defmodule MinAvgTwoSlice do
  def min_avg_two_slice(a) do
    size = length(a)
    a_map = a |> Enum.with_index() |> Map.new(fn {v, i} -> {i, v} end)
    initial_avg = (Map.get(a_map, 0) + Map.get(a_map, 1)) / 2

    {idx, _min_avg} =
      Enum.reduce(0..(size - 2), {0, initial_avg}, fn i, {idx, min_avg} ->
        two = (Map.get(a_map, i) + Map.get(a_map, i + 1)) / 2

        cur =
          if Map.has_key?(a_map, i + 2) do
            three = (Map.get(a_map, i) + Map.get(a_map, i + 1) + Map.get(a_map, i + 2)) / 3
            min(two, three)
          else
            two
          end

        if cur < min_avg, do: {i, cur}, else: {idx, min_avg}
      end)

    idx
  end
end