adventofcode/2024/day09/problem

Advent of Code

--- Day 9: Disk Fragmenter ---

   Another push of the button leaves you in the familiar hallways of some
   friendly [16]amphipods! Good thing you each somehow got your own personal
   mini submarine. The Historians jet away in search of the Chief, mostly by
   driving directly into walls.

   While The Historians quickly figure out how to pilot these things, you
   notice an amphipod in the corner struggling with his computer. He's trying
   to make more contiguous free space by compacting all of the files, but his
   program isn't working; you offer to help.

   He shows you the disk map (your puzzle input) he's already generated. For
   example:

 2333133121414131402

   The disk map uses a dense format to represent the layout of files and free
   space on the disk. The digits alternate between indicating the length of a
   file and the length of free space.

   So, a disk map like 12345 would represent a one-block file, two blocks of
   free space, a three-block file, four blocks of free space, and then a
   five-block file. A disk map like 90909 would represent three nine-block
   files in a row (with no free space between them).

   Each file on disk also has an ID number based on the order of the files as
   they appear before they are rearranged, starting with ID 0. So, the disk
   map 12345 has three files: a one-block file with ID 0, a three-block file
   with ID 1, and a five-block file with ID 2. Using one character for each
   block where digits are the file ID and . is free space, the disk map 12345
   represents these individual blocks:

 0..111....22222

   The first example above, 2333133121414131402, represents these individual
   blocks:

 00...111...2...333.44.5555.6666.777.888899

   The amphipod would like to move file blocks one at a time from the end of
   the disk to the leftmost free space block (until there are no gaps
   remaining between file blocks). For the disk map 12345, the process looks
   like this:

 0..111....22222
 02.111....2222.
 022111....222..
 0221112...22...
 02211122..2....
 022111222......

   The first example requires a few more steps:

 00...111...2...333.44.5555.6666.777.888899
 009..111...2...333.44.5555.6666.777.88889.
 0099.111...2...333.44.5555.6666.777.8888..
 00998111...2...333.44.5555.6666.777.888...
 009981118..2...333.44.5555.6666.777.88....
 0099811188.2...333.44.5555.6666.777.8.....
 009981118882...333.44.5555.6666.777.......
 0099811188827..333.44.5555.6666.77........
 00998111888277.333.44.5555.6666.7.........
 009981118882777333.44.5555.6666...........
 009981118882777333644.5555.666............
 00998111888277733364465555.66.............
 0099811188827773336446555566..............

   The final step of this file-compacting process is to update the filesystem
   checksum. To calculate the checksum, add up the result of multiplying each
   of these blocks' position with the file ID number it contains. The
   leftmost block is in position 0. If a block contains free space, skip it
   instead.

   Continuing the first example, the first few blocks' position multiplied by
   its file ID number are 0 * 0 = 0, 1 * 0 = 0, 2 * 9 = 18, 3 * 9 = 27, 4 * 8
   = 32, and so on. In this example, the checksum is the sum of these, 1928.

   Compact the amphipod's hard drive using the process he requested. What is
   the resulting filesystem checksum? (Be careful copy/pasting the input for
   this puzzle; it is a single, very long line.)

   Your puzzle answer was 6211348208140.

--- Part Two ---

   Upon completion, two things immediately become clear. First, the disk
   definitely has a lot more contiguous free space, just like the amphipod
   hoped. Second, the computer is running much more slowly! Maybe introducing
   all of that [17]file system fragmentation was a bad idea?

   The eager amphipod already has a new plan: rather than move individual
   blocks, he'd like to try compacting the files on his disk by moving whole
   files instead.

   This time, attempt to move whole files to the leftmost span of free space
   blocks that could fit the file. Attempt to move each file exactly once in
   order of decreasing file ID number starting with the file with the highest
   file ID number. If there is no span of free space to the left of a file
   that is large enough to fit the file, the file does not move.

   The first example from above now proceeds differently:

 00...111...2...333.44.5555.6666.777.888899
 0099.111...2...333.44.5555.6666.777.8888..
 0099.1117772...333.44.5555.6666.....8888..
 0099.111777244.333....5555.6666.....8888..
 00992111777.44.333....5555.6666.....8888..

   The process of updating the filesystem checksum is the same; now, this
   example's checksum would be 2858.

   Start over, now compacting the amphipod's hard drive using this new method
   instead. What is the resulting filesystem checksum?

   Your puzzle answer was 6239783302560.

   Both parts of this puzzle are complete! They provide two gold stars: **

   At this point, you should [18]return to your Advent calendar and try
   another puzzle.

   If you still want to see it, you can [19]get your puzzle input.

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