Day 13

Part 1

# input = File.stream!("13/example.txt")
input = File.stream!("13/input.txt")

defmodule U do
  @r ~r/: X[+=](\d+), Y[+=](\d+)/
  def parse(input) do
    input |> Stream.chunk_every(4) |> Enum.map(fn
      [a,b,p | _] ->
        [_, ax, ay] = Regex.run(@r, a)
        [_, bx, by] = Regex.run(@r, b)
        [_, px, py] = Regex.run(@r, p)
        [ax,ay,bx,by,px,py]= Enum.map([ax,ay,bx,by,px,py], &String.to_integer/1)
        {{ax, ay}, {bx, by}, {px, py}}
    end)
  end
end

data = U.parse(input)

defmodule P1 do
  def tokens({_, {bx, by}, {px, py}} = input) do
    # IO.inspect(input)
    b = max(:math.ceil(px / bx) |> trunc, :math.ceil(py / by) |> trunc)
    tokens(input, 0, b)
  end

  def tokens(_, _, 0), do: 0

  def tokens({{ax, ay}, {bx, by}, {px, py}} = input, a, b) do
    cx = ax * a + bx * b
    cy = ay * a + by * b

    cond do
      cx == px && cy == py -> a * 3 + b
      cx > px || cy > py -> tokens(input, a, b - 1)
      true -> tokens(input, a + 1, b)
    end
  end

  def solve(data) do
    data |> Enum.map(&tokens/1) |> Enum.sum()
  end
end

P1.solve(data)

Part 2

defmodule P2 do
  def tokens({{ax, ay}, {bx, by}, {px, py}}) do  
    px = px + 10000000000000
    py = py + 10000000000000
    det = ax * by - ay * bx
    if det == 0 do
      0
    else
      adet = px * by - py * bx
      bdet = ax * py - ay * px
      if rem(adet, det) != 0 || rem(bdet, det) != 0 do
        0
      else
        div(adet, det) * 3 + div(bdet, det)
      end
    end
  end

  def solve(data) do
    data |> Enum.map(&tokens/1) |> Enum.sum()
  end
end

P2.solve(data)