day9

Day 9

Problem description

Wandering around a secure area, you come across a datalink port to a new part of the network. After briefly scanning it for interesting files, you find one file in particular that catches your attention. It's compressed with an experimental format, but fortunately, the documentation for the format is nearby.

The format compresses a sequence of characters. Whitespace is ignored. To indicate that some sequence should be repeated, a marker is added to the file, like (10x2). To decompress this marker, take the subsequent 10 characters and repeat them 2 times. Then, continue reading the file after the repeated data. The marker itself is not included in the decompressed output.

If parentheses or other characters appear within the data referenced by a marker, that's okay - treat it like normal data, not a marker, and then resume looking for markers after the decompressed section.

For example:

• ADVENT contains no markers and decompresses to itself with no changes, resulting in a decompressed length of 6.
• A(1x5)BC repeats only the B a total of 5 times, becoming ABBBBBC for a decompressed length of 7.
• (3x3)XYZ becomes XYZXYZXYZ for a decompressed length of 9.
• A(2x2)BCD(2x2)EFG doubles the BC and EF, becoming ABCBCDEFEFG for a decompressed length of 11.
• (6x1)(1x3)A simply becomes (1x3)A - the (1x3) looks like a marker, but because it's within a data section of another marker, it is not treated any differently from the A that comes after it. It has a decompressed length of 6.
• X(8x2)(3x3)ABCY becomes X(3x3)ABC(3x3)ABCY (for a decompressed length of 18), because the decompressed data from the (8x2) marker (the (3x3)ABC) is skipped and not processed further.

What is the decompressed length of the file (your puzzle input)? Don't count whitespace.

Solution

How to run

Ensure you have some Erlang on your system. kerl might be helpful here. Though be warned, this shit takes forever. Then

make

Part 1

Not going to use the Famous Erlang Features (gen_server and all that) but maybe can use pattern matching or something. Pretty straightforward: read a string, if you're in a pattern, read the length of the copies forward, calculate what it would add, then continue.

Part 2

Lol after Day 8's non-part-2, this is considerably harder.

Or is it, idk. Trick may be to use mutual recursion: rather than drop the patterns and just naive add, you take the repeated bits, calculate the whole string method, then multiply upwards.

(10 seconds later)

lol.