R/day02.R

In Bisaloo/adventofcode21: What the Package Does (One Line, Title Case)

#' Day 02: Dive!
#'
#' [Dive!](https://adventofcode.com/2021/day/2)
#'
#' @name day02
#' @rdname day02
#' @details
#'
#' **Part One**
#'
#' Now, you need to figure out how to [pilot this
#' thing]{title="Tank, I need a pilot program for a B212 helicopter."}.
#'
#' It seems like the submarine can take a series of commands like
#' `forward 1`, `down 2`, or `up 3`:
#'
#' -   `forward X` increases the horizontal position by `X` units.
#' -   `down X` *increases* the depth by `X` units.
#' -   `up X` *decreases* the depth by `X` units.
#'
#' Note that since you\'re on a submarine, `down` and `up` affect your
#' *depth*, and so they have the opposite result of what you might expect.
#'
#' The submarine seems to already have a planned course (your puzzle
#' x). You should probably figure out where it\'s going. For example:
#'
#'     forward 5
#'     down 5
#'     forward 8
#'     up 3
#'     down 8
#'     forward 2
#'
#' Your horizontal position and depth both start at `0`. The steps above
#' would then modify them as follows:
#'
#' -   `forward 5` adds `5` to your horizontal position, a total of `5`.
#' -   `down 5` adds `5` to your depth, resulting in a value of `5`.
#' -   `forward 8` adds `8` to your horizontal position, a total of `13`.
#' -   `up 3` decreases your depth by `3`, resulting in a value of `2`.
#' -   `down 8` adds `8` to your depth, resulting in a value of `10`.
#' -   `forward 2` adds `2` to your horizontal position, a total of `15`.
#'
#' After following these instructions, you would have a horizontal position
#' of `15` and a depth of `10`. (Multiplying these together produces
#' `150`.)
#'
#' Calculate the horizontal position and depth you would have after
#' following the planned course. *What do you get if you multiply your
#' final horizontal position by your final depth?*
#'
#' ** Part Two **
#'
#' Based on your calculations, the planned course doesn\'t seem to make any
#' sense. You find the submarine manual and discover that the process is
#' actually slightly more complicated.
#'
#' In addition to horizontal position and depth, you\'ll also need to track
#' a third value, *aim*, which also starts at `0`. The commands also mean
#' something entirely different than you first thought:
#'
#' -   `down X` *increases* your aim by `X` units.
#' -   `up X` *decreases* your aim by `X` units.
#' -   `forward X` does two things:
#'     -   It increases your horizontal position by `X` units.
#'     -   It increases your depth by your aim *multiplied by* `X`.
#'
#' Again note that since you\'re on a submarine, `down` and `up` do the
#' opposite of what you might expect: \"down\" means aiming in the positive
#' direction.
#'
#' Now, the above example does something different:
#'
#' -   `forward 5` adds `5` to your horizontal position, a total of `5`.
#'     Because your aim is `0`, your depth does not change.
#' -   `down 5` adds `5` to your aim, resulting in a value of `5`.
#' -   `forward 8` adds `8` to your horizontal position, a total of `13`.
#'     Because your aim is `5`, your depth increases by `8*5=40`.
#' -   `up 3` decreases your aim by `3`, resulting in a value of `2`.
#' -   `down 8` adds `8` to your aim, resulting in a value of `10`.
#' -   `forward 2` adds `2` to your horizontal position, a total of `15`.
#'     Because your aim is `10`, your depth increases by `2*10=20` to a
#'     total of `60`.
#'
#' After following these new instructions, you would have a horizontal
#' position of `15` and a depth of `60`. (Multiplying these produces
#' `900`.)
#'
#' Using this new interpretation of the commands, calculate the horizontal
#' position and depth you would have after following the planned course.
#' *What do you get if you multiply your final horizontal position by your
#' final depth?*
#' @param x some data
#' @return For Part One, `f02a(x)` returns .... For Part Two,
#'   `f02b(x)` returns ....
#' @export
#' @examples
#' f02a(example_data_02())
#' f02b(example_data_02())
f02a <- function(x) {

  x$distance[x$direction == "up"] <- - x$distance[x$direction == "up"]

  sum(x$distance[x$direction != "forward"]) *
    sum(x$distance[x$direction == "forward"])

}


#' @rdname day02
#' @export
f02b <- function(x) {

  x$vertical_dist <- 0
  x$vertical_dist[x$direction == "down"] <- x$distance[x$direction == "down"]
  x$vertical_dist[x$direction == "up"] <- - x$distance[x$direction == "up"]

  x$horizontal_dist <- 0
  x$horizontal_dist[x$direction == "forward"] <- x$distance[x$direction == "forward"]

  aim <- cumsum(x$vertical_dist)

  depth <- sum(aim * x$horizontal_dist)

  depth * sum(x$horizontal_dist)

}

#' @param example Which example data to use (by position or name). Defaults to
#'   1.
#' @rdname day02
#' @export
example_data_02 <- function(example = 1) {
  l <- list(
    a = tibble::tribble(
      ~ direction, ~ distance,
      "forward", 5,
      "down", 5,
      "forward", 8,
      "up", 3,
      "down", 8,
      "forward", 2
    )
  )
  l[[example]]
}