107 lines
5.2 KiB
Org Mode
107 lines
5.2 KiB
Org Mode
#+title: December First challenge - Historian Hysteria
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* Blurb
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The *Chief Historian* is always present for the big Christmas sleigh launch, but
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nobody has seen him in months! Last anyone heard, he was visiting locations that
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are historically significant to the North Pole; a group of Senior Historians has
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asked you to accompany them as they check the places they think he was most
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likely to visit.
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As each location is checked, they will mark it on their list with a *star*. They
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figure the Chief Historian must be in one of the first fifty places they'll
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look, so in order to save Christmas, you need to help them get fifty stars on
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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 each day
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in the Advent calendar; the second puzzle is unlocked when you complete the
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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 already
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hit a problem: their list of locations to check is currently empty. Eventually,
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someone decides that the best place to check first would be the Chief
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Historian's office.
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Upon pouring into the office, everyone confirms that the Chief Historian is
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indeed nowhere to be found. Instead, the Elves discover an assortment of notes
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and lists of historically significant locations! This seems to be the planning
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the Chief Historian was doing before he left. Perhaps these notes can be used to
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determine which locations to search?
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Throughout the Chief's office, the historically significant locations are listed
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not by name but by a unique number called the location ID. To make sure they
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don't miss anything, The Historians split into two groups, each searching the
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office and trying to create their own complete list of location IDs.
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There's just one problem: by holding the two lists up side by side (your puzzle
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input), it quickly becomes clear that the lists aren't very similar. Maybe you
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can help The Historians reconcile their lists?
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* Example
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#+begin_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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#+end_example
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Maybe the lists are only off by a small amount! To find out, pair up the numbers
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and measure how far apart they are. Pair up the *smallest number in the left list*
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with *the smallest number in the right list*, then the *second-smallest left number*
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with the *second-smallest right number*, and so on.
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Within each pair, figure out *how far apart* the two numbers are; you'll need to
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*add up all of those distances*. For example, if you pair up a =3= from the left
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list with a =7= from the right list, the distance apart is =4=; if you pair up a =9=
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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 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 second-smallest number in the right list is another 3. The distance between them is 1.
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+ The third-smallest number in both lists is 3, so the distance between 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 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 up
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the distances between all of the pairs you found. In the example above, this
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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 total distance between your lists?
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* Part 2
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Your analysis only confirmed what everyone feared: the two lists of 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 most of the Chief's handwriting, but in the commotion you notice an interesting detail: a lot of location IDs appear in both lists! Maybe the other numbers aren't location IDs at all but rather misinterpreted handwriting.
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This time, you'll need to figure out exactly how often each number from the left list appears in the right list. Calculate a total similarity score by adding up each number in the left list after multiplying it by 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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#+begin_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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#+end_example
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For these example lists, here is the process of finding the similarity score:
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+ The first number in the left list is 3. It appears in the right list 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 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 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 similarity score increases by 9.
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+ The last number, 3, appears in the right list three times; the similarity score again increases by 9.
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So, for these example lists, the similarity score at the end of this process is 31 (9 + 4 + 0 + 0 + 9 + 9).
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