Day 4: Security Through Obscurity

Reference

library(tidyverse)

input <- read_csv(file = "inputs/2016/04.txt", col_names = "original")

Part One

Solution

chesum_eval <- function(x) {
    letters <- x |>
        str_split_1(pattern = "") |> 
        table() |> 
        sort(decreasing = TRUE) |> 
        names() |> 
        _[1:5] |> 
        paste0(collapse = "")
}

real_rooms <- input |> 
    mutate(sector_id      = as.numeric(str_extract(string = original, pattern = "[0-9]+")),
           checksum       = str_extract(string = original, pattern = "(?<=\\[)[a-z]*"),
           encrypted_name = str_extract(string = original, pattern = "[a-z\\-]*(?=-\\d+)"),    
           encrypted_name = str_remove_all(string = encrypted_name, pattern = "\\-"),
           chesum_eval    = map_chr(.x = encrypted_name, .f = chesum_eval),
           real_room      = checksum == chesum_eval)

real_rooms |> 
    filter(real_room == TRUE) |>
    summarise(total = sum(sector_id)) |> 
    pull(total) |> 
    cat()
## 185371

Full Table

Part Two

Solution

room_names <- input |> 
    mutate(sector_id = as.numeric(str_extract(string = original, pattern = "[0-9]+")),
           encrypted = str_remove(string = original, pattern = "-[0-9]+\\[[a-z]+\\]"))

decrypt <- function(string, sector_id) {
    decrypted <- character(0)
    
    for (element in str_split_1(string = string, pattern = "")) {
        if (element == "-")
            decrypted <- c(decrypted, " ")
        else {
            index <- (which(letters == element) + sector_id) %% 26
            decrypted <- c(decrypted, letters[index])
        }
    }
    
    return(paste(decrypted, collapse = ""))
}

room_names <- room_names |> 
    mutate(decrypted = map2_chr(.x = encrypted, .y = sector_id, .f = decrypt))

room_names |> 
    filter(decrypted == "northpole object storage") |> 
    pull(sector_id) |> 
    cat()
## 984

Full Table

room_names |> 
    rowid_to_column("row_number") |> 
    filter(decrypted == "northpole object storage") |> 
    pull("row_number") |> 
    cat()
## 476