Bash Expansion Interview Answer

bash_expansion.erl

expand(Bin) ->
    expand_1(Bin, [<<>>], []).

append(C, Prefixes) ->
    lists:map(fun (Prefix) ->
                      <<Prefix/binary, C>>
              end, Prefixes).

expand_1(Bin, Prefixes, Acc) ->
    expand_1(Bin, Prefixes, Prefixes, Acc).

expand_1(<<C, Rest/binary>>, Pre0, Cur0, Acc0) ->
    case expand_2(C, Pre0, Cur0, Acc0) of
        {Pre1, Acc1} ->
            expand_1(Rest, Pre1, Acc1);
        {Pre1, Cur1, Acc1} ->
            expand_1(Rest, Pre1, Cur1, Acc1)
    end;
expand_1(<<>>, _, Cur, Acc) ->
    lists:reverse(Cur, Acc).

expand_2(C, Pre, Cur, Acc) ->
    case C of
        $} -> {Cur ++ Acc, []};
        ${ -> {Cur, Cur, Acc};
        $, -> {Pre, Pre, Cur ++ Acc};
        _  -> {Pre, append(C, Cur), Acc}
    end.

bash_expansion.rb

def expand str
  _, res, _ = str.each_char.reduce([[''], [''], []]) do |(pre, cur, acc), c|
    case c
    when '}'; [acc + cur, acc + cur, []]
    when '{'; [cur, cur, acc]
    when ','; [pre, pre, acc + cur]
    else    ; [pre, cur.map { |x| x + c }, acc]
    end
  end
  res
end