Efficient date range overlap calculation in python?

Solution 1:

• Determine the latest of the two start dates and the earliest of the two end dates.
• Compute the timedelta by subtracting them.
• If the delta is positive, that is the number of days of overlap.

Here is an example calculation:

>>> from datetime import datetime
>>> from collections import namedtuple
>>> Range = namedtuple('Range', ['start', 'end'])

>>> r1 = Range(start=datetime(2012, 1, 15), end=datetime(2012, 5, 10))
>>> r2 = Range(start=datetime(2012, 3, 20), end=datetime(2012, 9, 15))
>>> latest_start = max(r1.start, r2.start)
>>> earliest_end = min(r1.end, r2.end)
>>> delta = (earliest_end - latest_start).days + 1
>>> overlap = max(0, delta)
>>> overlap
52

Solution 2:

Function calls are more expensive than arithmetic operations.

The fastest way of doing this involves 2 subtractions and 1 min():

min(r1.end - r2.start, r2.end - r1.start).days + 1

compared with the next best which needs 1 subtraction, 1 min() and a max():

(min(r1.end, r2.end) - max(r1.start, r2.start)).days + 1

Of course with both expressions you still need to check for a positive overlap.