It is and it isn't. The DFA is not deliberately constructed, it's more a "happy accident". A renaming/reframing of the `seen` variable would make it obvious that what is constructed is a manually conceived DFA. As presented, `seen` is "how many characters have we seen of the target string?". Reframing it as "There exists a state for each character in the target, `seen` represents which of these states we are presently in" makes it clear that it is a DFA. But as constructed it's an accidental DFA, rather than a deliberate one.