Advent of Code 2025

Python
1

I was very late to the (Christmas) party, discovering Advent of Code just this year.

It was proving quite fun until I got stuck on Day 5 Part 2.

The challenge (as I understand it) is to take this input (a list of integer ranges) and return the total number of unique integers which appear within it.

My approach was to sort the list by first number in range and then by second number, then merge all overlapping ranges so that all ranges within the list are distinct. Finally I iterate the list again and tot up the number of integers in each range to reach a final total value.

My code works on the sample code but apparently not on the much larger input above.

EDIT: This code does not work on the sample code (earlier iterations did).

def total_fresh_ids(ranges):

  total=0

  for i,_ in enumerate(ranges):
    ranges[i] = ranges[i].split('-')

  ranges.sort(key=lambda r: (int(r[0]), int(r[1])))
  #print(ranges)

  for i,x in enumerate(ranges):
    if i<len(ranges)-1:
      if int(x[1])>=int(ranges[i+1][0]):

        #print("merge", x, ranges[i+1])

        #merge with next
        ranges[i+1][0] = x[0]

        #print("merged", ranges[i+1])

        del ranges[i]

      #else:
        #print("no merge", x)

  for z in ranges:

    total += int(z[1])-int(z[0]) + 1
    #print(total) 

  return ("total:", total)

print(total_fresh_ids(ranges))
# output: ('total:', 346024201522906)

I converted the above input into a list in this format (before executing the above code) :

ranges = [ '70642195371793-72879218404633',
  '173870324743912-174003164605263',
  '201707880905704-210661896929678',
  '506861791312713-507300657398991',
  '323726482687518-323726482687518',
  '450429140158342-450429140158342',
  '81503580676246-81736399903236'
... etc ...
  '503254477307191-504094553585169',
  '367570185836554-369643343702845',
  '507300657398991-507790184520273',
  '264221915693854-268980593812546' ]

Can anyone help me understand what I’m doing wrong please?

2

It’s hard to say right now, considering it miscalculates for the sample. One option that’s always possible is that full puzzle input contains edge cases which sample omits.

The most risky in the current version I would call deleting items from list, which is iterated over.

3

Yes, I figured that using del to modify a list whilst iterating it using len might be an issue, so I refactored and this code does work for the sample code but still not for the full input:

def total_fresh_ids(ranges):

  total=0

  for i,_ in enumerate(ranges):
    ranges[i] = ranges[i].split('-')

  ranges.sort(key=lambda r: (int(r[0]), int(r[1])))
  #print(ranges)

  for i,x in enumerate(ranges):
    if i<len(ranges)-1:
      if int(x[1])>=int(ranges[i+1][0]):

        #print("merge", x, ranges[i+1])

        #merge with next
        ranges[i+1][0] = x[0]

        #print("merged", ranges[i+1])
        ranges[i] = ['empty']
        
      #else:
        #print("no merge", x)

  for z in ranges:
    if z == ['empty']:
        continue
    total += int(z[1])-int(z[0]) + 1
    #print(total) 

  return ("total:", total)

print(total_fresh_ids(ranges))
# output: ('total:', 335628055787604)

I can’t think of any possible edge cases that I might be missing… but I must be doing something wrong.

Sample input (should return 14):

sample = ['3-5', '10-14', '16-20', '12-18']
4

I’m getting incorrect result for following example:

['3-5', '9-22', '10-14', '16-20', '12-18']

I’m amazed with simplicity of the idea for the logic here.

1 Like
5

Ah, I see. Thanks! It’s a fairly simple logical error which I overlooked and have now rectified.

I passed - thank you for your invaluable insight!

1 Like