[R-sig-Geo] nearest distance in matrix

[Marius Gilbert]

Hello,

I'm trying to use R to simulate biological invasions, and got stuck with 
the following:

Is there a function that uses a matrix of occupied/empty values (1 or 0) 
as input, and producing an output matrix of the same size, with each cell 
containing the distance to the nearest occupied cell (1) of the input 
matrix ?

For example, the input matrix:

0    1    1    0
0    0    1    0
0    0    0    0
0    0    0    0

Would produce an output matrix like:

1    0    0    1
1.4  1    0    1
2.2  1.4  1    1.4
2.8  2.2  2.0  2.2	

For those familiar with AV Spatial Analyst, this function would be the 
equivalent of OutGrid = EucDistance (directionFN, allocationFN, 
maxDistance).

Many thanks in advance for any help or pointers,

Marius