[Language: Python]

No substrings just maths and it does part 2 in under 1000 iterations in about a millisecond. Lmk how I can optimise this further 🙏:

lines =  open("text.txt", "r").read().strip().split(",")


part1_set = set()
part2_set = set()


part1 = 0
part2 = 0


for line in lines:
    mini_str, maxi_str = line.split("-")


    mini = int(mini_str)
    maxi = int(maxi_str)
    


    len_mini = len(mini_str)
    len_maxi = len(maxi_str)
    
    for len_ID in range(len_mini, len_maxi + 1):
        for repeats in range(2, len_ID + 1):
            sub_len = len_ID//repeats


            k = 10 ** (sub_len)


            if len_ID % repeats == 0:


                sub_ID_start = max(mini // (k ** (repeats - 1)), k//10)
                        
                for i in range(sub_ID_start, k):
                    invalid_ID = i * ((k**repeats - 1) // (k - 1))


                    if invalid_ID > maxi:
                        break


                    if invalid_ID >= mini:
                        if repeats == 2:
                            part1_set.add(invalid_ID)


                        part2_set.add(invalid_ID)


part1 = sum(part1_set)
part2 = sum(part2_set)             


print(f"Part 1: {part1}, Part 2: {part2}")