Day12.cs

using System;
using System.Runtime.CompilerServices;
using AdventOfCode.CSharp.Common;

namespace AdventOfCode.CSharp.Y2020.Solvers;

public class Day12 : ISolver
{
    public static void Solve(ReadOnlySpan<byte> input, Solution solution)
    {
        // Part 1
        int x1 = 0, y1 = 0;
        int dx1 = 1, dy1 = 0;

        // Part 2
        int x2 = 0, y2 = 0;
        int dx2 = 10, dy2 = 1;

        var reader = new SpanReader(input);
        while (!reader.Done)
        {
            byte dir = reader.Peek();
            reader.SkipLength(1);
            int amount = reader.ReadPosIntUntil('\n');

            switch (dir)
            {
                case (byte)'N':
                    y1 += amount;
                    dy2 += amount;
                    break;
                case (byte)'S':
                    y1 -= amount;
                    dy2 -= amount;
                    break;
                case (byte)'E':
                    x1 += amount;
                    dx2 += amount;
                    break;
                case (byte)'W':
                    x1 -= amount;
                    dx2 -= amount;
                    break;
                case (byte)'L':
                    for (int i = amount; i > 0; i -= 90)
                    {
                        RotateLeft(ref dx1, ref dy1);
                        RotateLeft(ref dx2, ref dy2);
                    }
                    break;
                case (byte)'R':
                    for (int i = amount; i > 0; i -= 90)
                    {
                        RotateRight(ref dx1, ref dy1);
                        RotateRight(ref dx2, ref dy2);
                    }
                    break;
                case (byte)'F':
                    x1 += amount * dx1;
                    y1 += amount * dy1;

                    x2 += amount * dx2;
                    y2 += amount * dy2;
                    break;
            }
        }

        int part1 = Math.Abs(x1) + Math.Abs(y1);
        int part2 = Math.Abs(x2) + Math.Abs(y2);
        solution.SubmitPart1(part1);
        solution.SubmitPart2(part2);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static void RotateLeft(ref int dx, ref int dy)
    {
        int temp = dx;
        dx = -dy;
        dy = temp;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static void RotateRight(ref int dx, ref int dy)
    {
        int temp = dx;
        dx = dy;
        dy = -temp;
    }
}