"""AOC 2020 Day 13""" import pathlib import time TEST_INPUT = """939 7,13,x,x,59,x,31,19""" def read_input(input_path: str) -> str: """take input file path and return a str with the file's content""" with open(input_path, 'r') as input_file: input_data = input_file.read().strip() return input_data def extract(input_data: str) -> tuple: """take input data and return the appropriate data structure""" entries = input_data.split('\n') departure_ts = int(entries[0]) timetable = entries[1].split(',') ids = [(-index, int(bus_id)) for index, bus_id in enumerate(timetable) if bus_id != 'x'] return (departure_ts, ids) def egcd(a, b): """"extended euclidean algorithm""" if a == 0: return (b, 0, 1) g, y, x = egcd(b % a, a) return (g, x - (b // a) * y, y) def modinv(x, m): """calculate the modular multiplicative inverse""" g, inv, _ = egcd(x, m) assert g == 1 return inv % m def part1(entries: tuple) -> int: """part1 solver""" wait_time = float('inf') best_bus_id = -1 min_dep_time = entries[0] for _, bus_id in entries[1]: if min_dep_time % bus_id == 0: multiple = min_dep_time // bus_id else: multiple = (min_dep_time // bus_id) + 1 time_diff = (bus_id * multiple) - min_dep_time if time_diff < wait_time: wait_time = time_diff best_bus_id = bus_id return wait_time * best_bus_id def part2(entries: tuple) -> int: """part2 solver""" min_timestamp = 0 moduli_product = 1 for _, modulo in entries[1]: moduli_product *= modulo for remainder, modulo in entries[1]: ni = moduli_product // modulo modulo_inv = modinv(ni, modulo) min_timestamp += (remainder * ni * modulo_inv) return min_timestamp % moduli_product def test_input_day_13(): """pytest testing function""" entries = extract(TEST_INPUT) assert part1(entries) == 295 assert part2(entries) == 1068781 def test_bench_day_13(benchmark): """pytest-benchmark function""" benchmark(main) def main(): """main function""" input_path = str(pathlib.Path(__file__).resolve().parent.parent) + "/inputs/" + str(pathlib.Path(__file__).stem) start_time = time.time() input_data = read_input(input_path) entries = extract(input_data) print("Part 1: %d" % part1(entries)) print("Part 2: %d" % part2(entries)) end_time = time.time() print("Execution time: %f" % (end_time-start_time)) if __name__ == "__main__": main()