R/day13.R

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

#' Day 13: Transparent Origami
#'
#' [Transparent Origami](https://adventofcode.com/2021/day/13)
#'
#' @name day13
#' @rdname day13
#' @details
#'
#' **Part One**
#'
#' You reach another volcanically active part of the cave. It would be nice
#' if you could do some kind of thermal imaging so you could tell ahead of
#' time which caves are too hot to safely enter.
#'
#' Fortunately, the submarine seems to be equipped with a thermal camera!
#' When you activate it, you are greeted with:
#'
#'     Congratulations on your purchase! To activate this infrared thermal imaging
#'     camera system, please enter the code found on page 1 of the manual.
#'
#' Apparently, the Elves have never used this feature. To your surprise,
#' you manage to find the manual; as you go to open it, page 1 falls out.
#' It\'s a large sheet of [transparent
#' paper](https://en.wikipedia.org/wiki/Transparency_(projection))! The
#' transparent paper is marked with random dots and includes instructions
#' on how to fold it up (your puzzle input). For example:
#'
#'     6,10
#'     0,14
#'     9,10
#'     0,3
#'     10,4
#'     4,11
#'     6,0
#'     6,12
#'     4,1
#'     0,13
#'     10,12
#'     3,4
#'     3,0
#'     8,4
#'     1,10
#'     2,14
#'     8,10
#'     9,0
#'
#'     fold along y=7
#'     fold along x=5
#'
#' The first section is a list of dots on the transparent paper. `0,0`
#' represents the top-left coordinate. The first value, `x`, increases to
#' the right. The second value, `y`, increases downward. So, the coordinate
#' `3,0` is to the right of `0,0`, and the coordinate `0,7` is below `0,0`.
#' The coordinates in this example form the following pattern, where `#` is
#' a dot on the paper and `.` is an empty, unmarked position:
#'
#'     ...#..#..#.
#'     ....#......
#'     ...........
#'     #..........
#'     ...#....#.#
#'     ...........
#'     ...........
#'     ...........
#'     ...........
#'     ...........
#'     .#....#.##.
#'     ....#......
#'     ......#...#
#'     #..........
#'     #.#........
#'
#' Then, there is a list of *fold instructions*. Each instruction indicates
#' a line on the transparent paper and wants you to fold the paper *up*
#' (for horizontal `y=...` lines) or *left* (for vertical `x=...` lines).
#' In this example, the first fold instruction is `fold along y=7`, which
#' designates the line formed by all of the positions where `y` is `7`
#' (marked here with `-`):
#'
#'     ...#..#..#.
#'     ....#......
#'     ...........
#'     #..........
#'     ...#....#.#
#'     ...........
#'     ...........
#'     -----------
#'     ...........
#'     ...........
#'     .#....#.##.
#'     ....#......
#'     ......#...#
#'     #..........
#'     #.#........
#'
#' Because this is a horizontal line, fold the bottom half *up*. Some of
#' the dots might end up overlapping after the fold is complete, but dots
#' will never appear exactly on a fold line. The result of doing this fold
#' looks like this:
#'
#'     #.##..#..#.
#'     #...#......
#'     ......#...#
#'     #...#......
#'     .#.#..#.###
#'     ...........
#'     ...........
#'
#' Now, only `17` dots are visible.
#'
#' Notice, for example, the two dots in the bottom left corner before the
#' transparent paper is folded; after the fold is complete, those dots
#' appear in the top left corner (at `0,0` and `0,1`). Because the paper is
#' transparent, the dot just below them in the result (at `0,3`) remains
#' visible, as it can be seen through the transparent paper.
#'
#' Also notice that some dots can end up *overlapping*; in this case, the
#' dots merge together and become a single dot.
#'
#' The second fold instruction is `fold along x=5`, which indicates this
#' line:
#'
#'     #.##.|#..#.
#'     #...#|.....
#'     .....|#...#
#'     #...#|.....
#'     .#.#.|#.###
#'     .....|.....
#'     .....|.....
#'
#' Because this is a vertical line, fold *left*:
#'
#'     #####
#'     #...#
#'     #...#
#'     #...#
#'     #####
#'     .....
#'     .....
#'
#' The instructions made a square!
#'
#' The transparent paper is pretty big, so for now, focus on just
#' completing the first fold. After the first fold in the example above,
#' `17` dots are visible - dots that end up overlapping after the fold is
#' completed count as a single dot.
#'
#' *How many dots are visible after completing just the first fold
#' instruction on your transparent paper?*
#'
#' **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, `f13a(x)` returns .... For Part Two,
#'   `f13b(x)` returns ....
#' @export
#' @examples
#' f13a(example_data_13())
#' f13b(example_data_13())
f13a <- function(x) {

  positions_after_fold <- f13_fold(x$folds[[1]], x$positions)

  return(nrow(unique(positions_after_fold)))

}

f13_fold <- function(fold, positions) {

  for (i in c(1, 2)) {
    if (fold[i] >= max(positions[, i])/2) {
      positions[, i] <- ifelse(
        positions[, i] < fold[i],
        positions[, i],
        fold[i] * 2 - positions[, i]
      )
    } else {
      positions[, i] <- ifelse(
        positions[, i] < fold[i],
        fold[i] * 2 - positions[, i],
        positions[, i]
      )
    }
  }

  return(positions)

}

#' @rdname day13
#' @export
f13b <- function(x) {

  for (fold in x$folds) {
    x$positions <- f13_fold(fold, x$positions)
  }

  paper <- matrix(0, nrow = max(x$positions[, 1]) + 1, ncol = max(x$positions[, 2]) + 1)

  paper[x$positions+1] <- 1

  return(paper)
}


#' @param example Which example data to use (by position or name). Defaults to
#'   1.
#' @rdname day13
#' @export
example_data_13 <- function(example = 1) {
  l <- list(
    a = list(
      positions = matrix(ncol = 2, byrow = TRUE, c(
        6,10,
        0,14,
        9,10,
        0,3,
        10,4,
        4,11,
        6,0,
        6,12,
        4,1,
        0,13,
        10,12,
        3,4,
        3,0,
        8,4,
        1,10,
        2,14,
        8,10,
        9,0
      )),
      folds = list(c(7, 0), c(0, 5))
    )
  )
  l[[example]]
}