Advent of Code - 2025 - Day 6

var input = await File.ReadAllLinesAsync("input.txt");

var parsedInput = input
    .Select(line => line.Trim().Split(' ', StringSplitOptions.RemoveEmptyEntries))
    .ToArray();
var columns = new List<Column>();

for (var i = 0; i < parsedInput[0].Length; i++)
{
    var values = parsedInput[..^1].Select((line, idx) => line[i]).ToArray();
    var operation = parsedInput[^1][i][0];
    columns.Add(new Column(values, operation));
}

Console.WriteLine($"Part 1: {columns.Sum(col => col.Compute())}");

columns.Clear();
var lineLength = input[0].Length;
var caret = 0;
var lineCount = input.Length - 1;

while (caret < lineLength)
{
    var values = new string[lineCount];

    var segmentLength = 0;

    var endReached = false;
    var isSpace = false;

    //Calculate segment length from bottom row
    while (!endReached)
    {
        if (caret + segmentLength >= lineLength)
        {
            // End of line reached - add one more to align with previous segments
            segmentLength++;
            break;
        }

        var currentChar = input[lineCount][caret + segmentLength];

        if (isSpace && currentChar != ' ')
        {
            endReached = true;
            continue;
        }

        segmentLength++;
        isSpace = currentChar == ' ';
    }

    // Extract segments from each line
    for (var line = 0; line < lineCount; line++)
    {
        values[line] = input[line].Substring(caret, segmentLength - 1);
    }

    var operation = input[lineCount][caret];
    columns.Add(new Column(values, operation));
    caret += segmentLength;
}

Console.WriteLine($"Part 2: {columns.Sum(col => col.ComputeRtl())}");

record struct Column(string[] Values, char Operation)
{
    public readonly long Compute() => Operation switch
    {
        '+' => Values.Sum(long.Parse),
        '*' => Values.Aggregate(1L, (a, b) => a * long.Parse(b)),
        _ => throw new InvalidOperationException($"Unknown operation: {Operation}")
    };

    public readonly long ComputeRtl()
    {
        var parsedValues = new string[Values[0].Length];

        for (var col = 0; col < Values[0].Length; col++)
        {
            var sb = new System.Text.StringBuilder();
            for (var row = 0; row < Values.Length; row++)
            {
                sb.Append(Values[row][col]);
            }
            parsedValues[col] = sb.ToString().Trim();
        }

        return Operation switch
        {
            '+' => parsedValues.Sum(long.Parse),
            '*' => parsedValues.Aggregate(1L, (a, b) => a * long.Parse(b)),
            _ => throw new InvalidOperationException($"Unknown operation: {Operation}")
        };
    }
}