# A nice urn problem

While implementing counterfactual execution of Kappa traces, I came upon the following urn problem:

Let’s consider $$n$$ urns, each of them containing both white and black balls. What is an efficient way to pick uniformly one ball in every urn conditionned to the fact that one ball at least has to be black ?

One obvious algorithm would be to make a series of draws and to reject everyone but the first one featuring at least one black ball (rejection sampling). However, this can get very inefficient when black balls are rare in every box, which is typically the case in the application I had in mind.

## Formal statement

The problem can be reduced to a simpler form. Indeed, instead of directly picking one ball per urn, one can draw a vector $$x$$ of size $$n$$ whose coordinates are either $$0$$ for white and $$1$$ for black and then pick either a white or black ball uniformly in each urn depending on the corresponding coordinate of $$x$$. If we write $$p_{i}$$ the ratio of black balls in urn $$i$$, then the probability that vector $$x$$ is drawn is equal to $$F {\left(x\right)}$$ where:

$F {\left(x\right)} = \prod_{i} p_{i}^{x_{i}} {\left(1 - p_{i}\right)}^{{\left(1 - x_{i}\right)}}$

Then, the probability of drawing $$x$$ conditioned to the fact that $$x$$ contains at least one $$1$$ coordinate is equal to $$0$$ if $$x = 0$$ and $\frac{F {\left(x\right)}}{1 - \prod_{i} {\left(1 - p_{i}\right)}}$ otherwise.

## Solution

### Implementation in OCaml

 (* This function solves the following problem: Given n boxes containing white and black balls, pick one ball from each box conditionned to the fact that at least one ball has to be black. Input: [prob] is an array of size n whose ith element is the probability to draw a black ball from the ith box.Agent Returns: an array of n booleans, [true] representing a black ball and [false] a white ball. *) let draw_at_least_one probs = let draw p = let x = Random.float 1.0 in int_of_float (ceil (log1p (-. x) /. log1p (-. p))) in let array_min t = Array.fold_left min t.(0) t in let r = Array.map draw probs in let m = array_min r in Array.map (fun e -> e = m) r