2023-12-06 10:27:00 -05:00
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# Day 3: Gear Ratios
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## Part 1
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You and the Elf eventually reach a gondola lift station; he says the gondola lift will take you up to the **water source**,
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but this is as far as he can bring you. You go inside.
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It doesn't take long to find the gondolas, but there seems to be a problem: they're not moving.
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"Aaah!"
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You turn around to see a slightly-greasy Elf with a wrench and a look of surprise. "Sorry, I wasn't expecting anyone!
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The gondola lift isn't working right now; it'll still be a while before I can fix it." You offer to help.
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The engineer explains that an engine part seems to be missing from the engine, but nobody can figure out which one.
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If you can **add up all the part numbers** in the engine schematic, it should be easy to work out which part is missing.
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The engine schematic (your puzzle input) consists of a visual representation of the engine. There are lots of numbers
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and symbols you don't really understand, but apparently any number adjacent to a symbol, even diagonally, is a
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"part number" and should be included in your sum. (Periods (.) do not count as a symbol.)
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Here is an example engine schematic:
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```
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467..114..
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...*......
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..35..633.
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......#...
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617*......
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.....+.58.
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..592.....
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......755.
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...$.*....
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.664.598..
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```
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In this schematic, two numbers are **not** part numbers because they are not adjacent to a symbol: `114` (top right) and
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`58` (middle right). Every other number is adjacent to a symbol and so **is** a part number; their sum is `**4361**`.
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Of course, the actual engine schematic is much larger.
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**What is the sum of all of the part numbers in the engine schematic?**
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2023-12-07 15:45:58 -05:00
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## Part 2
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The engineer finds the missing part and installs it in the engine! As the engine springs to life, you jump in the
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closest gondola, finally ready to ascend to the water source.
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You don't seem to be going very fast, though. Maybe something is still wrong? Fortunately, the gondola has a phone
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labeled "help", so you pick it up and the engineer answers.
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Before you can explain the situation, she suggests that you look out the window. There stands the engineer, holding a
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phone in one hand and waving with the other. You're going so slowly that you haven't even left the station. You exit
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the gondola.
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The missing part wasn't the only issue - one of the gears in the engine is wrong. A **gear** is any `*` symbol that is
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adjacent to **exactly two part numbers**. Its **gear ratio** is the result of multiplying those two numbers together.
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This time, you need to find the gear ratio of every gear and add them all up so that the engineer can figure out
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which gear needs to be replaced.
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Consider the same engine schematic again:
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```
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467..114.. FAILED
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...*......
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..35..633.
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......#...
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617*......
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.....+.58.
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..592.....
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......755.
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...$.*....
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.664.598..
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```
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In this schematic, there are **two** gears. The first is in the top left; it has part numbers `467` and `35`, so its
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gear ratio is `16345`. The second gear is in the lower right; its gear ratio is `451490`. (The * adjacent to `617` is
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**not** a gear because it is only adjacent to one part number.) Adding up all of the gear ratios produces `**467835**`.
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**What is the sum of all of the gear ratios in your engine schematic?**
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