aoc2020day13.ex

defmodule Day13 do
  def read_input do
    [start_time, bus_times] = File.read!("input/day13.txt")
                              |> String.split("\n", trim: true)

    times = String.split(bus_times, ",", trim: true)
            |> Enum.reject(&(&1 == "x"))
            |> Enum.map(&String.to_integer/1)

    [String.to_integer(start_time), times]
  end

  def read_input_2 do
    File.read!("input/day13.txt")
    |> String.split("\n", trim: true)
    |> Enum.at(1)
    |> String.split(",", trim: true)
  end

  def part1 do
    [start_time, bus_times] = read_input()
    IO.inspect(start_time)
    IO.inspect(bus_times, charlists: :as_lists)

    [id, depart_time] = bus_times
                        |> Enum.map(&([&1, ceil(start_time / &1) * &1]))
                        |> Enum.min_by(fn [_id, time] -> time end)
                        |> IO.inspect

    IO.puts("result = #{id * (depart_time - start_time)}")
  end

  def find_part_2(seq, index \\ 0, result \\ 0, multiplication_so_far \\ 1)

  def find_part_2([], _index, result, _multiplication_so_far) do
    result
  end

  def find_part_2(["x" | tail], index, result, multiplication_so_far) do
    find_part_2(tail, index + 1, result, multiplication_so_far)
  end

  def find_part_2([shead | tail], index, result, multiplication_so_far) do
    IO.puts("index = #{index}; result=#{result}, mul=#{multiplication_so_far}, head=#{shead}")
    head = String.to_integer(shead)
    required_modulu = rem((head - index) + floor(index / head) * head, head)

    next_result = (0..multiplication_so_far * head)
                  |> Enum.find(&(rem(&1 * multiplication_so_far + result, head) == required_modulu))
                  |> Kernel.*(multiplication_so_far)
                  |> Kernel.+(result)


    find_part_2(tail, index + 1, next_result, multiplication_so_far * head)
  end

  def part2 do
    # find_part_2(["17", "x", "13", "19"])
    read_input_2
    |> find_part_2()
    |> IO.inspect
  end
end