Day 3

Mix.install([:kino])

Input

input = Kino.Input.textarea("")

Pre-work

defmodule Rucksacks do
  def item_in_both_compartments(rucksack) do
    compartment_size = rucksack |> length |> div(2)
    {comp0, comp1} = Enum.split(rucksack, compartment_size)
    comp0_set = MapSet.new(comp0)
    Enum.find(comp1, &MapSet.member?(comp0_set, &1))
  end

  def in_common([s0, s1 | ss]), do: in_common([MapSet.intersection(s0, s1) | ss])
  def in_common([s]), do: s |> MapSet.to_list() |> hd()

  def item_priority(item) when item >= ?a and item <= ?z, do: item - ?a + 1
  def item_priority(item), do: item - ?A + 27
end

rucksacks =
  input
  |> Kino.Input.read()
  |> String.split("\n")
  |> Enum.map(&amp;String.to_charlist/1)

Part 1

rucksacks
|> Enum.map(fn rucksack ->
  rucksack
  |> Rucksacks.item_in_both_compartments()
  |> Rucksacks.item_priority()
end)
|> Enum.sum()

Part 2

rucksacks
|> Enum.map(&MapSet.new/1)
|> Enum.chunk_every(3)
|> Enum.map(&Rucksacks.in_common/1)
|> Enum.map(&Rucksacks.item_priority/1)
|> Enum.sum()