advent-01.py

"""
URL for challenge: https://adventofcode.com/2020/day/1
"""

from functools import reduce
from operator import mul


def process_input():
    f = open("advent-01-input.txt")
    return [int(line) for line in f.readlines()]


def part1(target_amount, amounts):
    """
    By taking the intersection of the set of given amounts
    with the set of their inverses, the only amounts
    that will be left will be x and (target - x).
    """
    pair_entries = {target_amount - x for x in amounts}.intersection(amounts)
    num_entries = len(pair_entries)

    if num_entries == 1:
        return pair_entries.pop() ** 2

    if num_entries == 2:
        return reduce(mul, pair_entries)

    # This case is only possible when Part 1 is used by Part 2
    return None


def part2():
    amounts = process_input()
    inverse_amounts = [2020 - x for x in amounts]
    """
    Suppose x + inv = 2020 and x is assumed to be part of
    the solution, check if it is possible to find two
    numbers y, z from amounts-[x] s.t. y + z = inv
    (i.e. check existence of solution for part 1 with
    inputs inv and amounts-[x]).
    """
    for idx, inv in enumerate(inverse_amounts):
        sub_solution = part1(inv, amounts[:idx] + amounts[idx+1:])
        if sub_solution is not None:
            return amounts[idx] * sub_solution


def run():
    chall = int(input("Please enter either 1 or 2 for the challenges: "))
    if chall == 1:
        print(part1(2020, process_input()))
    elif chall == 2:
        print(part2())
    else:
        print("You need to enter either 1 or 2")
        exit(1)


run()