Solving Advent of Code Day 7: Splitting Tachyon Beams

Source: Dev.to

I originally posted this post on my blog.

This year, I’m following the Advent of Code. I challenged myself to write “functionalish” solutions. Here’s one of the puzzles I’ve liked the most.

On Day 7 of Advent of Code, we’re studying tachyon beams while we’re stuck in a teleporter.

We’re stuck in a teleporter because we’ve been trying to help the elves with all the Christmas preparation. But each step has complicated things even more, making it a mission impossible. I feel like I’m in an episode of The Mandalorian. But anyway, back to code…

I imagine this puzzle as a game where each iteration moves the beam until it hits the end of the manifold.

---

Moving a beam

A Beam is a list of positions in the manifold at a given iteration.

record Position(int X, int Y);
record Beam(IEnumerable<Location> Locations);

A manifold is an array‑of‑array of strings. Each cell could be an empty space or a splitter, e.g.:

var manifold = new string[][]
{
    new[] { ".", ".", "S", ".", "." },
    new[] { ".", ".", ".", ".", "." },
    new[] { ".", ".", "^", ".", "." },
    new[] { ".", ".", ".", ".", "." }
};

The method that moves a beam

static Beam Move(string[][] manifold, Beam beam)
{
    var newLocations = new HashSet<Position>();

    foreach (var current in beam.Locations)
    {
        var downward = manifold[current.X + 1][current.Y];
        if (downward == ".")
        {
            newLocations.Add(new Position(current.X + 1, current.Y));
        }
        else if (downward == "^")
        {
            // split the beam left and right
            newLocations.Add(new Position(current.X + 1, current.Y - 1));
            newLocations.Add(new Position(current.X + 1, current.Y + 1));
        }
    }

    return new Beam(newLocations);
}

Finding the entry position

static Beam Start(string[][] manifold)
{
    for (int i = 0; i ());
}

With those two methods a beam can be created and moved downward:

var start       = Start(manifold);
var newPosition = Move(manifold, start);
newPosition     = Move(manifold, newPosition);

All that’s left is to count splits and move the beam to the end.

---

Counting splits

We let the beam move on its own until it reaches the bottom of the manifold:

var beam = Start(manifold);
while (!HasReachedTheEnd(manifold, beam))
{
    beam = Move(manifold, beam);
}

The next step is to count when a beam splits. I do that inside Move().

Full solution (sample input is hard‑coded)

var manifold = new string[][]
{
    new[] { ".", ".", ".", ".", ".", ".", ".", "S", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", "^", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", "^", ".", "^", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", "^", ".", "^", ".", "^", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", "^", ".", "^", ".", ".", ".", "^", ".", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", ".", "^", ".", "^", ".", ".", ".", "^", ".", "^", ".", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", ".", "^", ".", ".", ".", "^", ".", ".", ".", ".", ".", "^", ".", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", "." },
    new[] { ".", "^", ".", "^", ".", "^", ".", "^", ".", "^", ".", ".", ".", "^", "." },
    new[] { ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", ".", "." }
};

var beam = Start(manifold);
while (!HasReachedTheEnd(manifold, beam))
{
    beam = Move(manifold, beam);
}

Console.WriteLine(beam.SplitCount);
Console.ReadLine();

static Beam Start(string[][] manifold)
{
    for (int i = 0; i ());
}

static bool HasReachedTheEnd(string[][] manifold, Beam beam)
{
    var anyBeam = beam.Locations.First();
    return anyBeam.X >= manifold.Length - 1;
}

static Beam Move(string[][] manifold, Beam beam)
{
    var splits       = 0;
    var newLocations = new HashSet<Position>();

    foreach (var current in beam.Locations)
    {
        var downward = manifold[current.X + 1][current.Y];
        if (downward == ".")
        {
            newLocations.Add(new Position(current.X + 1, current.Y));
        }
        else if (downward == "^")
        {
            splits++;
            newLocations.Add(new Position(current.X + 1, current.Y - 1));
            newLocations.Add(new Position(current.X + 1, current.Y + 1));
        }
    }

    return new Beam(beam.SplitCount + splits, newLocations);
}

record Position(int X, int Y);
record Beam(int SplitCount, IEnumerable<Position> Locations);

---

I thought “tachyon” was a made‑up word until I Googled it. TIL what a tachyon actually is.

A tachyon is. Win‑win.

Et voilà!

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