R/day25.R

In hturner/adventofcode21: Advent of Code Solutions 2021

#' Day 25: Sea Cucumber
#'
#' [Sea Cucumber](https://adventofcode.com/2021/day/25)
#'
#' @name day25
#' @rdname day25
#' @details
#'
#' **Part One**
#'
#' This is it: the bottom of the ocean trench, the last place the sleigh
#' keys could be. Your submarine\'s experimental antenna *still isn\'t
#' boosted enough* to detect the keys, but they *must* be here. All you
#' need to do is *reach the seafloor* and find them.
#'
#' At least, you\'d touch down on the seafloor if you could; unfortunately,
#' it\'s completely covered by two large herds of [sea
#' cucumbers](https://en.wikipedia.org/wiki/Sea_cucumber), and there isn\'t
#' an open space large enough for your submarine.
#'
#' You suspect that the Elves must have done this before, because just then
#' you discover the phone number of a deep-sea marine biologist on a
#' handwritten note taped to the wall of the submarine\'s cockpit.
#'
#' \"Sea cucumbers? Yeah, they\'re probably hunting for food. But don\'t
#' worry, they\'re predictable critters: they move in perfectly straight
#' lines, only moving forward when there\'s space to do so. They\'re
#' actually quite polite!\"
#'
#' You explain that you\'d like to predict when you could land your
#' submarine.
#'
#' \"Oh that\'s easy, they\'ll eventually pile up and leave enough space
#' for\-- wait, did you say submarine? And the only place with that many
#' sea cucumbers would be at the very bottom of the Mariana\--\" You hang
#' up the phone.
#'
#' There are two herds of sea cucumbers sharing the same region; one always
#' moves *east* (`>`), while the other always moves *south* (`v`). Each
#' location can contain at most one sea cucumber; the remaining locations
#' are *empty* (`.`). The submarine helpfully generates a map of the
#' situation (your puzzle input). For example:
#'
#'     v...>>.vv>
#'     .vv>>.vv..
#'     >>.>v>...v
#'     >>v>>.>.v.
#'     v>v.vv.v..
#'     >.>>..v...
#'     .vv..>.>v.
#'     v.v..>>v.v
#'     ....v..v.>
#'
#' Every *step*, the sea cucumbers in the east-facing herd attempt to move
#' forward one location, then the sea cucumbers in the south-facing herd
#' attempt to move forward one location. When a herd moves forward, every
#' sea cucumber in the herd first simultaneously considers whether there is
#' a sea cucumber in the adjacent location it\'s facing (even another sea
#' cucumber facing the same direction), and then every sea cucumber facing
#' an empty location simultaneously moves into that location.
#'
#' So, in a situation like this:
#'
#'     ...>>>>>...
#'
#' After one step, only the rightmost sea cucumber would have moved:
#'
#'     ...>>>>.>..
#'
#' After the next step, two sea cucumbers move:
#'
#'     ...>>>.>.>.
#'
#' During a single step, the east-facing herd moves first, then the
#' south-facing herd moves. So, given this situation:
#'
#'     ..........
#'     .>v....v..
#'     .......>..
#'     ..........
#'
#' After a single step, of the sea cucumbers on the left, only the
#' south-facing sea cucumber has moved (as it wasn\'t out of the way in
#' time for the east-facing cucumber on the left to move), but both sea
#' cucumbers on the right have moved (as the east-facing sea cucumber moved
#' out of the way of the south-facing sea cucumber):
#'
#'     ..........
#'     .>........
#'     ..v....v>.
#'     ..........
#'
#' Due to *strong water currents* in the area, sea cucumbers that move off
#' the right edge of the map appear on the left edge, and sea cucumbers
#' that move off the bottom edge of the map appear on the top edge. Sea
#' cucumbers always check whether their destination location is empty
#' before moving, even if that destination is on the opposite side of the
#' map:
#'
#'     Initial state:
#'     ...>...
#'     .......
#'     ......>
#'     v.....>
#'     ......>
#'     .......
#'     ..vvv..
#'
#'     After 1 step:
#'     ..vv>..
#'     .......
#'     >......
#'     v.....>
#'     >......
#'     .......
#'     ....v..
#'
#'     After 2 steps:
#'     ....v>.
#'     ..vv...
#'     .>.....
#'     ......>
#'     v>.....
#'     .......
#'     .......
#'
#'     After 3 steps:
#'     ......>
#'     ..v.v..
#'     ..>v...
#'     >......
#'     ..>....
#'     v......
#'     .......
#'
#'     After 4 steps:
#'     >......
#'     ..v....
#'     ..>.v..
#'     .>.v...
#'     ...>...
#'     .......
#'     v......
#'
#' To find a safe place to land your submarine, the sea cucumbers need to
#' stop moving. Again consider the first example:
#'
#'     Initial state:
#'     v...>>.vv>
#'     .vv>>.vv..
#'     >>.>v>...v
#'     >>v>>.>.v.
#'     v>v.vv.v..
#'     >.>>..v...
#'     .vv..>.>v.
#'     v.v..>>v.v
#'     ....v..v.>
#'
#'     After 1 step:
#'     ....>.>v.>
#'     v.v>.>v.v.
#'     >v>>..>v..
#'     >>v>v>.>.v
#'     .>v.v...v.
#'     v>>.>vvv..
#'     ..v...>>..
#'     vv...>>vv.
#'     >.v.v..v.v
#'
#'     After 2 steps:
#'     >.v.v>>..v
#'     v.v.>>vv..
#'     >v>.>.>.v.
#'     >>v>v.>v>.
#'     .>..v....v
#'     .>v>>.v.v.
#'     v....v>v>.
#'     .vv..>>v..
#'     v>.....vv.
#'
#'     After 3 steps:
#'     v>v.v>.>v.
#'     v...>>.v.v
#'     >vv>.>v>..
#'     >>v>v.>.v>
#'     ..>....v..
#'     .>.>v>v..v
#'     ..v..v>vv>
#'     v.v..>>v..
#'     .v>....v..
#'
#'     After 4 steps:
#'     v>..v.>>..
#'     v.v.>.>.v.
#'     >vv.>>.v>v
#'     >>.>..v>.>
#'     ..v>v...v.
#'     ..>>.>vv..
#'     >.v.vv>v.v
#'     .....>>vv.
#'     vvv>...v..
#'
#'     After 5 steps:
#'     vv>...>v>.
#'     v.v.v>.>v.
#'     >.v.>.>.>v
#'     >v>.>..v>>
#'     ..v>v.v...
#'     ..>.>>vvv.
#'     .>...v>v..
#'     ..v.v>>v.v
#'     v.v.>...v.
#'
#'     ...
#'
#'     After 10 steps:
#'     ..>..>>vv.
#'     v.....>>.v
#'     ..v.v>>>v>
#'     v>.>v.>>>.
#'     ..v>v.vv.v
#'     .v.>>>.v..
#'     v.v..>v>..
#'     ..v...>v.>
#'     .vv..v>vv.
#'
#'     ...
#'
#'     After 20 steps:
#'     v>.....>>.
#'     >vv>.....v
#'     .>v>v.vv>>
#'     v>>>v.>v.>
#'     ....vv>v..
#'     .v.>>>vvv.
#'     ..v..>>vv.
#'     v.v...>>.v
#'     ..v.....v>
#'
#'     ...
#'
#'     After 30 steps:
#'     .vv.v..>>>
#'     v>...v...>
#'     >.v>.>vv.>
#'     >v>.>.>v.>
#'     .>..v.vv..
#'     ..v>..>>v.
#'     ....v>..>v
#'     v.v...>vv>
#'     v.v...>vvv
#'
#'     ...
#'
#'     After 40 steps:
#'     >>v>v..v..
#'     ..>>v..vv.
#'     ..>>>v.>.v
#'     ..>>>>vvv>
#'     v.....>...
#'     v.v...>v>>
#'     >vv.....v>
#'     .>v...v.>v
#'     vvv.v..v.>
#'
#'     ...
#'
#'     After 50 steps:
#'     ..>>v>vv.v
#'     ..v.>>vv..
#'     v.>>v>>v..
#'     ..>>>>>vv.
#'     vvv....>vv
#'     ..v....>>>
#'     v>.......>
#'     .vv>....v>
#'     .>v.vv.v..
#'
#'     ...
#'
#'     After 55 steps:
#'     ..>>v>vv..
#'     ..v.>>vv..
#'     ..>>v>>vv.
#'     ..>>>>>vv.
#'     v......>vv
#'     v>v....>>v
#'     vvv...>..>
#'     >vv.....>.
#'     .>v.vv.v..
#'
#'     After 56 steps:
#'     ..>>v>vv..
#'     ..v.>>vv..
#'     ..>>v>>vv.
#'     ..>>>>>vv.
#'     v......>vv
#'     v>v....>>v
#'     vvv....>.>
#'     >vv......>
#'     .>v.vv.v..
#'
#'     After 57 steps:
#'     ..>>v>vv..
#'     ..v.>>vv..
#'     ..>>v>>vv.
#'     ..>>>>>vv.
#'     v......>vv
#'     v>v....>>v
#'     vvv.....>>
#'     >vv......>
#'     .>v.vv.v..
#'
#'     After 58 steps:
#'     ..>>v>vv..
#'     ..v.>>vv..
#'     ..>>v>>vv.
#'     ..>>>>>vv.
#'     v......>vv
#'     v>v....>>v
#'     vvv.....>>
#'     >vv......>
#'     .>v.vv.v..
#'
#' In this example, the sea cucumbers stop moving after `58` steps.
#'
#' Find somewhere safe to land your submarine. *What is the first step on
#' which no sea cucumbers move?*
#'
#' **Part Two**
#'
#' *(Use have to manually add this yourself.)*
#'
#' *(Try using `convert_clipboard_html_to_roxygen_md()`)*
#'
#' @param x some data
#' @return For Part One, `f25a(x)` returns .... For Part Two,
#'   `f25b(x)` returns ....
#' @export
#' @examples
#' f25a(example_data_25())
f25a <- function(x) {
  nr <- nrow(x)
  nc <- ncol(x)
  i <- 1
  repeat {
    y <- x
    move_east <- x == ">" & x[, c(2:nc, 1)] == "."
    x[move_east] <- "."
    x[, c(2:nc, 1)][move_east] <- ">"
    move_south <- x == "v" & x[c(2:nr, 1), ] == "."
    x[move_south] <- "."
    x[c(2:nr, 1),][move_south] <- "v"
    if (isTRUE(all.equal(x, y))) break
    i <- i + 1
  }
  i
}

#' @param example Which example data to use (by position or name). Defaults to
#'   1.
#' @rdname day25
#' @export
example_data_25 <- function(example = 1) {
  l <- list(
    a = matrix(c("v", ".", ">", ">", "v", ">", ".", "v", ".", ".",
                 "v", ">", ">", ">", ".", "v", ".", ".", ".", "v",
                 ".", "v", "v", ">", "v", "v", ".", ".", ">", ">",
                 ">", ".", ">", ".", ".", ".",  ">", ">", "v", ">",
                 "v", ".", ".", ".", "v", ">", ".", ">", ".", "v",
                 ".", ">", ">", ".", ".", "v", ".", ">", ".", "v",
                 ".", ">",  ".", "v", "v", ".", ".", "v", ".", ">",
                 "v", "v", "v", ".", ".",  "v", ".", ".", "v", ".",
                 ".", ">", ".", "v", ".", ".", ".", ".", "v", ">"),
               nrow = 9, ncol = 10)
  )
  l[[example]]
}