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

%% Faithful port of the PHP source: it accumulates the loop index i into
%% firstPart/secondPart rather than a[i], so this mirrors that behaviour
%% (including the quirk) rather than the classic tape-equilibrium formula.
tape_equilibrium(A) ->
    N = length(A),
    Total = lists:sum(A),
    {_, _, Min} = lists:foldl(fun(I, {First, Second, MinV}) ->
        First1 = First + I,
        Second1 = Second - I,
        Diff = abs(First1 - Second1),
        {First1, Second1, min(MinV, Diff)}
    end, {0, Total, 1 bsl 128}, lists:seq(0, N - 2)),
    Min.