adventofcode17: Advent of Code 2017

#' Day 02: Corruption Checksum
#'
#' [Corruption Checksum](http://adventofcode.com/2017/day/2)
#'
#' @name day02
#' @rdname day02
#' @details
#'
#' **Part One**
#'
#' As you walk through the door, a glowing humanoid shape yells in your
#' direction. "You there! Your state appears to be idle. Come help us
#' repair the corruption in this spreadsheet - if we take another
#' millisecond, we'll have to display an hourglass cursor!"
#'
#' The spreadsheet consists of rows of apparently-random numbers. To make
#' sure the recovery process is on the right track, they need you to
#' calculate the spreadsheet's *checksum*. For each row, determine the
#' difference between the largest value and the smallest value; the
#' checksum is the sum of all of these differences.
#'
#' For example, given the following spreadsheet:
#'
#'     5 1 9 5
#'     7 5 3
#'     2 4 6 8
#'
#' -   The first row's largest and smallest values are `9` and `1`, and
#'     their difference is `8`.
#' -   The second row's largest and smallest values are `7` and `3`, and
#'     their difference is `4`.
#' -   The third row's difference is `6`.
#'
#' In this example, the spreadsheet's checksum would be `8 + 4 + 6 = 18`.
#'
#' *What is the checksum* for the spreadsheet in your puzzle input?
#'
#' **Part Two**
#'
#' "Great work; looks like we're on the right track after all. Here's a
#' *star* for your effort." However, the program seems a little worried.
#' Can programs *be* worried?
#'
#' "Based on what we're seeing, it looks like all the User wanted is some
#' information about the *evenly divisible values* in the spreadsheet.
#' Unfortunately, none of us are equipped for that kind of calculation -
#' most of us specialize in bitwise operations."
#'
#' It sounds like the goal is to find the only two numbers in each row
#' where one evenly divides the other - that is, where the result of the
#' division operation is a whole number. They would like you to find those
#' numbers on each line, divide them, and add up each line's result.
#'
#' For example, given the following spreadsheet:
#'
#'     5 9 2 8
#'     9 4 7 3
#'     3 8 6 5
#'
#' -   In the first row, the only two numbers that evenly divide are `8`
#'     and `2`; the result of this division is `4`.
#' -   In the second row, the two numbers are `9` and `3`; the result is
#'     `3`.
#' -   In the third row, the result is `2`.
#'
#' In this example, the sum of the results would be `4 + 3 + 2 = 9`.
#'
#' What is the *sum of each row's result* in your puzzle input?
#'
#' @param spreadsheet a single string with a spreadsheet
#' @param f_filter a function to find a pair on each spreadsheet row
#' @param f_combine a function to apply on the pair from each line
#' @param x a vector of integers to filter
#' @export
#' @examples
#' s1 <- "5 1 9 5\n7 5 3\n2 4 6 8"
#' spreadsheet_checksum(s1, max_min_pair, function(x) max(x) - min(x))
#'
#' s2 <- "5 9 2 8\n9 4 7 3\n3 8 6 5"
#' spreadsheet_checksum(s2, evenly_divisible_pair, function(x) max(x) / min(x))
spreadsheet_checksum <- function(spreadsheet, f_filter, f_combine) {
  parse_spreadsheet(spreadsheet) %>%
    lapply(f_filter) %>%
    lapply(f_combine) %>%
    unlist() %>%
    sum()
}

#' @rdname day02
#' @export
evenly_divisible_pair <- function(x) {
  x %>%
    # Take all pairs
    utils::combn(2) %>%
    apply(2, list) %>%
    lapply(unlist) %>%
    # Keep just evenly divisible pairs
    keep_if(function(x) max(x) %% min(x) == 0) %>%
    unlist()
}

#' @rdname day02
#' @export
max_min_pair <- function(x) {
  range(x)
}

parse_spreadsheet <- function(x) {
  x %>%
    # Split lines
    strsplit("\n") %>%
    unlist() %>%
    # Find numbers on each line
    stringr::str_extract_all("\\d+") %>%
    lapply(as.numeric) %>%
    keep_if(function(x) length(x) != 0)
}