Fill down a row count until a new value is reached?

Newbie question - thanks for any help.

I have the following:

31
2
4
4
2
3
61
9
3
2
31
2
3
4

And would like a formula to number the rows until the next 1 occurs. So in the end, I want it to look like:

31
22
43
44
25
36
61
92
33
24
31
22
33
24

It's about 500,000 rows long, so any formula shortcut would be appreciated.

Thanks!

sorry if it's unclear that the 1's are in a separate column...

3 1
2
4
4
2
3
6 1
9
3
2
3 1
2
3
2

attached please find the file with both working option a macro and the formulas.

the macro requires less overhead since it does not need to be copied down

Thanks - but the numbers are in separate columns. Please see attached for an example.

Fotis - Thank you, that worked perfectly.