Advent of Code 2024 - Day 14

bjorng · December 14, 2024, 7:26am

This was a fun day.

I initially solved part 2 by printing out a map with the robots after each second. I then did some creative grepping to find the Christmas tree. Knowing what the Christmas tree looks like, I could create a unique pattern to use when searching for the Christmas tree programmatically.

The combined running time for both parts is 2.3 seconds.

github.com

bjorng/advent-of-code/blob/main/2024/day14/lib/day14.ex

defmodule Day14 do
  def part1(input, iter, width, height) do
    robots = parse(input)
    dim = {width, height}
    hw = div(width, 2)
    hh = div(height, 2)

    1..iter
    |> Enum.reduce(robots, fn _, robots ->
      Enum.map(robots, &move(&1, dim))
    end)
    |> Enum.map(&elem(&1, 0))
    |> Enum.reject(fn {x, y} ->
      x === hw or y === hh
    end)
    |> Enum.group_by(fn {x, y} ->
      {div(x, hw + 1), div(y, hh + 1)}
    end)
    |> Enum.map(&elem(&1, 1))
    |> Enum.map(&length/1)

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