Solved with a genetic algorithm.

Easy one!
y2021/day_07.ex

Not sure if this was accidentally swapped with Day 3 or something. Way too easy.

defmodule AdventOfCode.Y2021.Day07 do
  use AdventOfCode.Helpers.InputReader, year: 2021, day: 7

  def run_1, do: input!() |> parse() |> alignments() |> Enum.min()
  def run_2, do: input!() |> parse() |> alignments(&cost/1) |> Enum.min()
  def parse(data), do: data |> String.split(",") |> Enum.map(&String.to_integer/1)

  defp alignments(positions, cost_fn \\ &Function.identity/1) do
    for i <- positions do
      Enum.sum(for j <- positions, do: cost_fn.(abs(i - j)))
    end
  end

  defp cost(steps), do: (steps == 0 && 0) || div(steps * (steps + 1), 2)
end