For Day 9, the puzzle was fairly straightforward. Given a list of numbers and a “preamble” size n, find the first number after the nth number that is not the sum of two of the previous numbers in the list. This check can be done for an arbitrary number s like so:

def sum_in_window(nums, s):
    for num1 in nums:
        for num2 in nums:
            if num1 + num2 == s:
                return True
    return False

So the first part boils down to:

preamble_length = 25
for i in range(preamble_length, len(prog)):
    window = prog[i - preamble_length:i]
    if not sum_in_window(window, prog[i]):
        p1 = prog[i]

For the second part, the puzzle asks for a contiguous subset of numbers from the list that sum to the number found in part 1, and the sum of the smallest and larget numbers in that subset. This can be done like so:

# Iterate through list and keep rolling sum
for i in range(len(prog)):
    s = 0
    j = i
    while s < p1:
        s += prog[j]
        j += 1
    if s == p1:
        start = i
        end = j
p2 = min(prog[start:end]) + max(prog[start:end])

I’m sure there are more efficient ways to solve the problem, but I did not find this day’s puzzle particuarly interesting.