2024

Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 Day 9 Day 10 Day 11 Day 12 Day 13 Day 14 Day 15 Day 16

My part 2 is slow; I threw in some parellelisation and called it a day.

R

— R Day 7 Part 1 —

#' Find the total of valid calibrations
#'
#' Extract the target number as the first number on each line. Check if the
#' following sequence can total the target number, applying operators `+` or `*`
#' left-to-right in pairs. Sum the target numbers of all valid lines.
#'
#' @param input
#' Character vector of input lines with a target number followed by a sequence
#' of numbers to test against the target.
#'
#' @return
#' numeric(1) The total of targets that can be met
solve_day7_part1 <- function(input) {
  lines <- strsplit(input, ": | ")

  targets <- vapply(lines, \(line) line[[1]], character(1))
  targets <- as.numeric(targets)

  nums <- lapply(lines, \(line) line[-1])

  possible <- Map(is_possible, targets, nums)

  sum(targets[as.logical(possible)])
}

#' Check if a sequence of numbers can meet a target
#'
#' Uses recursion to follow branches of applying one of a choice of operators
#' to sequential pairs of numbers. If at any point we exceed the target number,
#' returns FALSE. Once we have worked through the sequence to get a single
#' number. Return whether that number is the target number.
#'
#' The `operator`s used are `+` and `*`
#'
#' @param target
#' numeric(1) Target to match.
#' @param nums
#' Sequence of numbers to operate on.
#' @param operator
#' Function which takes 2 arguments to apply to the first 2 numbers in `nums`
#' ie the next operator to try. When NULL (default), the function just calls
#' itself with each operator.
#'
#' @return
#' logical(1) Whether the target can be met with the given sequence of numbers.
is_possible <- function(target, nums, operator = NULL) {
  nums <- as.numeric(nums)

  if (length(nums) == 1) {
    return(nums == target)
  }

  if (!is.null(operator)) {
    new_num <- as.numeric(operator(nums[[1]], nums[[2]]))

    # prune
    if(new_num > target) {
      return(FALSE)
    }

    nums <- c(new_num, nums[-c(1, 2)])
  }

  # try each operator
  is_possible(target, nums, `+`) ||
    is_possible(target, nums, `*`)
}

aoc_source(day = 7, part = 1)

input = aoc_read(day = 7)

aoc_run(solve_day7_part1(input))

Elapsed: 0.796 seconds
Memory:  116 KB

— R Day 7 Part 2 —

#' Find the total of valid calibrations
#'
#' Extract the target number as the first number on each line. Check if the
#' following sequence can total the target number, applying operators `+`, `*`
#' or concatenation left-to-right in pairs. Sum the target numbers of all valid
#' lines.
#'
#' @param input
#' Character vector of input lines with a target number followed by a sequence
#' of numbers to test against the target.
#'
#' @return
#' numeric(1) The total of targets that can be met
solve_day7_part2 <- function(input) {
  lines <- strsplit(input, ": | ")

  targets <- vapply(lines, \(line) line[[1]], character(1))
  targets <- as.numeric(targets)

  nums <- lapply(lines, \(line) line[-1])

  # Parellelise
  future::plan(future::multisession, workers = 3)
  possible <- furrr::future_map2_lgl(targets, nums, is_possible)

  sum(targets[possible])
}

#' Check if a sequence of numbers can meet a target
#'
#' Uses recursion to follow branches of applying one of a choice of operators
#' to sequential pairs of numbers. If at any point we exceed the target number,
#' returns FALSE. Once we have worked through the sequence to get a single
#' number. Return whether that number is the target number.
#'
#' The `operator`s used are `+`, `*` and `paste0`.
#'
#' @param target
#' numeric(1) Target to match.
#' @param nums
#' Sequence of numbers to operate on.
#' @param operator
#' Function which takes 2 arguments to apply to the first 2 numbers in `nums`
#' ie the next operator to try. When NULL (default), the function just calls
#' itself with each operator.
#'
#' @return
#' logical(1) Whether the target can be met with the given sequence of numbers.
is_possible <- function(target, nums, operator = NULL) {
  nums <- as.numeric(nums)

  if (length(nums) == 1) {
    return(nums == target)
  }

  if (!is.null(operator)) {
    new_num <- as.numeric(operator(nums[[1]], nums[[2]]))

    # prune
    if(new_num > target) {
      return(FALSE)
    }

    nums <- c(new_num, nums[-c(1, 2)])
  }

  is_possible(target, nums, `+`) ||
    is_possible(target, nums, `*`) ||
    is_possible(target, nums, paste0)
}

aoc_source(day = 7, part = 2)

input = aoc_read(day = 7)

aoc_run(solve_day7_part2(input))

Elapsed: 22.332 seconds
Memory:  12436 KB