Interview question solution

stockpair

defmodule Test do
  @moduledoc """
  Documentation for `Test`.
  """
  @spec find_stack_pair(any) :: Integer
  def find_stack_pair(list) do
    list
    |> pairwise()
    |> IO.inspect(label: "after pairing")
    |> calculate()
    |> List.last()
  end

  defp pairwise(list)   do
    list
    |> Enum.chunk_every(2, 1, :discard)
  end

  defp calculate(list) do
    list
    |> Enum.map(&Enum.sum/1)
    |> Enum.sort()
    |> duplicates()
  end

  defp duplicates(list) do
    acc_dupes = fn(x, {elems, dupes}) ->
      case Map.has_key?(elems, x) do
        true -> {elems, Map.put(dupes, x, nil)}
        false -> {Map.put(elems, x, nil), dupes}
      end
    end
    acc_dupes
    list |> Enum.reduce({%{}, %{}}, acc_dupes) |> elem(1) |> Map.keys()
  end
end

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