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Let $G = (V, E)$ be a connected DAG (representing a circuit) with every vertex may be one of the following three types:

  1. Input variable, with in-degree $0$ and out-degree $\geqslant 1$.
  2. A gate, with in-degree $2$ and out-degree $\geqslant 1$.
  3. Output variable, with in-degree $1$ and out-degree $0$.

Does there exist a polynomial-time algorithm, which finds the minimal natural number $n$ and a function $f : V \rightarrow \{0, \cdots, n - 1\}$, such that:

  1. If $v \rightarrow w$ is an edge, $f(v) < f(w)$.
  2. If $f(v) = f(w)$, there doesn't exist an $u$ such that $u \rightarrow v$ and $u \rightarrow w$ are edges.

We denote the minimal possible $n$ by $M(G)$.

Without condition 2, the minimal number $n$ can be acquired by topological sorting $G$ and compute the "rank" of each gate by $\operatorname{rank}(u) = \max(\operatorname{rank}(v), \operatorname{rank}(w)) + 1$, where $v \rightarrow u, w \rightarrow u$.

Without the restriction of every gate has in-degree $\leqslant 2$, the problem is NP-hard. Given an undirected graph $H_0 = (V_0, E_0)$, we can construct a DAG $H_1 = (V_1, E_1)$, where $V_1 = V_0 \cup \{\alpha_e | e \in E_0\}$, and add edges $\alpha_e \rightarrow a, \alpha_e \rightarrow b$ for each $(a, b) \in E_0$. Then, we can compute the chromatic number of $H_0$ by $\chi(H_0) = M(H_1) - 1$, and thus determine whether $H_0$ is 3-colorable.

However, if $v \in H_0$ has degree $n$, then $v \in H_1$ has in-degree $n$. With $n \leq 2$ for every vertex, the computation of $\chi(H_0)$ is not hard, because $H_0$ will contain only simple patterns (lines, circles).

I wonder if the problem is NP-hard with this constraint of vertex type; if it is, do we have some polynomial-time approximating alogrithm?

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