2016-02: Bathroom Security

library(data.table)
library(stringi)

dt <- fread("input.txt", header = FALSE)

Part 1

mat <- matrix(1:9, 3, 3)

startX <<- 2
startY <<- 2
apply(dt, 1, function(x) {
  sapply(unlist(stri_match_all_regex(x, "[ULRD]")), function(y) {
    if (y == "U") { startY <<- startY - 1 }
    else if (y == "D") { startY <<- startY + 1 }
    else if (y == "L") { startX <<- startX - 1 }
    else { startX <<- startX + 1 }
    
    if(startX == 0) { startX <<- 1 }
    else if (startX == 4) {startX <<- 3}
    
    if(startY == 0) { startY <<- 1 }
    else if (startY == 4) {startY <<- 3}
  })
  return(rbind(startX, startY))
}) |> 
  t() |>
  apply(1, function(x) {
    return(mat[x[1], x[2]])
  }) |>
  paste0(collapse = "")

[1] "82958"

Part 2

mat <- as.matrix(fread("grid2.csv", header = FALSE, na.strings = ""))

startX <<- 3
startY <<- 1

apply(dt, 1, function(x) {
  sapply(unlist(stri_match_all_regex(x, "[ULRD]")), function(y) {
    if (y == "L") { 
      startY <<- startY - 1
      if (startY == 0 || is.na(mat[startX,startY])) { startY <<- startY + 1 }
    } else if (y == "R") { 
      startY <<- startY + 1 
      if (startY == 6 || is.na(mat[startX,startY])) { startY <<- startY - 1 }
    } else if (y == "U") { 
      startX <<- startX - 1 
      if (startX == 0 || is.na(mat[startX,startY])) { startX <<- startX + 1 }
    } else { 
      startX <<- startX + 1 
      if (startX == 6 || is.na(mat[startX,startY])) { startX <<- startX - 1 }
    }
  })
  return(rbind(startX, startY))
}) |> 
  t() |>
  apply(1, function(x) {
    return(mat[x[1], x[2]])
  }) |>
  paste0(collapse = "") |>
  stri_replace_all_regex(" ", "")

[1] "B3DB8"