Shuttle Search

defmodule Aoc2020.Day13 do
  @moduledoc "Shuttle Search"

  def run(), do: part_1()

  def part_1() do
    {timestamp, buses} = parse_input()

    Enum.map(buses, fn bus -> {bus, minutes_after_timestamp(timestamp, bus)} end)
    |> Enum.sort_by(&elem(&1, 1))
    |> List.first()
    |> (fn {bus, minutes} -> bus * minutes end).()
  end

  def parse_input() do
    File.read!("priv/inputs/2020/day13.txt")
    |> String.trim()
    |> String.split("\n")
    |> (fn [timestamp, line] ->
          {String.to_integer(timestamp), parse_buses(line)}
        end).()
  end

  def parse_buses(line) do
    line
    |> String.split(",")
    |> Enum.filter(&(&1 != "x"))
    |> Enum.map(&String.to_integer/1)
  end

  def minutes_after_timestamp(timestamp, bus) do
    multiple = floor(timestamp / bus)
    bus * (multiple + 1) - timestamp
  end
end