# Coding Codes

One of my sons has signed up with the S.H.O.E. Academy a fun site for espionage fans.

One of the regular tasks is to decipher messages using a code wheel

The is made up of two concentric circles each made up the letters of the alphabet with the outer ring in regular order and the inner ring, which you can spin, effectively randomized

The challenge for the agent is to find which letter in the inner ring to use to align with the ‘A’ in the outer ring. Then each letter in the message can be read off the inner ring and the match to the outer ring is the correct answer. For instance, the key letter in the picture above is ‘L’, and a code of ‘RMT YEFV’ transcribes to “get help”

This can be a pretty time-comnsuming task, particularly when the message is long and the initial cut-out of the wheel was a little poor meaning that matches are not immediately apparent. In addition, it is not something that can be memorized as their are 26 variations of the initial alignment possible

This seemed to be a good example of using R both to speed up the result and to impress on my son of the power of coding

## Letter Alignment

We need two vectors, one for each wheel. The first is fixed, the latter whilst having the letters is a proscribed order will commence from different positions

# No need for full tidyverse
library(stringr)

# Capital letters + space for easier legibility
vector_out <- c(LETTERS," ")

vector_in <- c("I","P","S","F","H","K","X","V","N","Q","E",
"T","U","O","A","D","G","J","L","Z","C","B","M",
"W","R","Y")

# Function to re-order vector_in based on key letter
code_fun <- function(key_letter) {

# Derive index of key letter
start <- str_which(vector_in,key_letter)

# Create two sections of inner wheel
part_1 <- vector_in[start:26]
part_2 <- vector_in[1:start-1]

# Combine them and add the space to match vector_out
c(part_1,part_2," ")

}

## create input for use in decoding function

# variable
key_letter <- "C"

vector_in_new <- code_fun(key_letter)
vector_in_new
##  [1] "C" "B" "M" "W" "R" "Y" "I" "P" "S" "F" "H" "K" "X" "V" "N" "Q" "E"
## [18] "T" "U" "O" "A" "D" "G" "J" "L" "Z" " "

## Decoding Message

This requires us to split the scrambled message into individual letters, find their index and match that to the other vector and thus decipher messsage

decode_fun <- function(text_in) {

# create two empty vectors
match_integers <- vector("integer",length(text_in))
match_characters <- vector("character",length(text_in))

# split input into individual letters
text_in_vector <- str_split(text_in,"")[[1]]

## these loops could be combined
for(i in 1:length(text_in_vector)) {
match_integers[i] <-str_which(vector_in_new,text_in_vector[i]) #4
}
match_integers

for(i in 1:length(text_in_vector)) {
match_characters[i] <-  vector_out[match_integers[i]]
}
match_characters

## collapse back to words

str_c(match_characters, collapse="")

}

text_in <- "GR VRRW BCMHAQ S MCVO XCHR OPR MCKK OPSU QKCMR SU YAKK NY MNXQAORT PCMHRTU CVW OPR QPNVRU CTR CKK BAIIRW UQL PNOKSVR MNWR SU PSIPONQU OSMHRO VAXBRT XSVAU CWXSUUSNV QTSMR"

# create message amd make  more legible
message <- str_to_title(decode_fun(text_in))
message
## [1] "We Need Backup I Cant Make The Call This Place Is Full Of Computer Hackers And The Phones Are All Bugged Spy Hotline Code Is Hightops Ticket Number Minus Admission Price"

And here is the agent in non-smiling mode