Day 20

Setup

https://adventofcode.com/2022/day/20

defmodule Load do
  def input do
    File.read!(Path.join(Path.absname(__DIR__), "input/20.txt"))
  end
end

defmodule Parse do
  def input(text) do
    text
    |> String.split("\n", trim: true)
    |> Enum.map(&Integer.parse/1)
    |> Enum.map(&elem(&1, 0))
  end
end

test_input =
  """
  1
  2
  -3
  3
  -2
  0
  4
  """
  |> Parse.input()

real_input =
  Load.input()
  |> Parse.input()

Part 1

defmodule Part1 do
  def mix(indexed_array) do
    length = Enum.count(indexed_array)
    indicies = 0..(length - 1)

    indicies
    |> Enum.reduce(indexed_array, fn orig_index, acc ->
      current_index = acc |> Enum.find_index(fn {_, idx} -> idx == orig_index end)
      {value, _} = Enum.at(acc, current_index)
      move_size = Integer.mod(value, length - 1)
      offset = if current_index + move_size >= length, do: 1, else: 0
      new_index = Integer.mod(current_index + move_size, length) + offset

      acc
      |> List.delete_at(current_index)
      |> List.insert_at(new_index, {value, orig_index})
    end)
  end

  def solve(array) do
    mixed =
      array
      |> Enum.with_index()
      |> mix()
      |> Enum.map(&elem(&1, 0))

    zero_index = Enum.find_index(mixed, fn val -> val == 0 end)

    [1000, 2000, 3000]
    |> Enum.map(fn offset -> rem(zero_index + offset, Enum.count(mixed)) end)
    |> Enum.map(fn idx -> Enum.at(mixed, idx) end)
    |> Enum.sum()
  end
end

Part1.solve(test_input)

Part1.solve(real_input)

Part 2

defmodule Part2 do
  def solve(input) do
    key = 811_589_153

    adjusted_input =
      input
      |> Enum.map(fn val -> val * key end)
      |> Enum.with_index()

    mixed =
      1..10
      |> Enum.reduce(adjusted_input, fn _, acc -> Part1.mix(acc) end)
      |> Enum.map(&elem(&1, 0))

    zero_index = Enum.find_index(mixed, fn val -> val == 0 end)

    [1000, 2000, 3000]
    |> Enum.map(fn offset -> rem(zero_index + offset, Enum.count(mixed)) end)
    |> Enum.map(fn idx -> Enum.at(mixed, idx) end)
    |> Enum.sum()
  end
end

Part2.solve(test_input)

Part2.solve(real_input)