60 lines
3.5 KiB
Markdown
60 lines
3.5 KiB
Markdown
# 2018 Day 8: Memory Maneuver
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Copyright (c) Eric Wastl
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#### [Direct Link](https://adventofcode.com/2018/day/8)
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## Part 1
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The sleigh is much easier to pull than you'd expect for something its weight. Unfortunately, neither you nor the Elves know which way the North Pole is from here.
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You check your wrist device for anything that might help. It seems to have some kind of navigation system! Activating the navigation system produces more bad news: "Failed to start navigation system. Could not read software license file."
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The navigation system's license file consists of a list of numbers (your puzzle input). The numbers define a data structure which, when processed, produces some kind of [tree](https://en.wikipedia.org/wiki/Tree_(data_structure)) that can be used to calculate the license number.
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The **tree** is made up of **nodes**; a single, outermost node forms the tree's root, and it contains all other nodes in the tree (or contains nodes that contain nodes, and so on).
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Specifically, a node consists of:
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- A **header**, which is always exactly two numbers:
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- The quantity of child nodes.
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- The quantity of metadata entries.
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- Zero or more **child nodes** (as specified in the header).
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- One or more **metadata entries** (as specified in the header).
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Each child node is itself a node that has its own header, child nodes, and metadata. For example:
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```
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2 3 0 3 10 11 12 1 1 0 1 99 2 1 1 2
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A----------------------------------
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B----------- C-----------
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D-----
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```
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In this example, each node of the tree is also marked with an underline starting with a letter for easier identification. In it, there are four nodes:
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- `A`, which has `2` child nodes (`B`, `C`) and `3` metadata entries (`1`, `1`, `2`).
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- `B`, which has `0` child nodes and `3` metadata entries (`10`, `11`, `12`).
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- `C`, which has `1` child node (`D`) and `1` metadata entry (`2`).
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- `D`, which has `0` child nodes and `1` metadata entry (`99`).
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The first check done on the license file is to simply add up all of the metadata entries. In this example, that sum is `1+1+2+10+11+12+2+99=138`.
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**What is the sum of all metadata entries?**
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## Part 2
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The second check is slightly more complicated: you need to find the value of the root node (`A` in the example above).
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The **value of a node** depends on whether it has child nodes.
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If a node has **no child nodes**, its value is the sum of its metadata entries. So, the value of node `B` is `10+11+12=33`, and the value of node `D` is `99`.
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However, if a node **does have child nodes**, the metadata entries become indexes which refer to those child nodes. A metadata entry of `1` refers to the first child node, `2` to the second, `3` to the third, and so on. The value of this node is the sum of the values of the child nodes referenced by the metadata entries. If a referenced child node does not exist, that reference is skipped. A child node can be referenced multiple time and counts each time it is referenced. A metadata entry of `0` does not refer to any child node.
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For example, again using the above nodes:
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- Node `C` has one metadata entry, `2`. Because node `C` has only one child node, `2` references a child node which does not exist, and so the value of node `C` is `0`.
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- Node `A` has three metadata entries: `1`, `1`, and `2`. The `1` references node `A`'s first child node, `B`, and the `2` references node `A`'s second child node, `C`. Because node `B` has a value of `33` and node `C` has a value of `0`, the value of node `A` is `33+33+0=66`.
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So, in this example, the value of the root node is `66`.
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**What is the value of the root node?** |