# Advent of Code 2023 - Day 25

By Eric Burden | January 3, 2024

It’s that time of year again! Just like last year, I’ll be posting my solutions to the Advent of Code puzzles. This year, I’ll be solving the puzzles in Kotlin. I’ll post my solutions and code to GitHub as well. If you haven’t given AoC a try, I encourage you to do so along with me!

Find the problem description HERE.

## The Input - A Proponent of Components

Let’s celebrate Christmas Day with one more input where each line is a unique piece of information and that information has nice, distinct separators! Thanks, Eric Wastl! Unfortunately, since what we want is an undirected graph of (component1 <--> component2), and the input is only guaranteed to include that relationship in one direction, we need to do a bit of work to make sure we have both the forward and reverse edges in our data structure.


/**
* Represents a single plugged in component
*
* These are derived from each of the three-letter names from the input. The
* idea is that comparing integers and copying them around is marginally faster
* than comparing and copying strings. I'm not actually sure if this really
* is faster, but it's a nice idea.
*
* @property id Unique integer identifier for this [Component].
*/
data class Component(val id: Int) {
companion object {
/**
* Parse a [Component] from a name in the input file
*
* Each three-letter name is converted to an integer by bit-shifting
* each character to the left and 'or'-ing all the bits together.
*
* @param str A component name from the input file.
* @return The [Component] represented by the three-letter name.
* @throws Exception When str isn't a three-letter string.
*/
fun fromString(str: String): Component {
require(str.length == 3) {
throw Exception("$str is not a valid [Component] name!") } val (c1, c2, c3) = str.toList().map { it.code } val id = c1.shl(16).or(c2.shl(8)).or(c3) return Component(id) } } } /** * This class represents an edge in the undirected [ComponentGraph] * * This class is useful because we want to represent edges between components * in our graph of components, and since the graph is undirected we want to * consider (A -> B) to be equal to (B -> A). * * @property c1 One [Component] in the edge. * @property c2 Another [Component] in the edge. */ data class ComponentEdge(val c1: Component, val c2: Component) /** * This class represents the graph of connections between [Component]s * * The graph here consists of undirected connections between components. We * know from the puzzle text that there are two groups of heavily interconnected * components separated by three nodes that can be "cut" to produce two * distinct networks. * * @property graph A mapping of one component to the list of components that are * reachable from that component. */ data class ComponentGraph(val graph: MutableMap<Component, MutableList<Component>>) { companion object { /** * Produce an empty [ComponentGraph] * * @return A mutable [ComponentGraph] with no nodes or edges. */ fun empty(): ComponentGraph { val graph = mutableMapOf<Component, MutableList<Component>>() return ComponentGraph(graph) } /** * Parse the lines from the input file into a [ComponentGraph] * * Each line represents a set of undirected connections, such that * "jqt: rhn xhk nvd" can be interpreted as (jqt <--> rhn), * (jqt <--> xhk), (jqt <--> nvd). * * @param lines The lines from the input file. * @return A [ComponentGraph] parsed from the input file. * @throws Exception If any component name fails to parse to a [Component]. */ fun fromInput(lines: List<String>): ComponentGraph { val graph = empty() lines.filter { it.isNotEmpty() }.flatMap { line -> val (keyStr, valueStr) = line.split(": ") val key = Component.fromString(keyStr) val values = valueStr.split(" ").map(Component::fromString) values.map { ComponentEdge(key, it) } }.forEach { graph.addEdge(it) } return graph } } /** * Add an edge to the [ComponentGraph] * * Technically, adds both the forward and reverse edges to the underlying * map. * * @param edge The [ComponentEdge] to add to the graph. */ fun addEdge(edge: ComponentEdge) { val (c1, c2) = edge val c1Values = graph.getOrDefault(c1, mutableListOf()) if (c2 !in c1Values) c1Values.add(c2) graph[c1] = c1Values val c2Values = graph.getOrDefault(c2, mutableListOf()) if (c1 !in c2Values) c2Values.add(c1) graph[c2] = c2Values } } class Day25(input: List<String>) { // Turn that input file into a real graph! private val parsed = ComponentGraph.fromInput(input) }  Why yes, I did jump through a few not totally necessary hoops there, but it’ll all be worth it when we solve the puzzle. Really, I probably should have gone with another builder pattern, something like a ComponentGraphBuilder so that the final ComponentGraph didn’t need to have a mutable inner data structure, but then I’d need to go back and re-factor again. ## Part One - If At First You Don’t Succeed So, according to our puzzle, we have two “clumps” of interconnected components with a three-component “bridge” between them, and we need to unplug those “bridge” components to bring down the star cost of running the snow-making machine. Sure hope that other half of the components aren’t really necessary.  // data class Component(val id: Int) { ... } data class ComponentEdge(val c1: Component, val c2: Component) { override fun equals(other: Any?): Boolean { if (this === other) return true if (other !is ComponentEdge) return false // Check equality regardless of the order of components return (c1 == other.c1 && c2 == other.c2) || (c1 == other.c2 && c2 == other.c1) } override fun hashCode(): Int { // Ensure the same hash code is generated regardless of the order // of components return c1.hashCode() * c2.hashCode() } } data class ComponentGraph(val graph: MutableMap<Component, MutableList<Component>>) { // companion object { ... } // fun addEdge(edge: ComponentEdge) { ... } /** * Return (one of) the component(s) that lies furthest from start * * Performs a comprehensive breadth-first search over the component graph, * starting with start, returning the last node found. In the event that * more than one component lies equally distant from start, there is no * guarantee as to which of these furthest components will be returned. * * @param start The component to start searching from. * @return One of the components that lies furthest from start. */ fun furthestComponentFrom(start: Component): Component { val queue = ArrayDeque(listOf(start)) val seen = mutableSetOf<Component>() var currentComponent = start while (queue.isNotEmpty()) { currentComponent = queue.removeLast() if (currentComponent in seen) continue seen.add(currentComponent) val nextComponents = graph.getOrDefault(currentComponent, listOf()) for (nextComponent in nextComponents) { if (nextComponent in seen) continue queue.addFirst(nextComponent) } } return currentComponent } /** * Return the set of edges in _a_ constrained shortest path from start to finish * * Performs a breadth-first search from start to finish, returning * the set of edges in the first path found. In the event that there is * more than one equally short path, there is no guarantee as to which is * returned. Any edge in excluded will not be traversed when finding the * shortest path. * * @param start The first component in the path. * @param finish The last component in the path. * @param excluded A set of edges that are considered impassable for this * traversal. */ fun pathExcludingEdges( start: Component, finish: Component, excluded: Set<ComponentEdge> ): Set<ComponentEdge>? { val queue = ArrayDeque(listOf(start to setOf<ComponentEdge>())) val seen = mutableSetOf<Component>() while (queue.isNotEmpty()) { val (component, path) = queue.removeLast() if (component in seen) continue if (component == finish) return path seen.add(component) val nextComponents = graph.getOrDefault(component, listOf()) for (nextComponent in nextComponents) { if (nextComponent in seen) continue val edge = ComponentEdge(component, nextComponent) if (edge in excluded) continue queue.addFirst(nextComponent to path + edge) } } return null } /** * Count the number of reachable components from start * * Performs yet another variant of a breadth-first search, searching * outwards from start to find all connected nodes. Any edge contained * in excluded is considered impassable and nodes on the other side of * that edge may not be discoverable (unless they can be reached another * way). * * @param start The component to start searching from. * @param excluded The set of edges that cannot be traversed in this search. * @return The count of reachable components from start. */ fun countReachableComponents( start: Component, excluded: Set<ComponentEdge> ): Int { val queue = ArrayDeque(listOf(start)) val seen = mutableSetOf<Component>() while (queue.isNotEmpty()) { val component = queue.removeLast() if (component in seen) continue seen.add(component) val nextComponents = graph.getOrDefault(component, listOf()) for (nextComponent in nextComponents) { if (nextComponent in seen) continue val edge = ComponentEdge(component, nextComponent) if (edge in excluded) continue queue.addFirst(nextComponent) } } return seen.size } } class Day25(input: List<String>) { // private val parsed = ... // Time to cut some components! For this final puzzle, we need to count // the number of components on either side of a three-component bridge // between two clusters of highly-connected components. fun solvePart1(): Int { // There are definitely some assumptions being made here, chief among // them that the two sub-graphs are relatively balanced on either side // of the cut-off (the hint being that we need to go from 100 stars // required to 50, i.e. remove half the components). We start by picking // a node at 'random', then finding the node furthest from it to serve // as a starting point. Then, we find the node furthest from start // and _assume_ it's on the other side of the three nodes we need to // disconnect. val aNode = parsed.graph.keys.take(1).single() val start = parsed.furthestComponentFrom(aNode) val finish = parsed.furthestComponentFrom(start) // Now, we find the shortest path from start to finish three times, // constraining the set of edges in these three paths to be unique (no // taking the same path twice). We still don't know _which_ three nodes // need to be disconnected, but if start and finish are on opposite // sides, we can be reasonably certain that they're in the list of // excluded edges we accumulate. val path1 = parsed.pathExcludingEdges(start, finish, setOf()) ?: throw Exception("Could not find one path from$start to $finish!") val path2 = parsed.pathExcludingEdges(start, finish, path1) ?: throw Exception("Could not find two paths from$start to $finish!") val path3 = parsed.pathExcludingEdges(start, finish, path1 + path2) ?: throw Exception("Could not find three paths from$start to \$finish!")
val excludedEdges = path1 + path2 + path3

// Now, with our list of excluded edges containing the only paths from
// one sub-graph to another, we perform one more search over each side
// of the cut-off, counting components in each. This bit assumes that
// the components in each sub-graph are connected _enough_ that we'll
// still be able to reach all of them despite our overly generous set
// of excluded edges.
val group1Size = parsed.countReachableComponents(start, excludedEdges)
val group2Size = parsed.countReachableComponents(finish, excludedEdges)

// Finally, we return the product of the two group sizes.
return group1Size * group2Size
}
}


Not going to lie, we played it kind of fast and loose with this one. I’m certain there are more general ways of identifying and removing the bridge or of just counting the interconnected clusters without needing to perform seven breadth-first searches over this graph, but I didn’t have the patience for it. On the plus side, that’s all fifty stars earned. Huzzah!

## Part Two - Auld Lang Syne

If you haven’t heard, Day 25 - Part Two is always the “go back and finish the rest of the puzzles” challenge, so no extra coding needed here. Instead, I’d like to use this space to reflect on the Advent of Code puzzles for this year and my experience with them.

Has it really been four years since my first Advent of Code? Clearly the event has been around much longer, but it really seems like yesterday that a friend of mine was talking about this yearly set of coding puzzles he liked to do. This year was even more of a community event for me than in years past, with lots of activity on Mastodon of all places. I was also able to work with the organizer for the Carolina Code Conference and some other community members to brainstorm a “contest” for free conference tickets around Advent of Code participation. It was really nice to have some local folks also working on the puzzles and chatting about them in Slack.

Kotlin was a nice surprise as a language! I’d heard good things and it definitely lives up to the hype. I’ve written just enough Java to be dangerous as well as a bit of Clojure, but Kotlin was the first JVM language I’ve tried that I really enjoyed. I’d have to say the highlight was just how unsurprising the language is when compared to the syntax of other languages I’m used to. There are a couple of weird things (Why does passing anonymous functions require curly braces? What’s the deal with companion objects for class methods?), but for the most part, the syntax really managed to get out of my way for a language I’d not written more than 100 lines in previously. I will say the “non-IntelliJ” tooling left a bit to be desired. I started out trying to use the Kotlin LSP in both NeoVim (my preferred editor) and VSCode, but there were definitely some rough edges and performance issues there. I opted for Maven as a build system, and once I figured out that setup, it was fine. Figuring it out was a bit of a bear, though.

The other star of the show this year was ChatGPT. No, I didn’t ask it to solve the puzzles for me. I did ask a lot of questions like “How can I add a method to a class in Kotlin, not to an instance of that class?” and “Does the standard library have a ‘reduce’ function for lists?”. Compared to Googling and digging through the docs, having the (mostly) right information delivered to me in a concise format was really handy.

Just like last year, I’d like to send out a huge thank you to Eric Wastl and his crew of helpers for putting this event/phenomenon on and keeping it running. If you can, please consider donating to this effort to help keep the magic going, I guarantee there will be a lot of coders who will be glad you did.

If you’re interested, you can find all the code from this blog series on GitHub, please feel free to submit a pull request if you’re really bored. Happy Holidays!