2022-25: Full of Hot Air

This doesn’t look too terrible. The numbering system is weird, but I can work with it. It seems too easy for Day 25.

library(mistlecode)

To install `mistlecode` yourself, run `devtools::install_github('guslipkin/mistlecode')`.

 Also loading:  cipheR data.table dplyr purrr slider stringr tidyverse

options(scipen = 999)

dt <- readLines("input.txt")

Part 1

I was able to get Bob’s number pretty quickly. Then I realized I hadn’t fully read the question and I had to translate Bob’s number back into SNAFU. I had a few ideas on how to do it, but really struggled for some reason. I tried a recursive method first, but quickly hit C stack limits and couldn’t quite wrap my mind around how to brute-force it. I did realize that I could jump start the guessing by finding the smallest number greater than Bob’s that’s made entirely out of 2s. By starting there, I could subtract towards my goal, rather than trying to add, which would have been much more difficult. It was then just a matter of getting the code straight in my head, and using the un_snafu function to check my values every cycle.

un_snafu <- function(x) {
  x <- 
    x |>
    str_split_1("") |>
    rev()

  x <- case_when(
    x == "-" ~ -1,
    x == "=" ~ -2,
    TRUE ~ as.numeric(x)
  )

  sum((5 ^ (0:(length(x)-1))) * x)
}

bob <- 
  dt |>
  sapply(un_snafu) |>
  suppressWarnings() |>
  sum()

subtract <- function(x) {
  case_when(
    x == "2" ~ "1",
    x == "1" ~ "0",
    x == "0" ~ "-",
    x == "-" ~ "="
  )
}

max_places <-
  sapply(1:100, \(x) paste0(rep("2", x), collapse = "")) |>
  sapply(un_snafu)
x <- names(which(max_places > bob)[1])
place <- 1

repeat {
  x2 <- x
  
  s <- substr(x2, place, place)
  if (s != "=") { substr(x2, place, place) <- subtract(s) } 
  else { place <- place + 1 }
  
  u <- un_snafu(x2)
  if (u > bob) { x <- x2 }
  else if (u < bob) { place <- place + 1 }
  else { print(x2); break }
} |>
  suppressWarnings()

[1] "2-02===-21---2002==0"

Part 2

Caution

This puzzle does not unlock until you have completed the rest of the year. I’ll be back one day…