Advent of Code: Day #09 - Encoding Error

We're breaking the eXchange-Masking Addition System (XMAS - Wow I only just got that joke, that's awful) today!

The XMAS encryption algorithm sends 25 numbers before the data starts. Every number after that must be equal to the sum of any two of the previous 25 numbers. We're tasked with finding the first number that is not valid.

Reading in the data

Today I just read each number into an array.

def read_data(path):
    dataset = []
    
    with open(path, 'r') as f:
        for line in f.readlines():
            dataset.append(int(line))

    return dataset

The Solution

I used the combinations function form itertools to genetrate every 2 digit pair of the last 25 numbers and then took the sum of them. Then it's as simple as chekcing if current number is in this list, if it's not we return it.

def task_one(numbers, p_size=25):
    for i, n in enumerate(numbers[p_size:]):
        possibilities = [sum(group) for group in
                         combinations(numbers[i:i + p_size], 2)]
        if n not in possibilities:
            return n

The Solution

For task two we must find a contiguous set of numbers that sum to the invalid number we found in part one. I took a brute force approach to this and generated every combination and check if they sum to the invalid number.

def task_two(numbers, invalid):
    for i in range(len(numbers)):
        for j in range(i + 2, len(numbers)):
            if sum(numbers[i:j]) == invalid:
                return min(numbers[i:j]) + max(numbers[i:j])

A super small condensed version of the code.

from itertools import combinations
d = [int(i) for i in open("input.txt").read().split("\n")]
print([n for i, n in enumerate(d[25:]) if n not in [sum(g) for g in combinations(d[i:i + 25], 2)]][0])
print([min(d[i:j]) + max(d[i:j]) for i in range(len(d)) for j in range(i + 2, len(d)) if sum(d[i:j]) == 2089807806][0])