Day01.cs

using AdventOfCode.CSharp.Common;
using System;
using System.Collections.Generic;

namespace AdventOfCode.CSharp.Y2016.Solvers;

public class Day01 : ISolver
{
    private const uint Up = 1;
    private const uint Down = unchecked((uint)-Up);
    private const uint Right = 1 << 16;
    private const uint Left = unchecked((uint)-Right);

    public static void Solve(ReadOnlySpan<byte> input, Solution solution)
    {
        // assume the origin is at 1 << 15, 1 << 15
        const ushort xOrigin = 1 << 15;
        const ushort yOrigin = 1 << 15;

        // pack the x and y ushorts into a uint
        uint pos = unchecked((uint)xOrigin << 16 | yOrigin);
        uint dir = Up;

        var seenLocations = new HashSet<uint> { pos };
        int distanceToFirstRepeatedLocation = -1;
        var reader = new SpanReader(input.TrimEnd((byte)'\n'));
        while (true)
        {
            dir = MakeTurn(dir, reader.Peek() == 'L');
            reader.SkipLength(1);

            uint distance = (uint)reader.ReadPosIntUntil(',');
            if (distanceToFirstRepeatedLocation == -1)
            {
                for (uint i = 0; i < distance; i++)
                {
                    pos = unchecked(pos + dir);
                    if (!seenLocations.Add(pos))
                    {
                        distanceToFirstRepeatedLocation = ManhattanDistance(pos);
                    }
                }
            }
            else
            {
                pos = unchecked(pos + dir * distance);
            }

            if (reader.Done)
                break;

            reader.SkipLength(1);
        }

        int distanceToDestination = ManhattanDistance(pos);

        solution.SubmitPart1(distanceToDestination);
        solution.SubmitPart2(distanceToFirstRepeatedLocation);

        static uint MakeTurn(uint dir, bool isLeft) => dir switch
        {
            Left => isLeft ? Down : Up,
            Down => isLeft ? Right : Left,
            Right => isLeft ? Up : Down,
            Up => isLeft ? Left : Right,
            _ => default,
        };

        static int ManhattanDistance(uint pos)
        {
            int xAbs = Math.Abs((int)(pos >> 16) - xOrigin);
            int yAbs = Math.Abs((int)(pos & 0xFFFF) - yOrigin);
            return xAbs + yAbs;
        }
    }
}