2023 - Day 3

2023/03/PART1.md

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+--- Day 3: Gear Ratios ---
+
+You and the Elf eventually reach a gondola lift station; he says the gondola
+lift will take you up to the water source, but this is as far as he can bring
+you. You go inside.
+
+It doesn't take long to find the gondolas, but there seems to be a problem:
+they're not moving.
+
+"Aaah!"
+
+You turn around to see a slightly-greasy Elf with a wrench and a look of
+surprise. "Sorry, I wasn't expecting anyone! The gondola lift isn't working
+right now; it'll still be a while before I can fix it." You offer to help.
+
+The engineer explains that an engine part seems to be missing from the engine,
+but nobody can figure out which one. If you can add up all the part numbers in
+the engine schematic, it should be easy to work out which part is missing.
+
+The engine schematic (your puzzle input) consists of a visual representation of
+the engine. There are lots of numbers and symbols you don't really understand,
+but apparently any number adjacent to a symbol, even diagonally, is a "part
+number" and should be included in your sum. (Periods (.) do not count as a
+symbol.)
+
+Here is an example engine schematic:
+
+```
+467..114..
+...*......
+..35..633.
+......#...
+617*......
+.....+.58.
+..592.....
+......755.
+...$.*....
+.664.598..
+```
+
+In this schematic, two numbers are not part numbers because they are not
+adjacent to a symbol: 114 (top right) and 58 (middle right). Every other number
+is adjacent to a symbol and so is a part number; their sum is 4361.
+
+Of course, the actual engine schematic is much larger. What is the sum of all of
+the part numbers in the engine schematic?