Bart Broere

Freelance Developer

Advent of Code 2022 - Day 9 - part 1

30 Nov 2023

To get ready for Advent of Code 2023, I continued where I stopped last year: day 9.

Here’s me struggling for 1 hour and 40 minutes spread across two days, because I tried to be clever. For viewing pleasure it has been sped up.

What goes wrong here is that I assumed incorrectly that a negative number to the power of zero would be -1. Quick maths turned out not to be good maths. The following bit of experimentation led me to believe that.

result = -5 ** 0  
assert result == -1

You may have already guessed that the order of operation fooled me. The ** operator is executed before the - operator is applied to the result. The behaviour is different if I do the same using a variable:

x = -5
result = x ** 0
assert result == 1

The solution that eventually led to the correct answer is in the code block below. After submitting the correct answer, I did some cleanup:

from advent_of_code import *
import requests_cache

requests_cache.install_cache()

test_input = """R 4
U 4
L 3
D 1
R 4
D 1
L 5
R 2"""
input_9 = fetch_input(9)
# input_9 = test_input
movements = [x.split(' ') for x in input_9.splitlines()]
movements = [(x[0], int(x[1])) for x in movements]
visited = set()
head = (0, 0)
tail = (0, 0)

directions = {
# What coordinates change for each movement
#         x, y
    "U": (1, 0),
    "D": (-1, 0),
    "L": (0, -1),
    "R": (0, 1),
}


def modify_location(location, direction):
    if isinstance(direction, str):
        change = directions[direction]
    else:
        change = direction
    return location[0] + change[0], location[1] + change[1]


def direction_to_move(head, tail):
    x = head[0] - tail[0]
    y = head[1] - tail[1]
    
    # head and tail are at the same location, don't move
    if x == 0 and y == 0:  
        return 0, 0
    
    # head and tail are less than one square apart (including diagonally)
    elif max(abs(x), abs(y)) == 1:  
        return 0, 0
    
    # head and tail are too far apart, decide which direction to move the tail
    else:
        if x < 0:
            x = x ** 0 * -1
        else:
            x = 0 if not x else 1
        if y < 0:
            y = -1
        else:
            y = 0 if not y else 1
        return x, y


tail_visited = set()
for direction, length in movements:
    for _ in range(length):
        head = modify_location(head, direction)
        tail = modify_location(tail, direction_to_move(head, tail))
        # keep track of where the tail has been:
        tail_visited.add(tail)

submit_answer(level=1, day=9, answer=len(tail_visited))

The lessons I learned: Don’t try to be clever and check your maths.

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