Day 9: Disk Fragmenter

library(tidyverse)
library(stringr)
library(tictoc)
library(gmp)

dat <- read_lines("aoc09.txt") |>
  strsplit("") |>
  first() |>
  as.numeric()

spec <- data.frame(n = dat) |>
  mutate(type = c(rep(c("data", "free"), floor(n()/2)), "data"),
         i = 1:n(),
         id = floor((i + 1)/2) - 1)

head(spec)

build_disk <- function(id, type, n) {
  data.frame(dat = rep(ifelse(type == "data", id, NA), n))
}

the_disk <- pmap(list(spec$id, spec$type, spec$n), build_disk) |>
  bind_rows()

head(the_disk)

Part 1

Move the blocks:

tic()
start <- 1
end   <- nrow(the_disk)

while (start < end) {
  if (!is.na(the_disk[start, "dat"])) {
    # Find the next free space
    start <- start + 1
  }
  else {
    while (start < end && is.na(the_disk[end, "dat"])) {
      # Find the next data
      end <- end - 1
    }
    if (start < end) {
      # Finally move the data!
      the_disk[start, "dat"] <- the_disk[end, "dat"]
      the_disk[end, "dat"] <- NA
      end <- end - 1
      start <- start + 1
    }
  }
}
toc()

10.42 sec elapsed

Calculate the checksums:

pos <- 0
the_disk$pos <- rep(NA, nrow(the_disk))

for (i in 1:nrow(the_disk)) {
  if (!is.na(the_disk[i, "dat"])) {
    the_disk[i, "pos"] <- pos
    pos <- pos + 1
  }
}

the_disk <- the_disk |>
  mutate(check = pos * dat)

summary(the_disk$check)

     Min.   1st Qu.    Median      Mean   3rd Qu.      Max.      NA's 
        0  41929318 123670738 125973979 208262546 261908897     44944 

sum(the_disk$check |> as.bigz(), na.rm = TRUE)

Big Integer ('bigz') :
[1] 6242766523059

Part 2

TBC…