R과 함께 코드 2020-03의 출현

지침

1 단계

2 단계

R 솔루션

1부

library(purrr)

n_trees <- function(
  by_x = 1, 
  by_y = 3
){
  ipt <- read.delim( "2020-03-aoc.txt", header = FALSE, stringsAsFactors = FALSE)
  # Building the path
  # x need to go from 1 to nrow(ipt), 1 by 1
  x <- seq(1, nrow(ipt), by = by_x)
  # y needs to start at 1, and go by 3 steps until we have length(x) steps
  y <- seq(1, by = by_y, length.out = length(x))
  list(
    x = x, 
    y = y
  ) %>%
    pmap_dbl(~ {
      # Which x step are we in?
      row <- ipt[..1,] 
      # If the row isn't wide enough, expand it until it is
      # There is probably a better way to do that but I'm not 
      # sure I want to spend 5 minutes trying stuff that will
      # save me half a micro second
      while(nchar(row) < ..2){
        row <- paste0(row, row)
      }
      # Split the damn thing
      row <- strsplit(row, "")[[1]]
      # look for the y, is it a tree?
      if (row[..2] == ".") return(0)
      if (row[..2] == "#") return(1)
    }
    ) %>% sum()
}
n_trees()

## [1] 278

두 번째 부분

data.frame(
  x = c(1, 1, 1, 1, 2),
  y = c(1, 3, 5, 7, 1)
) %>%
  pmap_dbl(~ n_trees(..1, ..2)) %>%
  reduce(`*`)

## [1] 9709761600