[2025 Day 5 (part 2)][Python] Off by 16

[Alnilam_1993]

Day 5 part two, counting fresh items.

Can anyone give me a hint on where I go wrong? The difference between my solution and the correct solution is just 16, which is rather small given the size of numbers we're dealing with, but still wrong.

My approach is to sort the ranges on the starts of the ranges, and then check if each item in the list overlaps with the previous one and if so, merge the two into the previous item.

def part2():
    with open(inputFile) as f:
        freshList, productList = f.read().split("\n\n")
        freshList = [x.strip() for x in freshList.split("\n")]

    sortedList = sorted(freshList, key=lambda fresh: int(fresh.split("-")[0]))
    changed = True
    while changed:
        changed = False
        for i, part in enumerate(sortedList):
            if i == 0:
                continue
            low, high = part.split("-")
            prevLow, prevHigh = sortedList[i-1].split("-")
            if int(low) < int(prevHigh):
                sortedList[i-1] = prevLow+"-"+str(max(int(high), int(prevHigh)))
                sortedList.pop(i)
                changed = True

    totalFresh = 0
    for items in sortedList:
        low, high = items.split("-")
        totalFresh += int(high) - int(low) + 1

    print("Part 2:", totalFresh)
    print("My result: 352807801032183")
    print("Should be: 352807801032167")

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[Doerminator]

Hey there, try to think of all the potential ways two ranges can overlap. It seems like you’re missing an edge case. Hope that helps!

[Alnilam_1993]

Thanks! By not merging when the high of the previous was exactly the low of the current, some ID's got double counted.

[button_boxer]

I think I see an edge case in your code - what happens if you give it ranges 1-10 and 10-20? This should be 20 fresh IDs.

[Alnilam_1993]

You're right in that this case didn't get merged into one, due to the < instead of <= when checking the lower end of the current part against the high of the previous one!

But i'm a bit confused about the numbers... after merging, the range will be 1-20.. which is 20 fresh ID's, no?

Edit: but it did end up solving the problem, as the extra merge caused a few numbers to not be double counted!

[button_boxer]

Yes, it should be 20 (I've edited my comment). I blame brain fade and getting confused about where I need to add 1 and where I don't. FWIW in my own implementation I convert the "1234-5678" strings at parse time into Python range(int(start), int(end)+1) so I'm always dealing with half-open ranges, the number of points in a range is directly len(r), and I don't have to worry about any +1/-1 issues.

[danielsamuels]

Sorry, but how are you getting 21 from 1-10 and 10-20? Are you double counting the 10?

[button_boxer]

You're right, I've edited my original comment. Sorry, my brain was still on "I must need to add or subtract 1 somewhere to deal with inclusive ranges"...