Question

I have a list of tuples where each tuple is a (start-time, end-time). I am trying to merge all overlapping time ranges and return a list of distinct time ranges.
For example

[(1, 5), (2, 4), (3, 6)] --->  [(1,6)]
[(1, 3), (2, 4), (5, 8)] --->  [(1, 4), (5,8)]

Here is how I implemented it.

# Algorithm
# initialranges: [(a,b), (c,d), (e,f), ...]
# First we sort each tuple then whole list.
# This will ensure that a<b, c<d, e<f ... and a < c < e ... 
# BUT the order of b, d, f ... is still random
# Now we have only 3 possibilities
#================================================
# b<c<d: a-------b           Ans: [(a,b),(c,d)]
#                  c---d
# c<=b<d: a-------b          Ans: [(a,d)]
#               c---d
# c<d<b: a-------b           Ans: [(a,b)]
#         c---d
#================================================
def mergeoverlapping(initialranges):
    i = sorted(set([tuple(sorted(x)) for x in initialranges]))

    # initialize final ranges to [(a,b)]
    f = [i[0]]
    for c, d in i[1:]:
        a, b = f[-1]
        if c<=b<d:
            f[-1] = a, d
        elif b<c<d:
            f.append((c,d))
        else:
            # else case included for clarity. Since 
            # we already sorted the tuples and the list
            # only remaining possibility is c<d<b
            # in which case we can silently pass
            pass
    return f

I am trying to figure out if

1. Is the a an built-in function in some python module that can do this more efficiently? or
2. Is there a more pythonic way of accomplishing the same goal?

Your help is appreciated. Thanks!

Answer 1

A few ways to make it more efficient, Pythonic:

1. Eliminate the set() construction, since the algorithm should prune out duplicates during in the main loop.
2. If you just need to iterate over the results, use yield to generate the values.
3. Reduce construction of intermediate objects, for example: move the tuple() call to the point where the final values are produced, saving you from having to construct and throw away extra tuples, and reuse a list saved for storing the current time range for comparison.

Code:

def merge(times):
    saved = list(times[0])
    for st, en in sorted([sorted(t) for t in times]):
        if st <= saved[1]:
            saved[1] = max(saved[1], en)
        else:
            yield tuple(saved)
            saved[0] = st
            saved[1] = en
    yield tuple(saved)

data = [
    [(1, 5), (2, 4), (3, 6)],
    [(1, 3), (2, 4), (5, 8)]
    ]

for times in data:
    print list(merge(times))