Find the number of observations since the opposite event occured in R using datatables

I am hoping to create a new variable in datatable that adds a column telling me the number of days since the opposite (or other event occured).

The dataset I have looks like the following:

  date     event  id obs_since_event_1 obs_since_event_2
2000-07-06     2  1        NA                 NA
2000-07-07     1  1        NA                 1
2000-07-09     0  1        1                  2
2000-07-10     0  1        2                  3
2000-07-15     2  1        3                  4
2000-07-16     1  1        4                  1
2000-07-20     0  1        1                  2
2000-07-21     1  1        2                  3
2000-07-06     1  2        NA                 NA
2000-07-07     2  2        1                  NA
2000-07-15     0  2        2                  1
2000-07-16     0  2        3                  2
2000-07-17     2  2        4                  3
2000-07-18     1  2        5                  1

And I am hoping to add a column called days_since_opposite. which records number of days since the opposite event occurred (the opposite events being 1 and 2). I already have the number of days since either an event 1 or 2 occurred. Now I need to work out an if statement that works in datatables that will provide me with the corresponding values in the final column.

  date     event  id obs_since_event_1 obs_since_event_2 days_since_opposite
2000-07-06     2  1        NA                 NA                  NA
2000-07-07     1  1        NA                 1                   NA
2000-07-09     0  1        1                  2                   NA
2000-07-10     0  1        2                  3                   NA
2000-07-15     2  1        3                  4                   3
2000-07-16     1  1        4                  1                   1
2000-07-20     0  1        1                  2                   NA
2000-07-21     1  1        2                  3                   3

I hope this is clear. I also have different ids to reckon with but not sure if it impacts the results.

I tried something along the following lines but it did not work:

data[,days_since_opposite:=ifelse(event==1,obs_since_event_2,ifelse(event==2,obs_since_event_1,0)),]

Thanks in advance