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

max_double_slice_sum(A) ->
    Size = length(A),
    case Size < 3 of
        true -> 0;
        false ->
            Arr = array:from_list(A),
            P1 = build_p1(Arr, Size),
            P2 = build_p2(Arr, Size),
            lists:max([array:get(I, P1) + array:get(I, P2) || I <- lists:seq(1, Size - 2)])
    end.

build_p1(Arr, Size) ->
    P0 = array:set(1, 0, array:new(Size)),
    lists:foldl(fun(I, Acc) ->
        V = max(0, array:get(I - 1, Acc) + array:get(I - 1, Arr)),
        array:set(I, V, Acc)
    end, P0, lists:seq(2, Size - 2)).

build_p2(Arr, Size) ->
    P0 = array:set(Size - 2, 0, array:new(Size)),
    lists:foldl(fun(J, Acc) ->
        V = max(0, array:get(J + 1, Acc) + array:get(J + 1, Arr)),
        array:set(J, V, Acc)
    end, P0, lists:seq(Size - 3, 1, -1)).