R/day5.R

In moodymudskipper/adventofcode2021: My Solutions for Advent Of Code 2021

#' day 5
#'
#' @export
day5 <- function() {

  # data
  file <- system.file("extdata/day5.txt", package = "adventofcode2021")
  input <- unglue::unglue_data(readLines(file), "{x1},{y1}->{x2},{y2}", convert = TRUE) + 1
  grid <- with(input, matrix(0, nrow = max(x1, x2), ncol = max(y1, y2)))

  # part1
  input1 <- subset(input, x1 == x2 | y1 == y2)
  update_grid <- function(grid, coords) with(coords, {
    grid[x1:x2, y1:y2] <- grid[x1:x2, y1:y2] + 1
    grid})
  grid1 <- Reduce(update_grid, x = split(input1, 1:nrow(input1)), init = grid)
  part1 <- sum(grid1 >= 2)

  # part 2
  update_grid <- function(grid, coords) with(coords, {
    if(x1 == x2 | y1 == y2)
      grid[x1:x2, y1:y2] <- grid[x1:x2, y1:y2] + 1
    else
      diag(grid[x1:x2, y1:y2]) <- diag(grid[x1:x2, y1:y2]) + 1
    grid})
  grid2 <- Reduce(update_grid, x = split(input, 1:nrow(input)), init = grid)
  part2 <- sum(grid2 >= 2)

  # alternate solution
  input <- unglue::unglue_data(readLines(file), "{x1},{y1}->{x2},{y2}", convert = TRUE) + 1

  solve <- \(x) x |>
    split(1:nrow(x)) |>
    lapply(\(row) data.frame(x = row$x1:row$x2, y = row$y1:row$y2, n = 1)) |>
    do.call(what = rbind) |>
    stats:::aggregate.formula(formula = n ~ x + y, FUN = sum) |>
    subset(n > 1) |>
    nrow()

  # part1
  input1 <- subset(input, x1 == x2 | y1 == y2)
  part1 <- solve(input1)

  # part2 (same but using unfiltered input)
  part2 <- solve(input)

  list(part1 = part1, part2 = part2)
}