-module(equi_leader).
-export([equi_leader/1]).

equi_leader(A) ->
    N = length(A),
    {LeaderSize, LeaderValue} = leader_scan(A),
    Candidate = case LeaderSize > 0 of
        true -> LeaderValue;
        false -> -1
    end,
    LeaderCount = length([X || X <- A, X =:= Candidate]),
    Leader = case LeaderCount > N / 2 of
        true -> Candidate;
        false -> -1
    end,
    {_, Count} = lists:foldl(fun({K, V}, {LCount, Equi}) ->
        LCount1 = case V =:= Leader of
            true -> LCount + 1;
            false -> LCount
        end,
        LeftHalf = (K + 1) div 2,
        RightHalf = (N - K - 1) div 2,
        RCount = LeaderCount - LCount1,
        Equi1 = case LCount1 > LeftHalf andalso RCount > RightHalf of
            true -> Equi + 1;
            false -> Equi
        end,
        {LCount1, Equi1}
    end, {0, 0}, lists:zip(lists:seq(0, N - 1), A)),
    Count.

leader_scan(A) ->
    lists:foldl(fun(V, {Size, Value}) ->
        case Size of
            0 -> {1, V};
            _ ->
                case Value =:= V of
                    true -> {Size + 1, Value};
                    false -> {Size - 1, Value}
                end
        end
    end, {0, undefined}, A).