How could I create a variable called test_index to enumerate (from 1 to ...) the tests for each record_id ?

And then create another variable called days_since_prior_test to calculate the difference in elapsed days between the test_date for that particular row and the test_date for the row with the same record_id and test_index - 1 ?

This would be simple if my data were in wide format i.e. one patient per row but it's not and I'd rather not reshape if unless necessary.

Thanks,
Jonathan
record_id test_date test_index days_since_prior_test
ID-072 2009-07-16
ID-072 2009-07-29
ID-073 2009-07-22
ID-073 2009-08-03
ID-074 2004-06-08
ID-074 2009-02-18
ID-074 2010-03-09
ID-074 2012-08-17
ID-076 1999-01-07
ID-076 2009-06-08
ID-076 2009-07-30
ID-076 2012-11-09
ID-077 2009-07-29
ID-077 2009-07-31
ID-086 2009-12-16