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Rewrite as four-dimensional route finding The grid isn't just a 2D grid. The constraints make it 4D: * X * Y * Last direction * Number of steps in that direction By tracking all four dimensions, we can find the shortest route for _all_ combinations of the constraint. Previously, we were dropping routes that were currently longer but ended up shorter because they could take subsequent steps that other routes couldn't.
author IBBoard <dev@ibboard.co.uk>
date Sun, 22 Sep 2024 11:30:53 +0100
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1 --- Day 3: Gear Ratios ---
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3 A part is missing from a system. If you can add up all the part numbers in the schematic, it should be easy to work out which part is missing.
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5 The schematic (your puzzle input) consists of a visual representation of the system. 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.)
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7 Here is an example schematic:
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9 467..114..
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10 ...*......
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11 ..35..633.
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12 ......#...
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13 617*......
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14 .....+.58.
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15 ..592.....
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16 ......755.
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17 ...$.*....
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18 .664.598..
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20 In this schematic, two numbers are not part numbers because they are not adjacent to a symbol: 114 (top right) and 58 (middle right). Every other number is adjacent to a symbol and so is a part number; their sum is 4361.
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22 Of course, the actual schematic is much larger. What is the sum of all of the part numbers in the schematic?
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24 --- Part Two ---
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26 The missing part wasn't the only issue - one of the gears in the schematic is wrong. A gear is any * symbol that is adjacent to exactly two part numbers. Its gear ratio is the result of multiplying those two numbers together.
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28 This time, you need to find the gear ratio of every gear and add them all up to figure out which gear needs to be replaced.
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30 Consider the same schematic again:
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32 467..114..
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33 ...*......
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34 ..35..633.
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35 ......#...
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36 617*......
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37 .....+.58.
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38 ..592.....
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39 ......755.
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40 ...$.*....
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41 .664.598..
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43 In this schematic, there are two gears. The first is in the top left; it has part numbers 467 and 35, so its gear ratio is 16345. The second gear is in the lower right; its gear ratio is 451490. (The * adjacent to 617 is 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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45 What is the sum of all of the gear ratios in your schematic?