Advent of Code 2024, Day 8

In part 2, I lost time because I didn’t include the antinodes on the antennas themselves at first.

Part 1

import sys

def valid_location(lines, i, j):
    return 0 <= i < len(lines) and 0 <= j < len(lines[i])

def get_antinodes(lines, locations):
    r = set()
    for ai, aj in locations:
        for bi, bj in locations:
            if (ai, aj) == (bi, bj): continue
            diffi = bi-ai
            diffj = bj-aj
            if valid_location(lines, ai-diffi, aj-diffj):
                r.add((ai-diffi, aj-diffj))
            if valid_location(lines, bi+diffi, bj+diffj):
                r.add((bi+diffi, bj+diffj))
    return r

with open(sys.argv[1]) as f:
    lines = f.readlines()
    lines = [line.strip() for line in lines]

locations = {}
for i in range(len(lines)):
    for j in range(len(lines[i])):
        letter = lines[i][j]
        if letter == ".": continue
        if letter in locations:
            locations[letter].add((i, j))
        else:
            locations[letter] = {(i, j)}

antinodes = set()
for locations in locations.values():
    antinodes.update(get_antinodes(lines, locations))
print(len(antinodes))

Part 2

import sys

def valid_location(lines, i, j):
    return 0 <= i < len(lines) and 0 <= j < len(lines[i])

def get_antinodes(lines, locations):
    r = set()
    for ai, aj in locations:
        for bi, bj in locations:
            if (ai, aj) == (bi, bj): continue
            diffi = bi-ai
            diffj = bj-aj
            n = 0
            while valid_location(lines, ai+n*diffi, aj+n*diffj):
                r.add((ai+n*diffi, aj+n*diffj))
                n -= 1
            n = 1
            while valid_location(lines, ai+n*diffi, aj+n*diffj):
                r.add((ai+n*diffi, aj+n*diffj))
                n += 1
    return r

with open(sys.argv[1]) as f:
    lines = f.readlines()
    lines = [line.strip() for line in lines]

locations = {}
for i in range(len(lines)):
    for j in range(len(lines[i])):
        letter = lines[i][j]
        if letter == ".": continue
        if letter in locations:
            locations[letter].add((i, j))
        else:
            locations[letter] = {(i, j)}

antinodes = set()
for locations in locations.values():
    antinodes.update(get_antinodes(lines, locations))
print(len(antinodes))

2024-12-13