adventofcode17: Advent of Code 2017

#' Day 01: Inverse Captcha
#'
#' [Inverse Captcha](http://adventofcode.com/2017/day/1)
#'
#' @name day01
#' @rdname day01
#' @details
#'
#' **Part One**
#'
#' The night before Christmas, one of Santa's Elves calls you in a panic.
#' "The printer's broken! We can't print the *Naughty or Nice List*!" By
#' the time you make it to sub-basement 17,
#' there are only a few minutes until midnight. "We have a big problem,"
#' she says; "there must be almost *fifty* bugs in this system, but nothing
#' else can print The List. Stand in this square, quick! There's no time to
#' explain; if you can convince them to pay you in *stars*, you'll be able
#' to--" She pulls a lever and the world goes blurry.
#'
#' When your eyes can focus again, everything seems a lot more pixelated
#' than before. She must have sent you inside the computer! You check the
#' system clock: *25 milliseconds* until midnight. With that much time, you
#' should be able to collect all *fifty stars* by December 25th.
#'
#' Collect stars by solving puzzles. Two puzzles will be made available on
#' each ~~day~~ millisecond in the advent calendar; the second puzzle is
#' unlocked when you complete the first. Each puzzle grants *one star*.
#' Good luck!
#'
#' You're standing in a room with "digitization quarantine" written in LEDs
#' along one wall. The only door is locked, but it includes a small
#' interface. "Restricted Area - Strictly No Digitized Users Allowed."
#'
#' It goes on to explain that you may only leave by solving a
#' [captcha](https://en.wikipedia.org/wiki/CAPTCHA) to prove you're *not* a
#' human. Apparently, you only get one millisecond to solve the captcha:
#' too fast for a normal human, but it feels like hours to you.
#'
#' The captcha requires you to review a sequence of digits (your puzzle
#' input) and find the *sum* of all digits that match the *next* digit in
#' the list. The list is circular, so the digit after the last digit is the
#' *first* digit in the list.
#'
#' For example:
#'
#' -   `1122` produces a sum of `3` (`1` + `2`) because the first digit
#'     (`1`) matches the second digit and the third digit (`2`) matches the
#'     fourth digit.
#' -   `1111` produces `4` because each digit (all `1`) matches the next.
#' -   `1234` produces `0` because no digit matches the next.
#' -   `91212129` produces `9` because the only digit that matches the next
#'     one is the last digit, `9`.
#'
#' *What is the solution* to your captcha?
#'
#' **Part Two**
#'
#' You notice a progress bar that jumps to 50% completion. Apparently, the
#' door isn't yet satisfied, but it did emit a *star* as encouragement. The
#' instructions change:
#'
#' Now, instead of considering the *next* digit, it wants you to consider
#' the digit *halfway around* the circular list. That is, if your list
#' contains `10` items, only include a digit in your sum if the digit
#' `10/2 = 5` steps forward matches it. Fortunately, your list has an even
#' number of elements.
#'
#' For example:
#'
#' -   `1212` produces `6`: the list contains `4` items, and all four
#'     digits match the digit `2` items ahead.
#' -   `1221` produces `0`, because every comparison is between a `1` and a
#'     `2`.
#' -   `123425` produces `4`, because both `2`s match each other, but no
#'     other digit has a match.
#' -   `123123` produces `12`.
#' -   `12131415` produces `4`.
#'
#' *What is the solution* to your new captcha?
#'
#' @param x a number
#' @return the specified sum of digits
#' @export
#' @examples
#' sum_of_digits_matching_next(91212129)
#' sum_of_digits_matching_halfway_around(12131415)
sum_of_digits_matching_next <- function(x) {
  x <- as.character(x)
  digits <- extract_digits(x)

  # Find indices of next digits
  positions <- seq_len(nchar(x))
  next_positions <- c(tail(positions, -1), head(positions, 1))

  # Find matches
  matches_next <- digits[positions] == digits[next_positions]
  sum(digits[matches_next])
}

#' @rdname day01
#' @export
sum_of_digits_matching_halfway_around <- function(x) {
  x <- as.character(x)
  digits <- extract_digits(x)

  # Find indices of digits halfway around the circle
  positions <- seq_len(nchar(x))
  half_size <- nchar(x) / 2
  next_positions <- c(tail(positions, -half_size), head(positions, half_size))

  # Find matches
  matches_next <- digits[positions] == digits[next_positions]
  sum(digits[matches_next])
}

extract_digits <- function(x) {
  as.numeric(str_tokenize(x))
}