# Brzozowski's algorithm for DFA minimization

Brzozowski's DFA minimization algorithm builds a minimal DFA for DFA $G$ by:

1. reversing all the edges in $G$, making the initial state an accept state, and the accept states initial, to get an NFA $N'$ for the reverse language,
2. using powerset construction to get $G'$ for the reverse language,
3. reversing the edges (and initial-accept swap) in $G'$ to get an NFA $N$ for the original language, and
4. doing powerset construction to get $G_{\min}$.

Of course, since some DFA's have an exponential large reverse DFA, this algorithm runs in exponential time in worst case in terms of the size of the input, so lets keep track of the size of the reverse DFA.

If $N$ is the size of the input DFA, $n$ is the size of the minimal DFA, and $m$ the size of the minimal reverse DFA, then what is the run time of Brzozowski's algorithm in terms of $N$,$n$, and $m$?

In particular, under what relationship between $n$ and $m$ does Brzozowski's algorithm outperform Hopcroft's or Moore's algorithms?

I have heard that on typical examples in practice/application, Brzozowski's algorithm outperforms the others. Informally, what are these typical examples like?

• it would be helpful if you include the O(f(n)) estimates of these algorithms. are they all O(n log(n)) in "average" case? if so the debate over their relative performance might be mostly an applied test depending on statistical characteristics/structure of the input... it seems likely that Brzozowski runs quickly when the reverse NFA is "not large"...?
– vzn
Aug 13, 2012 at 17:43
• Be careful with the execution of the algorithm, you might be tempted to introduce a virtual start state when performing 1. and 3., which will lead to incorrect results - see here. (Its not wrong in the question, you just have to be careful not getting it wrong.) Oct 18, 2012 at 19:59

Most of the below is from Parsing Theory by Sippu and Soisalon-Soininen.

Let $Q$ be the set of states of the DFA. Let $T$ be the input alphabet. Let $|M|=O(|T| \cdot |Q|)$ be the size of the machine. Exercise 3.40 gives a $O(|T|\cdot|Q|^2)$ algorithm for state minimization. As Wikipedia describes, Hopcroft's algorithm has a running time of $O(|T| \cdot |Q| \cdot \log|T|)$ and Moore's algorithm has a running time of $O(|T|^2 \cdot |Q|)$.

Theorem 3.30 states that the subset construction can be done in $O(2^{|T|+\log|T|+\log|Q|})$ yielding an automaton of size $O(2^{|T|+\log|Q|})$ (actually, if the resulting automaton has $|T'|$ states, the running time is $(|T'| + |T|\cdot|M|) \cdot |Q|$). The two reversals and the second determinisation are therefore inconsequential in the running time, so the asymptotic running time of Brzozowski's algorithm is the same as that of the subset construction.

This means that in the worst case, Brzozowski's algorithm is exponentially slower than the other three algorithms. Note that the worst case really does occur: the classic example of the NFA for the language $(a|b)^*a^k$ has $k+1$ states and its corresponding minimal DFA has $O(2^k)$ states, while the reverse of the NFA is deterministic, so running Brzozowski's algorithm on this reversed NFA triggers the worst-case behavior.

However, if the subset construction yields an automata of size $|T'|=O(|T|)$, then its running time is also $O(|T|^2 \cdot |Q|^2)$, which is often the case on real-life inputs. Furthermore, if proper care is taken when computing the closure of a state, then this can be done much faster in most cases (that is, cases where the closure is small), saving a factor $|T|$ in practice (for essentially the same reason that transitive closures can be computed quite quickly on real-world examples). Furthermore, if the input and intermediate automatons are sparse, which means that states have few transitions, then a factor $|Q|$ is saved, which gives a $O(|T| \cdot |Q|)$ running time on 'good' inputs.

Unfortunately, I'm not familiar enough with Hopcroft's or Moore's algorithms to give an analysis of their running times in typical cases. Wikipedia talks about a $O(|T| \log \log |T|)$ running time in some cases, which would make the three algorithms comparable.

Here's partial answer regarding your third question. In fact, perhaps Brzozowski's algorithm really doesn't outperform all the other algorithms so clearly in DFA minimization.

In , the authors investigate the practical performance of DFA/NFA minimization algorithms. The algorithms are Hopcroft's, Brzozowski's, and two variants of Watson's. They conclude that there's no clear winner, but Hopcroft's algorithm performs better for DFAs with small alphabets. For NFAs, Brzozowski is clearly the fastest one.

The paper itself is quite short and clearly written. There's also additional discussion and references that might be helpful.

• Thank you, I will take a look at the paper and see if I can use the references to find a complete answer. Aug 10, 2012 at 23:27

De Felice and Nicaud show that Brzozowski's algorithms is asymptotically hyper-polynomial. David has shown that, for several distributions on final states, Hopcroft's algorithm is slower that Moore's algorithm.

References

S. De Felice and C. Nicaud, "Brzozowski Algorithm is Generically Super-Polynomial for Deterministic Automata". In Proceedings of 17th International Conference on Developments in Language Theory (DLT 2013), Lecture Notes in Computer Science, pp. 170–190, 2013. (PDF)

J. David, "Average complexity of Moore’s and Hopcroft’s algorithms". Theoretical Computer Science, 417:50–65, 2012. (Science Direct)