defmodule Ladder do
  import Bitwise

  def ladder(a, b) do
    mod = (1 <<< Enum.max(b)) - 1
    limit = Enum.max(a)
    fib = build_fib(limit, mod)

    a
    |> Enum.zip(b)
    |> Enum.map(fn {ai, bi} -> Map.get(fib, ai + 1) &&& (1 <<< bi) - 1 end)
  end

  defp build_fib(limit, mod) do
    Enum.reduce(2..(limit + 1)//1, %{0 => 0, 1 => 1}, fn i, fib ->
      value = (Map.get(fib, i - 1) + Map.get(fib, i - 2)) &&& mod
      Map.put(fib, i, value)
    end)
  end
end