Advent of Code 2022 Day 9

— Day 9: Rope Bridge —

For a rope, the tail has to follow the head as it moves around the grid.

Simulate your complete hypothetical series of motions. How many positions does the tail of the rope visit at least once?

import numpy as np

with open('data-2022-09.txt', 'r') as f:
# with open('data-2022-09-test.txt', 'r') as f:
    inp = f.read().splitlines()

Check the limits of the range.

# add up all the directions but the actual extent will be smaller than this
maxes = {'D': 0, 'U': 0, 'L': 0, 'R': 0}

for i in inp:
    d,n = i.split(' ')
    maxes[d] += int(n)

maxes

# reset and try again just tracking the head
maxes = {'D': 0, 'U': 0, 'L': 0, 'R': 0}

pos = {'x': 0, 'y': 0}

for i in inp:
    d,n = i.split(' ')
    if d == 'L':
        pos['x'] -= int(n)
        if pos['x'] < maxes[d]: maxes[d] = pos['x']
    if d == 'R':
        pos['x'] += int(n)
        if pos['x'] > maxes[d]: maxes[d] = pos['x']
    if d == 'U':
        pos['y'] += int(n)
        if pos['y'] > maxes[d]: maxes[d] = pos['y']
    if d == 'D':
        pos['y'] -= int(n)
        if pos['y'] < maxes[d]: maxes[d] = pos['y']

pos
maxes

{'D': -54, 'U': 276, 'L': -222, 'R': 65}

Let’s do 1000x1000 rather than 3000x3000

# head range and tail range
rH = np.zeros((1000,1000))
rT = np.zeros((1000,1000))

# location of head and tail
xH = 500
yH = 500
xT = 500
yT = 500

# move the tail
def move_tail(arr, xH, yH, xT, yT):
    if xH - 1 <= xT <= xH + 1 and yH - 1 <= yT <= yH + 1:
        # this is fine, tail is close enough
        pass
    else: 
        if yH - yT == 2: 
            yT += 1
            xT = xH
        elif yT - yH == 2:
            yT -= 1
            xT = xH
        elif xH - xT == 2:
            xT += 1
            yT = yH
        elif xT - xH == 2:
            xT -= 1
            yT = yH
    arr[xT,yT] += 1
    return arr, xT, yT


for i in inp:
    d,n = i.split(' ')
    for m in range(int(n)):
        if d == 'L':
            xH -= 1
        if d == 'R':
            xH += 1
        if d == 'U':
            yH += 1
        if d == 'D':
            yH -= 1
        rH[xH,yH] += 1
        rT, xT, yT = move_tail(rT, xH, yH, xT, yT)

int(np.sum(rT > 0))

# 6486

6486

— Part Two —

Simulate your complete series of motions on a larger rope with ten knots. How many positions does the tail of the rope visit at least once?

# new arrays for tracking
# tail = np.zeros((1000,1000))
tail = []

# separate positions for each rope piece
posi = [[0,0] for _ in range(10)]

# record tail position
tail.append(posi[9])

# this is pretty much the same as above 
# but we need to use it to assess the movement of each rope piece
def move_next(p, q):
    xH = p[0]
    yH = p[1]
    xT = q[0]
    yT = q[1]
    if xH - 1 <= xT <= xH + 1 and yH - 1 <= yT <= yH + 1:
        # this is fine, don't move
        pass
    else: 
        if yH - yT == 2: 
            yT += 1
            xT = xH
        elif yT - yH == 2:
            yT -= 1
            xT = xH
        elif xH - xT == 2:
            xT += 1
            yT = yH
        elif xT - xH == 2:
            xT -= 1
            yT = yH
    return [xT, yT]


# loop through the instructions
for i in inp[:]:   
    # create understandable instructions
    d,n = i.split(' ')
    # we need to move the head of the rope the right number of times
    for _ in range(int(n)):
        if d == 'L':
            posi[0][0] -= 1
        if d == 'R':
            posi[0][0] += 1
        if d == 'U':
            posi[0][1] += 1
        if d == 'D':
            posi[0][1] -= 1
        # we need to move the rest of the knots
        # print()
        # print(posi[0])
        for k in range(1,10):
            posi[k] = move_next(posi[k-1], posi[k])
            # print(posi[k])
            if k == 9 and posi[9] not in tail: 
                tail.append(posi[9])

            
# np.sum(tail > 0)
len(tail)
posi
# 2357 too low
# 6551

[[-180, 254],
 [-180, 255],
 [-180, 256],
 [-180, 257],
 [-180, 258],
 [-180, 259],
 [-180, 260],
 [-179, 260],
 [-178, 260],
 [-178, 259]]