Day 08: Advent Snail

Solution

This looks like it is a barcode, we tried adding the markers for a data matrix, but this leads nowhere. Given that it is a 25x25 image, it is likely it is a QR code but somehow encrypted or scrambled.

After a lot of searching (including finding out about this amusing service), we realize this is a QR code, but in the shape of a spiral (like the shell of a snail).

There would be various directions to start from and we could spiral either clockwise or counterclockwise, but reasoning from the central pixel, and knowing we would have to start with 7 black pixels for the QR code corner marker, only one orientation remains:

We write the following script to do the transformation:

from PIL import Image


def getdirection(d):
    ''' change direction, clockwise '''
    if d % 4 == 0:  # up
        return (-1, 0)
    if d % 4 == 1:  # right
        return (0, 1)
    if d % 4 == 2:  # down
        return (1, 0)
    else:  # left
        return (0, -1)

# input image converted to bit string, 1=black, 0=white
qr = "1101011001101011001101101111000110011111111010000101101101000001001101001010010011100100011000111000110100100000011000101001101000011111100000000111101000111110110000010011001011110011011010001101011000001110011011101100010001110100110100000001011111110101101011000101010001001010001101011001001001100100111100101111110010111110100011011111111100011101001011111110000110001101000000100110111001000001100000101110100011000110100001101000101110110100001100101111111100000010100110101111101111100001100110110011000110110101100111100110110101001000011100101111101010000011001010111011101110110001101100011101101100000110010100000"

# create 2D array
size = 25
qr = [qr[i: i + size] for i in range(0, len(qr), size)]
qr = map(list, qr)

# start in middle
middle = size / 2
i = middle
j = middle

# save transformed bitstring
outpixels = ""

# paint middle pixel
outpixels += qr[middle][middle]

# spiral
p = 0
stepsize = 1
direction = 0
while p <= size*size:
    for _ in range(0, 2):  # same length twice
        d = getdirection(direction)
        for k in range(0, stepsize):
            i += d[0]
            j += d[1]
            try:
                outpixels += qr[i][j]
            except IndexError:
                break
        p += stepsize
        direction += 1
    stepsize += 1

# create an image
outimg = Image.new('RGB', (size, size), "white")
pixels = outimg.load()

for i in range(0, size*size):
    if outpixels[i] == "1":
        pix = (0, 0, 0)
    else:
        pix = (255, 255, 255)
    pixels[i / size, i % size] = pix

# save image
outimg = outimg.resize((500, 500))
outimg.save("day08-snail-out.png")

Which outputs the following QR code:

Day 08: Advent Snail

Challenge

Solution

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