# Advent of Code 2020 in Python, Day 1

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I’m late to Advent of Code this year, but I thought I’d post my solution anyway. I can’t say if I’ll continue doing more of these (and posting them here) or not — I’m well-rested and have the energy after a week of vacation tonight, but I return to work on Monday and may get busy with other things in life.

The prompt for day 1 asks you, in the first part, to find the two integers in the input data that add to 2020, and multiply them together and return the product.

In my first solution, I wrote two functions. The first takes an integer `term`, a list of integer `terms`, and a desired `total` and returns the first integer from the list of terms that satisfies `term + x = total`.

``````def find_addend_to_sum_to_num(term1, terms, total=2020):
"""
Given a term (integer) and a list of terms, find the other term in the list
of terms to satisfy:
term + ____ = total
Returns an integer
"""
matches = [x for x in terms if total - x == term1]
if len(matches) < 1:
return None
return matches
``````

A second function takes the list of all `terms` and a desired `total` as arguments. It splits the `terms` into three lists: `lower` has all terms less than half of `total`; `upper` has all terms greater than half of `total`; and `half` has all terms that are half of `total`. I split it because any solution for two terms will either involve one number less than half of the total and one number greater than half of the total, or two numbers that are both exactly half of the total. If there are two items in `half`, that’s the solution. Otherwise, loop through the terms that are less than half of the `total` and use `find_addend_to_sum_to_num` to find a matching term from the `upper` terms.

``````def find_terms_adding_to_num(terms, total=2020):
"""
Given a list of `terms`, find the two that add to `total`
Returns a tuple of integers
"""
lower = [x for x in terms if x <= total / 2]
upper = [x for x in terms if x > total / 2]
half = [x for x in lower if x == total / 2]

if len(half) >= 2:
return (half, half)

for term in lower:
match = find_addend_to_sum_to_num(term, upper, total=2020)
if match:
return (term, match)

return (None, None)
``````

Part 2 is a variation on the theme, asking for three terms summing to 2020 instead of two.

At first I wrote another function to solve part 2 that works on a similar principle to `finding_terms_adding_to_num`, except instead of looping through all the lower terms it recursively loops through every term, since that trick doesn’t work if you’re dividing it in more than two pieces, and then loops through every other term in a `while` loop, checking for a matching third term with `find_addend_to_sum_to_num`.

``````def solve_part_2(terms, total=2020):
term1 = terms.pop()
working_terms = list(terms)
while len(working_terms) > 0:
term2 = working_terms.pop()
if term1 + term2 < total:
match = find_addend_to_sum_to_num(term1 + term2, working_terms,
total=total)
if match:
return (term1, term2, match)

return solve_part_2(terms, total=total)
``````

These solutions worked, but I realized I could refactor the recursive solution to part 2 to work for any number of terms and solve both parts with one function.

``````def find_terms_adding_to_x(terms, n=2, total=2020):
""" Given a list of integer `terms`, find the `n` terms that add
to `total`
Return those `n` terms as a tuple of integers.
"""
if len(terms) == 0:
return []

if n == 1:
if total in terms:
return [total]
return []

working_terms = list(terms)
term1 = working_terms.pop()
remaining_terms = list(working_terms)
n=n - 1,
total=total - term1)
if matches:
return [term1] + matches

return find_terms_adding_to_x(working_terms, n=n, total=total)
``````

This simplifies things significantly — it can find any number of terms that sum to some total, and it no longer needs the separate `find_addends_to_sum_to_num` function. It works in basically the same way as the `solve_part_2` function, except it replaces the `while` loop with pure recursion.

This wrapper function calls it and returns the solution:

``````def solve(terms, n=2, total=2020):
""" Given a list of integer `terms`, find the `n` terms that add
to `total` and return the product of those terms multiplied together """
return reduce(mul, find_terms_adding_to_x(terms, n=n, total=total))
``````