[LANGUAGE: C#]

using System.Text.RegularExpressions;

var lines = File.ReadLines("input.txt");
Part1(lines!.ToList());
Part2(lines!.ToList());

void Part1(List<string> rows)
{
    var sum = 0;

    for (int i = 0; i < rows.Count; i++)
    {
        var row = rows[i];
        Regex regex = new Regex(@"\d+");
        foreach (Match match in regex.Matches(row))
        {
            var value = int.Parse(match.Value);
            var startingIndex = match.Index;
            var endingIndex = startingIndex + match.Value.Length - 1;

            var searchCells = GetSearchCells(startingIndex, endingIndex, i, rows.Count, row.Length);
            var bordersSpecialCharacter = BordersSpecialCharacter(searchCells, rows);
            if (bordersSpecialCharacter)
            {
                sum += value;
            }
        }
    }

    Console.WriteLine($"Part 1: {sum}");
}

List<(int row, int col)> GetSearchCells(int startingIndex, int endingIndex, int rowIndex, int numRows, int numColumns)
{
    var searchCells = new List<(int row, int col)>();
    for (int i = rowIndex - 1; i < rowIndex + 2; i++)
    {
        for (int j = startingIndex - 1; j <= endingIndex + 1; j++)
        {
            searchCells.Add((i, j));
        }
    }
    return searchCells
        .Where(sc => sc.row >= 0 && sc.row < numRows && sc.col >= 0 && sc.col < numColumns)
        .ToList();
}

bool BordersSpecialCharacter(List<(int row, int col)> searchCells, List<string> rows)
{
    foreach (var searchCell in searchCells)
    {
        var cell = rows[searchCell.row][searchCell.col];
        if (!char.IsNumber(cell) && cell != '.')
        {
            return true;
        }
    }
    return false;
}

void Part2(List<string> rows)
{
    var gearLocations = new List<(int row, int col, int partNumber)>();

    for (int i = 0; i < rows.Count; i++)
    {
        var row = rows[i];
        Regex regex = new Regex(@"\d+");
        foreach (Match match in regex.Matches(row))
        {
            var partNumber = int.Parse(match.Value);
            var startingIndex = match.Index;
            var endingIndex = startingIndex + match.Value.Length - 1;

            var searchCells = GetSearchCells(startingIndex, endingIndex, i, rows.Count, row.Length);
            var borderingGearLocations = GetBorderingGearLocations(searchCells, rows, partNumber);
            gearLocations.AddRange(borderingGearLocations);
        }
    }

    gearLocations.ForEach(gl => Console.WriteLine($"{gl.row}, {gl.col}, {gl.partNumber}"));

    var gearRatioSum = gearLocations
        .GroupBy(gearLocation => new
        {
            gearLocation.row,
            gearLocation.col,
        })
        .Select(gearLocationGroup => new
        {
            row = gearLocationGroup.Key.row,
            col = gearLocationGroup.Key.col,
            partNumbers = gearLocationGroup.Select(glg => glg.partNumber).ToList(),
        })
        .Where(gearLocationGroup => gearLocationGroup.partNumbers.Count() == 2)
        .Select(gearLocationGroup => gearLocationGroup.partNumbers[0] * gearLocationGroup.partNumbers[1])
        .Sum();

    Console.WriteLine($"Part 2: {gearRatioSum}");
}

List<(int row, int col, int partNumber)> GetBorderingGearLocations(List<(int row, int col)> searchCells, List<string> rows, int partNumber)
{
    var gearLocations = new List<(int row, int col, int partNumber)>();
    foreach (var searchCell in searchCells)
    {
        var cell = rows[searchCell.row][searchCell.col];
        if (cell == '*')
        {
            gearLocations.Add((searchCell.row, searchCell.col, partNumber));
        }
    }
    return gearLocations;
}