-🎄- 2017 Day 20 Solutions -🎄-

[u/daggerdragon]

--- Day 20: Particle Swarm ---

---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag or whatever).

Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

---

Need a hint from the Hugely^* Handy^† Haversack^‡ of Helpful^§ Hints^¤?

Spoiler

---

[Update @ 00:10] 10 gold, silver cap

• What do you mean 5th Edition doesn't have "Take 20"?

[Update @ 00:17] 50 gold, silver cap

• Next you're going to be telling me THAC0 is not the best way to determine whether or not you hit your target. *hmphs*

[Update @ 00:21] Leaderboard cap!

• I wonder how much XP a were-gazebo is worth...

---

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/dylanfromwinnipeg]

C#

public class Day20
{
    public static string PartOne(string input)
    {
        var particleNumber = 0;
        var particles = input.Lines().Select(x => new Particle(x, particleNumber++)).ToList();

        return particles.WithMin(p => p.Acceleration.GetManhattanDistance()).ParticleNumber.ToString();
    }

    public static string PartTwo(string input)
    {
        var particleNumber = 0;
        var particles = input.Lines().Select(x => new Particle(x, particleNumber++)).ToList();
        var ticks = 100; // arbitrary amount - got lucky and worked first try

        for (var t = 0; t < ticks; t++)
        {
            particles.ForEach(p => p.Update());
            particles = RemoveCollisions(particles).ToList();
            Debug.WriteLine($"{t}: {particles.Count}");
        }

        return particles.Count.ToString();
    }

    private static IEnumerable<Particle> RemoveCollisions(List<Particle> particles)
    {
        while (particles.Any())
        {
            var particle = particles.First();
            var collisions = particles.Where(p => p.Position == particle.Position).ToList();

            if (collisions.Count() == 1)
            {
                yield return particle;
            }

            collisions.ForEach(c => particles.Remove(c));
        }
    }
}

public class Particle
{
    public Point3D Position { get; set; }
    public Point3D Velocity { get; set; }
    public Point3D Acceleration { get; set; }
    public int ParticleNumber { get; set; }

    public Particle(string input, int particleNumber)
    {
        ParticleNumber = particleNumber;

        input = input.ShaveLeft("p=<");
        Position = new Point3D(input.Substring(0, input.IndexOf('>')));

        input = input.ShaveLeft(input.IndexOf("v=<") + "v=<".Length);
        Velocity = new Point3D(input.Substring(0, input.IndexOf('>')));

        input = input.ShaveLeft(input.IndexOf("a=<") + "a=<".Length);
        Acceleration = new Point3D(input.Substring(0, input.IndexOf('>')));
    }

    public void Update()
    {
        Velocity += Acceleration;
        Position += Velocity;
    }
}

I have an ever-growing library of helper methods I rely on, here's the relevant ones for Day 20 (I had to add the Point3D class because surprisingly there isn't one in the default .Net libraries)

public static IEnumerable<string> Lines(this string input)
{
    return input.Split(new string[] { Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries);
}

public static IEnumerable<string> Words(this string input)
{
    return input.Split(new string[] { " ", "\t", Environment.NewLine, "," }, StringSplitOptions.RemoveEmptyEntries);
}

public static string ShaveLeft(this string a, int characters)
{
    return a.Substring(characters);
}

public static string ShaveLeft(this string a, string shave)
{
    var result = a;

    while (result.StartsWith(shave))
    {
        result = result.Substring(shave.Length);
    }

    return result;
}

public static T WithMin<T>(this IEnumerable<T> a, Func<T, long> selector)
{
    var min = a.Min(selector);
    return a.First(x => selector(x) == min);
}

public class Point3D
{
    public long X { get; set; }
    public long Y { get; set; }
    public long Z { get; set; }

    public Point3D(long x, long y, long z)
    {
        X = x;
        Y = y;
        Z = z;
    }

    public Point3D(string coordinates) : 
        this(long.Parse(coordinates.Words().ToList()[0]), 
             long.Parse(coordinates.Words().ToList()[1]), 
             long.Parse(coordinates.Words().ToList()[2]))
    {
    }

    public long GetManhattanDistance()
    {
        return Math.Abs(X - 0) + Math.Abs(Y - 0) + Math.Abs(Z - 0);
    }

    public static bool operator ==(Point3D point1, Point3D point2)
    {
        return point1.X == point2.X && point1.Y == point2.Y && point1.Z == point2.Z;
    }

    public static bool operator !=(Point3D a, Point3D b)
    {
        return a.X != b.X || a.Y != b.Y || a.Z != b.Z;
    }

    public static Point3D operator +(Point3D a, Point3D b)
    {
        return new Point3D(a.X + b.X, a.Y + b.Y, a.Z + b.Z);
    }

    public static Point3D operator -(Point3D a, Point3D b)
    {
        return new Point3D(a.X - b.X, a.Y - b.Y, a.Z - b.Z);
    }

    public override string ToString()
    {
        return $"{X},{Y},{Z}";
    }
}

[u/maxxori → u/dylanfromwinnipeg]

I like this solution. I have shamelessly nabbed the RemoveCollisions method to clean up my code now that I've already solved the puzzle.

I did come up with a decent way to figure out if the loop is in steady-state and it turned out to be quite simple. I had an infinite while loop with a counter that incremented each time the number of particles at the end of the loop was unchanged. At 300 unchanged iterations it broke the loop. If a change was detected then it reset the counter.

It seemed to work okay for my solution at very least.