defmodule Dominator do
  def dominator(a) do
    {size, value, index} =
      a
      |> Enum.with_index()
      |> Enum.reduce({0, nil, nil}, fn {v, k}, {size, value, index} ->
        cond do
          size == 0 -> {1, v, k}
          value != v -> {size - 1, value, index}
          true -> {size + 1, value, index}
        end
      end)

    candidate = if size > 0, do: value, else: -1
    count = Enum.count(a, &(&1 == candidate))

    if count > div(length(a), 2), do: index, else: -1
  end
end