - 1][y];
                switch (gridValue)
                {
                    case 'S':
                        numbersGrid[x][y] = 1;
                        break;
                    case '.':
                        numbersGrid[x][y] += numbersGrid[x - 1][y];
                        break;
                    case '^':
                        numbersGrid[x][y - 1] += numbersGrid[x - 1][y];
                        numbersGrid[x][y + 1] += numbersGrid[x - 1][y];
                        break;
                }
            }
        }

        return numbersGrid[^1];
    }
}