df has an integer column 'X'. For each value, count the distance back to the previous zero (or the start, whichever is closer). Return this as a Series.

X = [7,2,0,3,4,2,5,0,3,4]  ->  [1,2,0,1,2,3,4,0,1,2]

Sample data: df:

   X
0  7
1  2
2  0
3  3
4  4
5  2
... (10 rows total)