adventofcode/2024/day01/problem

Advent of Code

br0xen (AoC++) 4*

--- Day 1: Historian Hysteria ---

   The Chief Historian is always present for the big Christmas sleigh launch,
   but nobody has seen him in months! Last anyone heard, he was visiting
   locations that are historically significant to the North Pole; a group of
   Senior Historians has asked you to accompany them as they check the places
   they think he was most likely to visit.

   As each location is checked, they will mark it on their list with a star.
   They figure the Chief Historian must be in one of the first fifty places
   they'll look, so in order to save Christmas, you need to help them get
   fifty stars on their list before Santa takes off on December 25th.

   Collect stars by solving puzzles. Two puzzles will be made available on
   each day in the Advent calendar; the second puzzle is unlocked when you
   complete the first. Each puzzle grants one star. Good luck!

   You haven't even left yet and the group of Elvish Senior Historians has
   already hit a problem: their list of locations to check is currently
   empty. Eventually, someone decides that the best place to check first
   would be the Chief Historian's office.

   Upon pouring into the office, everyone confirms that the Chief Historian
   is indeed nowhere to be found. Instead, the Elves discover an assortment
   of notes and lists of historically significant locations! This seems to be
   the planning the Chief Historian was doing before he left. Perhaps these
   notes can be used to determine which locations to search?

   Throughout the Chief's office, the historically significant locations are
   listed not by name but by a unique number called the location ID. To make
   sure they don't miss anything, The Historians split into two groups, each
   searching the office and trying to create their own complete list of
   location IDs.

   There's just one problem: by holding the two lists up side by side (your
   puzzle input), it quickly becomes clear that the lists aren't very
   similar. Maybe you can help The Historians reconcile their lists?

   For example:

 3   4
 4   3
 2   5
 1   3
 3   9
 3   3

   Maybe the lists are only off by a small amount! To find out, pair up the
   numbers and measure how far apart they are. Pair up the smallest number in
   the left list with the smallest number in the right list, then the
   second-smallest left number with the second-smallest right number, and so
   on.

   Within each pair, figure out how far apart the two numbers are; you'll
   need to add up all of those distances. For example, if you pair up a 3
   from the left list with a 7 from the right list, the distance apart is 4;
   if you pair up a 9 with a 3, the distance apart is 6.

   In the example list above, the pairs and distances would be as follows:

     • 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.
     • 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.
     • The third-smallest number in both lists is 3, so the distance between
       them is 0.
     • The next numbers to pair up are 3 and 4, a distance of 1.
     • The fifth-smallest numbers in each list are 3 and 5, a distance of 2.
     • 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.

   To find the total distance between the left list and the right list, add
   up the distances between all of the pairs you found. In the example above,
   this is 2 + 1 + 0 + 1 + 2 + 5, a total distance of 11!

   Your actual left and right lists contain many location IDs. What is the
   total distance between your lists?

   Your puzzle answer was 2066446.

--- Part Two ---

   Your analysis only confirmed what everyone feared: the two lists of
   location IDs are indeed very different.

   Or are they?

   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.

   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.

   Here are the same example lists again:

 3   4
 4   3
 2   5
 1   3
 3   9
 3   3

   For these example lists, here is the process of finding the similarity
   score:

     • 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.
     • 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.
     • 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).
     • The fourth number, 1, also does not appear in the right list.
     • The fifth number, 3, appears in the right list three times; the
       similarity score increases by 9.
     • The last number, 3, appears in the right list three times; the
       similarity score again increases by 9.

   So, for these example lists, the similarity score at the end of this
   process is 31 (9 + 4 + 0 + 0 + 9 + 9).

   Once again consider your left and right lists. What is their similarity
   score?

   Your puzzle answer was 24931009.

   Both parts of this puzzle are complete! They provide two gold stars: **

   At this point, you should return to your Advent calendar and try
   another puzzle.

   If you still want to see it, you can get your puzzle input.

References