Day 13

Mix.install([{:utils, path: "#{__DIR__}/utils"}])

Solution

{:ok, contents} = File.read("#{__DIR__}/inputs/day13.txt")

parse_ints = fn line ->
  ~r/\D+(\d+)\D+(\d+)/
  |> Regex.run(line)
  |> tl()
  |> Enum.map(&String.to_integer/1)
end

parse = fn line ->
  line
  |> String.split("\n", trim: true)
  |> Enum.map(parse_ints)
end

adjust_prize = fn [a, b, [px, py]], extra -> [a, b, [px + extra, py + extra]] end

try_div = fn a, b ->
  if rem(a, b) != 0, do: nil, else: div(a, b)
end

solve = fn [[ax, ay], [bx, by], [px, py]] ->
  a_coeff = ax * by - ay * bx
  pa_coeff = px * by - py * bx
  a = try_div.(pa_coeff, a_coeff)

  b_coeff = bx * ay - by * ax
  pb_coeff = px * ay - py * ax
  b = try_div.(pb_coeff, b_coeff)

  if is_nil(a) or is_nil(b), do: 0, else: 3 * a + b
end

contents
|> String.split("\n\n")
|> Enum.map(parse)
|> Enum.map(&adjust_prize.(&1, 10_000_000_000_000))
|> Enum.map(solve)
|> Enum.sum()