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Day 1: Historian Hysteria

by @spamegg1

Puzzle description​

https://adventofcode.com/2024/day/1

Solution Summary​

  1. Parse the input to split it into two lists (left/right), each sorted in increasing order.
  2. Find the distance scores (for part1) and the similarity scores (for part2).
  3. Sum the scores.

Parsing​

Our parser iterates over the lines, extracts the pair of numbers from each line, then splits them into two lists (lefts and rights), and separately sorts the lists. Therefore it looks like this:

def parse(input: String): (Seq[Long], Seq[Long]) =
  // Extract pairs of numbers from each line
  val pairs = input
    .linesIterator
    .map(line => line.split("   ").map(_.toLong))
    .toSeq

  // Group the left and right members from each pair, sort them
  val lefts = pairs.map(_.head).toSeq.sorted
  val rights = pairs.map(_.last).toSeq.sorted
  (lefts, rights)

Part 1​

Now that the lefts and rights are sorted in increasing order, we can zip them, so that the first smallest on the left is paired with the first smallest on the right, the second smallest on the left is paired with the second smallest on the right, and so on. Then we can find the distances between them, and sum the distances:

def part1(input: String): Long =
  val (lefts, rights) = parse(input)
  lefts
    .zip(rights)
    .map((left, right) => math.abs(left - right)) // distances
    .sum
end part1

Part 2​

Very similar, but instead of distances, we find a left number's similarity on the right list. We do this by counting how many times the left number occurs on the right list, then multiply that count by the number itself. Finally we sum the similarity scores of all the left numbers:

def part2(input: String): Long =
  val (lefts, rights) = parse(input)
  lefts
    .map(left => rights.count(_ == left) * left) // similarity scores
    .sum
end part2

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