# Uniformly random efficient sampling of shortest s-t paths, with optimal random bits

The answers give methods of randomly sampling shortest $s\text{-}t$ paths. However, they use a lot of seemingly unnecessary random bits.

My question is:

Can the solution be improved to use a single random number in interval $[0,w(t))$, where $w(t)$ is the total number of shortest paths from $s\text{-}t$.

Alternatively, can the solution be improved to use $\left\lceil \log_2 w(t)\right\rceil$ random bits?

## 2 Answers

D. W. computes for each node $v \in S$ the number of paths $n(v)$ from $v$ to $t$. Using this information, it is easy to decode a number in the range $[0,n(s))$ to a unique path from $s$ to $t$ in $S$ (and so in the original graph). More generally, at each node $v$ we will come up with a procedure to map $[0,n(v))$ to a unique path from $v$ to $t$. Let the children of $v$ be $v_1,\ldots,v_k$. The idea is to write $$[0,n(v)) = [0,n(v_1)) \cup [n(v_1),n(v_1)+n(v_2)) \cup \cdots [n(v_1)+\cdots + n(v_{k-1}), n(v_1)+\cdots+n(v_k)),$$ and use the $i$th component to encode paths going through $v_i$. By subtracting $n(v_1) + \cdots + n(v_{i-1})$, we then reduce the problem to decoding a number in $[0,n(v_i))$ to a unique path from $v_i$ to $t$. Pseudocode left to the reader.

The answer is yes, if you'll grant me some reasonable assumptions.

Generate a random seed (uniformly at random), then use a cryptographically secure pseudorandom generator to stretch it to an arbitrarily long sequence of pseudorandom bits. Under reasonable cryptographic assumptions, that pseudorandom sequence will be computationally indistinguishable from true randomness and so will be just as good as real randomness.

The cryptographic assumptions required are widely believed to be true, e.g., that AES is secure. The likelihood of a practical break of AES is almost certainly far smaller than the likelihood of a bug in your code -- so from a practical perspective, it's in the noise.

From a theoretical perspective, the situation is different. This answer is only implemented at practical implementation, not at proving theorems about complexity classes.