-module(word_split).
-export([words/1, to_list/1]).


% data WordState = {chunk, S} | {segment, L, C, R}

nil() ->
    nil.

singleton(S) ->
    {single, S}.

conc(A, B) ->
    {conc, A, B}.

to_list(nil) ->
    [];
to_list({single, S}) ->
    [S];
to_list({conc, A, B}) ->
    to_list(A) ++ to_list(B).

maybe_word("") ->
    nil();
maybe_word(S) ->
    singleton(S).

process_char($ ) ->
    {segment, "", nil(), ""};
process_char(C) ->
    {chunk, [C]}.

combine({chunk, S1}, {chunk, S2}) ->
    {chunk, S1 ++ S2};
combine({chunk, S}, {segment, L, C, R}) ->
    {segment, L ++ S, C, R};
combine({segment, L, C, R}, {chunk, S}) ->
    {segment, L, C, R ++ S};
combine({segment, L1, C1, R1}, {segment, L2, C2, R2}) ->
    {segment, L1, conc(C1, conc(maybe_word(R1 ++ L2), C2)), R2}.

words([X|XS]) ->
    T = lists:foldl(fun(A, B) -> combine(B, A) end, process_char(X),
		    lists:map(fun(C) -> process_char(C) end, XS)),
    case T of
	{chunk, C} ->
	    maybe_word(C);
	{segment, L, C, R} ->
	    conc(maybe_word(L), conc(C, maybe_word(R)))
    end.