16: Permutation Promenade

library(mistlecode)

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

 Also loading:  cipheR data.table dplyr purrr slider stringr tidyverse glue rlang R6

x <-
  readLines("input.txt") |>
  str_split_1(",") |>
  add_processor()

Part 1

I hate zero indexed stuff… But it’s fun to keep using create_assembly

registers <-
  1:16 |>
  as.list() |>
  `names<-`(letters[1:16])

functions <- list(
  "proc" = \(x, y) {
    y <- stringr::str_split_fixed(x, "/", 2)
    yy <- stringr::str_split_fixed(y[,1], "", 2)
    x <- c(yy, y[,-1])
    
    x[1] <- case_match(
      x[1],
      "s" ~ "spin",
      "x" ~ "exchange",
      "p" ~ "partner"
    )
    self[[x[1]]](x[2], x[3])
    
    private$.inc()
  },
  "spin" = \(x, y) {
    x <- as.integer(x)
    for (r in letters[1:16]) {
      self[[r]] <- (self[[r]] + as.integer(x)) %% length(registers)
      if (self[[r]] == 0) self[[r]] <- length(registers)
    }
  },
  "exchange" = \(x, y) {
    x <- as.integer(x) + 1
    y <- as.integer(y) + 1
    pos <- sapply(letters[1:16], \(r) self[[r]])
    x <- names(which(pos == x))
    y <- names(which(pos == y))
    self$partner(x, y)
  },
  "partner" = \(x, y) {
    tmp <- self[[y]]
    self[[y]] <- self[[x]]
    self[[x]] <- tmp
  }
)

a <- create_assembly(registers, functions)$run(x)
as.list(a)[letters[1:16]] |>
  unlist() |>
  sort() |>
  names() |>
  paste0(collapse = "")

[1] "ceijbfoamgkdnlph"

Part 2

It’s going to loop at some point and I need to track that. Ugh. Just track the new ones until it repeats, then modulo. Slow, but it works.

seen <- c()

i <- 0
repeat {
  a <- create_assembly(registers, functions)$run(x)
  registers <- as.list(a)[letters[1:16]]
  new_a <-
    registers |>
    unlist() |>
    sort() |>
    names() |>
    paste0(collapse = "")
  if (new_a %in% seen) break else seen <- c(seen, new_a)
  i <- i + 1
}

seen[1e9 %% i]

[1] "pnhajoekigcbflmd"