AoC 2016 Day 9: Decompressinator

Source: Explosives in Cyberspace

Part 1: A file is compressed by including compression markers of the form (#x#)... where the first number tells how many characters to copy and the second is how many times to copy them. So A(2x3)HA! becomes AHAHAHA!.

For part 1, we could likely just expand the actual strings. But I’m guessing part 2 is going to do something crazy recursive. So let’s just calculate the length directly.

re_compressed_block = re.compile(r'\((?P<length>\d+)x(?P<count>\d+)\)')

def decompressed_length(content):
    index = 0
    output_length = 0

    while True:
        m = re_compressed_block.search(content, pos = index)
        if m:
            # Content before the current block
            output_length += m.start() - index

            # Expanded content
            length = int(m.group('length'))
            count = int(m.group('count'))

            output_length += length * count

            # Skip past this block for the next iteration
            index = m.end() + length
        else:
            break

    output_length += len(content) - index

    return output_length

if os.path.exists(args.input):
    with open(args.input, 'r') as fin:
        content = re.sub('\s+', '', fin.read())
else:
    content = args.input

print('v1:', decompressed_length(content, 1))

Part 2: If a decompressed string contains a marker, decompress that as well. Repeat.

Called that.

re_compressed_block = re.compile(r'\((?P<length>\d+)x(?P<count>\d+)\)')

def decompressed_length(content, version):
    index = 0
    output_length = 0

    while True:
        m = re_compressed_block.search(content, pos = index)
        if m:
            # Content before the current block
            output_length += m.start() - index

            # The current block, expand recursively for version 2
            length = int(m.group('length'))
            count = int(m.group('count'))

            if version == 1:
                output_length += length * count
            elif version == 2:
                block = content[m.end() : m.end() + length]
                expanded_block_length = decompressed_length(block, version = 2)
                output_length += expanded_block_length * count

            # Skip past this block for the next iteration
            index = m.end() + length
        else:
            break

    output_length += len(content) - index

    return output_length

print('v1:', decompressed_length(content, 1))
print('v2:', decompressed_length(content, 2))

My output for v2 was over 10 billion, so keeping that all in memory is unlikely to have ended well…

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