exhausted the masks of a given length ?

hello every gurus how are you 2017.

i have one question, could you please help:

line 347 menu.inc

elseif ($i < (1 << $length)) {
      // We have exhausted the masks of a given length, so decrease the length.
      --$length;
    }

why say “// We have exhausted the masks of a given length, so decrease the length.” ? could some one help to clarify it? thanks.

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