Can you trust it?

How do you know it’s right?

hpdr prints out some HQL when you call it with a couple of datetime objects, but how can you be sure what it prints out is accurate?

You could pretty print it and try to reason it out, but the whole purpose of hpdr is to eliminate that kind of tedious, error-prone approach.

Reversing the algorithm

hpdr is tested by comparing the number of seconds between the begin and end datetime objects with the number of seconds represented by each clause in the HQL output added together. Let’s look at a simple example.

#!/usr/bin/python2.7

from datetime import datetime

begin = datetime(2016, 02, 02, 18)
end = datetime(2016, 05, 11, 3, 56)
print((end - begin).total_seconds())

This prints 8502960.0 (seconds).

When we have hpdr print out the range, we get:

(
 YYYY=2016 AND
 (
     (MM=02 AND DD=02 AND HH>=18)
  OR (MM=02 AND DD>02)
  OR (MM>02 AND MM<05)
  OR (MM=05 AND DD<11)
  OR (MM=05 AND DD=11 AND HH<03)
  OR (MM=05 AND DD=11 AND HH=03 AND MIN<56)
 )
)

We can calcuate how many seconds each clause in HQL query represents by starting at the earliest possible datetime for the begin and end times, and then triangulating the durations each HQL condition represents.

condition group	seconds	from (inclusive)	to (exclusive)
MM=02 DD=02 HH>=18	21600	2016-02-02 18:00	2016-02-03 00:00
MM=02 DD>02	2332800	2016-02-03 00:00	2016-03-01 00:00
MM>02 MM<05	5270400	2016-03-01 00:00	2016-05-01 00:00
MM=05 DD<11	864000	2016-05-01 00:00	2016-05-11 00:00
MM=05 DD=11 HH<03	10800	2016-05-11 00:00	2016-05-11 03:00
MM=05 DD=11 HH=03 MIN<56	3360	2016-05-11 03:00	2016-05-11 03:56
total	8502960

If we further prohibit any empty condition groups – those which evaluate to 0 seconds – we can be fairly certain the results are correct.