How to keep counting although it start at 1 again

My df looks as follows:

import pandas as pd
d = {'col1': [1,2,3,3,1,2,2,3,4,1,1,2]
df= pd.DataFrame(data=d)

Now I want to add a new column with the following schemata:

Once it starts again at 1 it should just keep counting.

At the moment I am at the point where I just add a column with difference:

df['diff'] = df['col1'].diff()

How to extend this approach?

>Solution :

Try with

df.col1.diff().ne(0).cumsum()
Out[94]: 
0     1
1     2
2     3
3     3
4     4
5     5
6     5
7     6
8     7
9     8
10    8
11    9
Name: col1, dtype: int32