144 lines
5.9 KiB
Plaintext
144 lines
5.9 KiB
Plaintext
Advent of Code
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br0xen (AoC++) 4*
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--- Day 1: Historian Hysteria ---
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The Chief Historian is always present for the big Christmas sleigh launch,
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but nobody has seen him in months! Last anyone heard, he was visiting
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locations that are historically significant to the North Pole; a group of
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Senior Historians has asked you to accompany them as they check the places
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they think he was most likely to visit.
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As each location is checked, they will mark it on their list with a star.
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They figure the Chief Historian must be in one of the first fifty places
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they'll look, so in order to save Christmas, you need to help them get
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fifty stars on their list before Santa takes off on December 25th.
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Collect stars by solving puzzles. Two puzzles will be made available on
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each day in the Advent calendar; the second puzzle is unlocked when you
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complete the first. Each puzzle grants one star. Good luck!
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You haven't even left yet and the group of Elvish Senior Historians has
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already hit a problem: their list of locations to check is currently
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empty. Eventually, someone decides that the best place to check first
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would be the Chief Historian's office.
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Upon pouring into the office, everyone confirms that the Chief Historian
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is indeed nowhere to be found. Instead, the Elves discover an assortment
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of notes and lists of historically significant locations! This seems to be
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the planning the Chief Historian was doing before he left. Perhaps these
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notes can be used to determine which locations to search?
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Throughout the Chief's office, the historically significant locations are
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listed not by name but by a unique number called the location ID. To make
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sure they don't miss anything, The Historians split into two groups, each
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searching the office and trying to create their own complete list of
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location IDs.
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There's just one problem: by holding the two lists up side by side (your
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puzzle input), it quickly becomes clear that the lists aren't very
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similar. Maybe you can help The Historians reconcile their lists?
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For example:
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3 4
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4 3
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2 5
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1 3
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3 9
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3 3
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Maybe the lists are only off by a small amount! To find out, pair up the
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numbers and measure how far apart they are. Pair up the smallest number in
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the left list with the smallest number in the right list, then the
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second-smallest left number with the second-smallest right number, and so
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on.
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Within each pair, figure out how far apart the two numbers are; you'll
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need to add up all of those distances. For example, if you pair up a 3
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from the left list with a 7 from the right list, the distance apart is 4;
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if you pair up a 9 with a 3, the distance apart is 6.
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In the example list above, the pairs and distances would be as follows:
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• The smallest number in the left list is 1, and the smallest number in
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the right list is 3. The distance between them is 2.
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• The second-smallest number in the left list is 2, and the
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second-smallest number in the right list is another 3. The distance
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between them is 1.
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• The third-smallest number in both lists is 3, so the distance between
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them is 0.
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• The next numbers to pair up are 3 and 4, a distance of 1.
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• The fifth-smallest numbers in each list are 3 and 5, a distance of 2.
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• Finally, the largest number in the left list is 4, while the largest
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number in the right list is 9; these are a distance 5 apart.
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To find the total distance between the left list and the right list, add
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up the distances between all of the pairs you found. In the example above,
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this is 2 + 1 + 0 + 1 + 2 + 5, a total distance of 11!
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Your actual left and right lists contain many location IDs. What is the
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total distance between your lists?
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Your puzzle answer was 2066446.
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--- Part Two ---
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Your analysis only confirmed what everyone feared: the two lists of
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location IDs are indeed very different.
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Or are they?
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The Historians can't agree on which group made the mistakes or how to read
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most of the Chief's handwriting, but in the commotion you notice an
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interesting detail: a lot of location IDs appear in both lists! Maybe the
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other numbers aren't location IDs at all but rather misinterpreted
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handwriting.
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This time, you'll need to figure out exactly how often each number from
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the left list appears in the right list. Calculate a total similarity
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score by adding up each number in the left list after multiplying it by
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the number of times that number appears in the right list.
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Here are the same example lists again:
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3 4
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4 3
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2 5
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1 3
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3 9
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3 3
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For these example lists, here is the process of finding the similarity
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score:
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• The first number in the left list is 3. It appears in the right list
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three times, so the similarity score increases by 3 * 3 = 9.
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• The second number in the left list is 4. It appears in the right list
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once, so the similarity score increases by 4 * 1 = 4.
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• The third number in the left list is 2. It does not appear in the
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right list, so the similarity score does not increase (2 * 0 = 0).
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• The fourth number, 1, also does not appear in the right list.
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• The fifth number, 3, appears in the right list three times; the
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similarity score increases by 9.
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• The last number, 3, appears in the right list three times; the
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similarity score again increases by 9.
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So, for these example lists, the similarity score at the end of this
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process is 31 (9 + 4 + 0 + 0 + 9 + 9).
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Once again consider your left and right lists. What is their similarity
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score?
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Your puzzle answer was 24931009.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, you should return to your Advent calendar and try
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another puzzle.
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If you still want to see it, you can get your puzzle input.
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References
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