defmodule EquiLeader do
  def equi_leader(a) do
    {leader_size, value} =
      Enum.reduce(a, {0, nil}, fn v, {size, value} ->
        cond do
          size == 0 -> {1, v}
          value != v -> {size - 1, value}
          true -> {size + 1, value}
        end
      end)

    count = length(a)
    candidate = if leader_size > 0, do: value, else: -1
    leader_count = Enum.count(a, &(&1 == candidate))
    leader = if leader_count > div(count, 2), do: candidate, else: -1

    {equi_leaders, _l_leader_count} =
      a
      |> Enum.with_index()
      |> Enum.reduce({0, 0}, fn {v, k}, {equi_leaders, l_leader_count} ->
        left_half = div(k + 1, 2)
        right_half = div(count - k - 1, 2)
        l_leader_count = if v == leader, do: l_leader_count + 1, else: l_leader_count
        r_leader_count = leader_count - l_leader_count

        equi_leaders =
          if l_leader_count > left_half and r_leader_count > right_half do
            equi_leaders + 1
          else
            equi_leaders
          end

        {equi_leaders, l_leader_count}
      end)

    equi_leaders
  end
end