day07.livemd

Advent of Code 2021 - Day 7

Puzzle description

The Treachery of Whales

A giant whale has decided your submarine is its next meal

Suddenly, a swarm of crabs (each in its own tiny submarine) zooms in to rescue you!

crabs_positions =
  File.read!("inputs/day07.txt")
  |> String.trim()
  |> String.split(",")
  |> Enum.map(&String.to_integer/1)

The crab submarines all need to be aligned before they'll have enough power to blast a large enough hole in the ocean floor for your submarine to get through (sensors indicate a massive underground cave system).

Crab submarines can only move horizontally and have limited fuel, so you need to find a way to make all of their horizontal positions match while requiring them to spend as little fuel as possible.

defmodule Part1 do
  def align_crabs(crabs) do
    min = Enum.min(crabs)
    max = Enum.max(crabs)

    Enum.map(min..max, fn c ->
      Enum.map(crabs, fn p -> abs(p - c) end)
      |> Enum.sum()
    end)
    |> Enum.min()
  end
end

Determine the horizontal position that the crabs can align to using the least fuel possible. How much fuel must they spend to align to that position?

Part1.align_crabs(crabs_positions)

Perhaps you (again) misunderstood?

Crab submarine engines don't burn fuel at a constant rate.

Instead, each change of 1 step in horizontal position costs 1 more unit of fuel than the last.

As each crab moves, moving further becomes more expensive.

defmodule Part2 do
  def align_crabs(crabs) do
    min = Enum.min(crabs)
    max = Enum.max(crabs)

    Enum.map(min..max, fn c ->
      Enum.map(crabs, fn p ->
        distance = abs(p - c)
        Enum.sum(0..distance)
      end)
      |> Enum.sum()
    end)
    |> Enum.min()
  end
end

Part2.align_crabs(crabs_positions)

Check

Part1.align_crabs(crabs_positions) == 352_331 && Part2.align_crabs(crabs_positions) == 99_266_250