# Advent of Code 2018 - Day 8, in Kotlin

Kotlin solutions to parts 1 and 2 of Advent of Code 2018, Day 8: 'Memory Maneuver'

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Day 8 brings us a tree, and a recursive function to create it.

If you’d rather just view code, the GitHub Repository is here.

Problem Input

Our input is a single `String` of space-separated integers. Since the input is turned into a tree structure, and we need to read the problem first, there’s nothing to be done here. We’ll consume the input in Part 1 below.

#### ⭐ Day 8, Part 1

The puzzle text can be found here.

It seems that most of the work in this is going to be turning the input into a structure we can use. For this, let’s define a class to represent a `Node`. Nodes have child nodes (also of type `Node`), and some metadata. We’ll make this a regular `class`, because we don’t need any of the extra things that come along with a `data class`.

``````class Node(children: List<Node>, metadata: List<Int>)
``````

We are not making the arguments `children` and `metadata` vals because we don’t actually need them for anything yet. You’ll see in a minute how we’ll consume that data, and return useful results. But first, we need to parse our input into a `Node` and its children. Thankfully, the problem states there is only one top level node, meaning this is definitely a Tree and not a more complicated version of a directed graph.

The way we’re going to parse this is to write a recursive function. Recursive functions are functions that call themselves. Recursive functions work well with tree structures because it’s easy to picture how to break the problem into smaller and smaller steps. First, let’s outline how it will work, and then code it up:

1. To start, receive an `Iterator<Int>`. This lets us consume `Ints` off the `Iterator`, and pass the remaining unconsumed down the call stack.
2. Get the number of children from the iterator.
3. Get the amount of metadata from the iterator.
4. For each child we are supposed to have, call ourselves with what’s left in the iterator (Go back to step 1) to generate children.
5. When that returns, consume the number of metadata entries from the Iterator
6. Package that all up into a `Node` and return.

This should give us the tree structure we’re looking for. We’ll implement this on the companion object of `Node`:

``````private class Node(children: List<Node>, metadata: List<Int>) {

companion object {

fun of(values: Iterator<Int>): Node {
val numChildren: Int = values.next()
val children = (0 until numChildren).map { Node.of(values) }
}

}

}
``````

One thing to watch out for with recursive functions is that at some point you’ll run out of stack space and the call will fail. Thankfully, our call depth here isn’t so bad that we have to worry about it. Otherwise, we would have to find a different way to solve the problem.

Now that we have a node, we can implement a function on it to calculate the metadata’s total. But hold on. Do we really need a function for that? Not really, because the answer itself never changes. And in this case (because it’s a puzzle) I know we’ll always need to know the answer. So let’s just define a new `val` on `Node` to calculate this for us:

``````val metadataTotal: Int =
``````

What that says is “sum the metadata of the current node, and add that to the sum of the metadata of all the child nodes we have (if any)”.

Now to create our root node and inspect this value:

``````class Day08(rawInput: String) {
private val tree: Node = Node.of(rawInput.split(" ").map { it.toInt() }.iterator())

fun solvePart1(): Int =
}
``````

To turn our `String` input into an `Iterator`, we split it apart every time we have a space. This gives us a `List<String>`, and then we map each `String` into an `Int`, and then get the `Iterator` off of the `List<Int>`.

Star earned! Onward!

#### ⭐ Day 8, Part 2

The puzzle text can be found here.

Thankfully, we have almost everything we need already. The only real work we have is to add another `val` to `Node` to calculate its value.

``````val value: Int =
else metadata.sumBy { children.getOrNull(it - 1)?.value ?: 0 }
``````

In the case where the `Node` doesn’t have any children, we just sum the metadata. In the case where it does have children, sum their values. The trick here is with indexing into our `children` list. If you read the problem statement, it is one-indexed (starts at 1!), not zero-indexed like we’re used to. And because we can ask for any child number, even if it doesn’t exist, we’ll need to account for that as well. For this we use `getOrNull` (Thanks Karel!), which accounts for this. If we get a null, we just assume the value is zero. Summing all this gives us our answer!

``````fun solvePart2(): Int =
tree.value
``````

Star 2 earned! I think this was a nice gentle introduction to recursive functions.