Advent of Code 2023 - Day 18

var input = File.ReadAllLines("input.txt");

var polygon1 = new List<(long Row, long Col)>();
(long Row, long Col) currentPosition1 = (0, 0);
var circumference1 = 0.0;

var polygon2 = new List<(long Row, long Col)>();
(long Row, long Col) currentPosition2 = (0, 0);
var circumference2 = 0.0;

foreach (var line in input)
{
    polygon1.Add(currentPosition1);
    var move = line.Split(' ');

    var length1 = int.Parse(move[1]);
    currentPosition1 = move[0] switch
    {
        "R" => (currentPosition1.Row, currentPosition1.Col + length1),
        "D" => (currentPosition1.Row + length1, currentPosition1.Col),
        "L" => (currentPosition1.Row, currentPosition1.Col - length1),
        "U" => (currentPosition1.Row - length1, currentPosition1.Col),
        _ => throw new Exception("Unknown direction")
    };

    circumference1 += length1;

    polygon2.Add(currentPosition2);
    var hex = move[2].TrimStart('(').TrimEnd(')');
    var length2 = long.Parse(hex[1..^1], System.Globalization.NumberStyles.HexNumber);

    currentPosition2 = hex.Last() switch
    {
        '0' => (currentPosition2.Row, currentPosition2.Col + length2), // R
        '1' => (currentPosition2.Row + length2, currentPosition2.Col), // D
        '2' => (currentPosition2.Row, currentPosition2.Col - length2), // L
        '3' => (currentPosition2.Row - length2, currentPosition2.Col), // U
        _ => throw new Exception("Unknown direction")
    };

    circumference2 += length2;
}

Console.WriteLine($"Part 1: {Area(polygon1) + circumference1 / 2 + 1}");
Console.WriteLine($"Part 2: {Area(polygon2) + circumference2 / 2 + 1}");
return;

// Shoelace formula, found it on the internet:
// https://rosettacode.org/wiki/Shoelace_formula_for_polygonal_area#C#
static double Area(List<(long Row, long Col)> polygon)
{
    var n = polygon.Count;
    var result = 0.0;
    for (var i = 0; i < n - 1; i++)
    {
        result += polygon[i].Row * polygon[i + 1].Col - polygon[i + 1].Row * polygon[i].Col;
    }

    result = Math.Abs(result + polygon[n - 1].Row * polygon[0].Col - polygon[0].Row * polygon[n - 1].Col) / 2.0;
    return result;
}