Day13

Section

example = """
Button A: X+94, Y+34
Button B: X+22, Y+67
Prize: X=8400, Y=5400

Button A: X+26, Y+66
Button B: X+67, Y+21
Prize: X=12748, Y=12176

Button A: X+17, Y+86
Button B: X+84, Y+37
Prize: X=7870, Y=6450

Button A: X+69, Y+23
Button B: X+27, Y+71
Prize: X=18641, Y=10279
"""

defmodule Day13 do
  def parse_input(input, part \\ :one) do
    machines = String.split(input, "\n\n", trim: true)

    Enum.map(machines, fn raw_machine ->
      [ax, ay, bx, by, px, py] =
        String.split(raw_machine, "\n", trim: true)
        |> Enum.map(&Regex.scan(~r/\d+/, &1))
        |> List.flatten()
        |> Enum.map(&String.to_integer/1)

      if part == :two do
        [ax, ay, bx, by, px + 10_000_000_000_000, py + 10_000_000_000_000]
      else
        [ax, ay, bx, by, px, py]
      end
    end)
  end

  def part1(input) do
    machines = parse_input(input)

    machines
    |> Enum.map(fn machine ->
      machine
      |> min_token()
      |> Enum.map(fn {a, b} -> a * 3 + b end)
      |> Enum.sort()
      |> List.first()
    end)
    |> Enum.filter(& &1)
    |> Enum.sum()
  end

  def min_token(machine) do
    [ax, ay, bx, by, px, py] = machine

    for a <- 1..lo(div(px, ax), div(py, ay)),
        b <- 1..lo(div(px, bx), div(py, by)),
        a * ax + b * bx == px,
        a * ay + b * by == py do
      {a, b}
    end
  end

  def lo(a, b) do
    if a > b, do: a, else: b
  end

  def part2(input) do
    machines = parse_input(input, :two)

    machines
    |> Enum.map(fn machine ->
      [ax, ay, bx, by, px, py] = machine

      a = div(px * by - bx * py, ax * by - bx * ay)
      b = div(px * ay - py * ax, ay * bx - by * ax)

      a * ax + b * bx == px &amp;&amp; a * ay + b * by == py &amp;&amp; {a, b}
    end)
    |> Enum.filter(&amp; &amp;1)
    |> Enum.map(fn {a, b} -> a * 3 + b end)
    |> Enum.sum()
  end
end

Day13.part1(example)
Day13.part2(example)