R/day05.R

In tjmahr/adventofcode20: Advent of Code 2020

#' Day 05: Binary Boarding
#'
#' [Binary Boarding](https://adventofcode.com/2020/day/5)
#'
#' @name day05
#' @rdname day05
#' @details
#'
#' **Part One**
#'
#' boarding pass! You aren\'t sure which seat is yours, and all of the
#' flight attendants are busy with the flood of people that suddenly made
#' it through passport control.
#'
#' You write a [quick program]{title="No problem!"} to use your phone\'s
#' camera to scan all of the nearby boarding passes (your puzzle input);
#' perhaps you can find your seat through process of elimination.
#'
#' Instead of [zones or
#' groups](https://www.youtube.com/watch?v=oAHbLRjF0vo), this airline uses
#' *binary space partitioning* to seat people. A seat might be specified
#' like `FBFBBFFRLR`, where `F` means \"front\", `B` means \"back\", `L`
#' means \"left\", and `R` means \"right\".
#'
#' The first 7 characters will either be `F` or `B`; these specify exactly
#' one of the *128 rows* on the plane (numbered `0` through `127`). Each
#' letter tells you which half of a region the given seat is in. Start with
#' the whole list of rows; the first letter indicates whether the seat is
#' in the *front* (`0` through `63`) or the *back* (`64` through `127`).
#' The next letter indicates which half of that region the seat is in, and
#' so on until you\'re left with exactly one row.
#'
#' For example, consider just the first seven characters of `FBFBBFFRLR`:
#'
#' -   Start by considering the whole range, rows `0` through `127`.
#' -   `F` means to take the *lower half*, keeping rows `0` through `63`.
#' -   `B` means to take the *upper half*, keeping rows `32` through `63`.
#' -   `F` means to take the *lower half*, keeping rows `32` through `47`.
#' -   `B` means to take the *upper half*, keeping rows `40` through `47`.
#' -   `B` keeps rows `44` through `47`.
#' -   `F` keeps rows `44` through `45`.
#' -   The final `F` keeps the lower of the two, *row `44`*.
#'
#' The last three characters will be either `L` or `R`; these specify
#' exactly one of the *8 columns* of seats on the plane (numbered `0`
#' through `7`). The same process as above proceeds again, this time with
#' only three steps. `L` means to keep the *lower half*, while `R` means to
#' keep the *upper half*.
#'
#' For example, consider just the last 3 characters of `FBFBBFFRLR`:
#'
#' -   Start by considering the whole range, columns `0` through `7`.
#' -   `R` means to take the *upper half*, keeping columns `4` through `7`.
#' -   `L` means to take the *lower half*, keeping columns `4` through `5`.
#' -   The final `R` keeps the upper of the two, *column `5`*.
#'
#' So, decoding `FBFBBFFRLR` reveals that it is the seat at *row `44`,
#' column `5`*.
#'
#' Every seat also has a unique *seat ID*: multiply the row by 8, then add
#' the column. In this example, the seat has ID `44 * 8 + 5 = 357`.
#'
#' Here are some other boarding passes:
#'
#' -   `BFFFBBFRRR`: row `70`, column `7`, seat ID `567`.
#' -   `FFFBBBFRRR`: row `14`, column `7`, seat ID `119`.
#' -   `BBFFBBFRLL`: row `102`, column `4`, seat ID `820`.
#'
#' As a sanity check, look through your list of boarding passes. *What is
#' the highest seat ID on a boarding pass?*
#'
#' **Part Two**
#'
#' *Ding!* The \"fasten seat belt\" signs have turned on. Time to find your
#' seat.
#'
#' It\'s a completely full flight, so your seat should be the only missing
#' boarding pass in your list. However, there\'s a catch: some of the seats
#' at the very front and back of the plane don\'t exist on this aircraft,
#' so they\'ll be missing from your list as well.
#'
#' Your seat wasn\'t at the very front or back, though; the seats with IDs
#' +1 and -1 from yours will be in your list.
#'
#' *What is the ID of your seat?*
#'
#' @param x some data
#' @return For Part One, `f05a(x)` returns .... For Part Two,
#'   `f05b(x)` returns ....
#' @export
#' @examples
#' x <- c("FBFBBFFRLR", "BFFFBBFRRR", "FFFBBBFRRR", "BBFFBBFRLL")
#' convert_seat_string_to_number(x)
convert_seat_string_to_number <- function(x) {
  # originally used
  # stringr::str_replace_all(x, "F|L", "0")
  # stringr::str_replace_all(x, "R|B", "1")

  chartr2 <- function(x, old, new) chartr(old, new, x)
  b <- x %>%
    chartr2("F", "0") %>%
    chartr2("L", "0") %>%
    chartr2("R", "1") %>%
    chartr2("B", "1") %>%
    strtoi(base = 2)
  b
}