Finding indices efficiently

Hey all,

I have a three dimensional int array of 1s and 0s. I need a reasonably efficient way to, given an index i,j,k of a cell containing 0, find the index of the nearest cell that contains a 1. Here I define nearest as euclidian distance, ie sqrt((i-x)^2 + (j-y)^2 +(k-z)^2)). It is quite a simple matter to brute force it, by nesting three loops and testing each cell that contains a one for proximity to the cell of interest and then taking the minimal one, but give that this array may have several 10s of millions of cells this may take longer than I would like.

So: any suggestions on making this a little more efficient? Ideally I would like to find a way to only search for ones within a certain radius of a given cell, but I am stuck with the same problem of determining which cells are in that radius without looping through the whole array.