# Is the gas enough?

I have been trying to use the more general frameworks when I encounter dynamic programming questions, so I can demonstrate the power of abstraction later on.

Problem1

Let D=(V,A) be a directed acyclic graph. w(e) is the weight of an arc. A s-t path e_1,\ldots,e_m is called a \alpha-deficient path if \sum_{i=1}^k w(e_i) + \alpha \geq 0 for all 1 \leq k\leq m. Find the smallest \alpha, such that there is a \alpha-deficient s-t path.

One can view this problem as how much gas would one require to travel from s to t. If one knows how much gas one can gain or lose between arcs.

Let D(v) be the value of the minimum deficiency path from v to t, and D(t)=0.

\displaystyle D(v) = \min_{(v,u)\in A} \max(D(u)-w((v,u)),0)

Because the graph is acyclic, there is a nice ordering allowing us to compute this nicely.

Note this is exactly the best weight problem[1] under the semiring (\mathbb{R}\cup \{\infty\},\min,\otimes,\infty,0), where a \otimes b = \max(a-b,0).

One should prove it's a semiring before using it. Everything seems obvious except the distributive property of the \otimes operation. Here is a proof for one of the two distributive laws, the other is left as an exercise to the reader.

\displaystyle \begin{aligned} (a\oplus b) \otimes c &= \max(\min(a,b)-c,0)\\ &= \max(\min(a-c,b-c),0)\\ &= \min(\max(a-c,0),\max(b-c,0))\\ &= (a \otimes c) \oplus (b \otimes c) \end{aligned}

Can we extend the problem to directed graph with cycles? Yes. This semiring has the property that it is k-closed for any graph G for k depending on G and w. It means we can't keep going though a cycle and keep producing better solutions. We can run a generic single source shortest distance algorithm[2].

# References

[1] L. Huang, Advanced dynamic programming in semiring and hypergraph frameworks, (2008).

[2] M. Mohri, Semiring frameworks and algorithms for shortest-distance problems, J. Autom. Lang. Comb. 7 (2002) 321–350.

Posted by Chao Xu on 2014-02-25.
Tags: .