defmodule Abundance do
  def is_abundant?(n) do
    cache = Process.get(:abundance, %{})

    case cache |> Map.get(n) do
      nil ->
        sum = Euler.Algebra.divisors(n) |> Enum.sum()
        is? = sum > n
        Process.put(:abundance, Map.put(cache, n, is?))
        is?
      is? -> is? 
    end
  end
end

# sum of all numbers that can be expressed as sum of two abundant numbers
sum = for n <- 1..28123,
  mid = div(n, 2),
  Enum.any?(1..mid, &(Abundance.is_abundant?(&1) and Abundance.is_abundant?(n - &1))),
  reduce: 0
  do
    s -> s + n
end


(((28123*28124)/2) - sum)
|> IO.puts()