[2018-03-26] Challenge #355 [Easy] Alphabet Cipher https://www.reddit.com/r/dailyprogrammer/comments/879u8b/20180326_challenge_355_easy_alphabet_cipher/. Demo at http://rextester.com/ZZIN40507

cipher.R

getEncoding <- function(letter1, letter2) {
  # Find the number for each letter.
  index1 <- match(letter1, letters)
  index2 <- match(letter2, letters)

  # Find the index number within the shifted letters by adding the two indices and taking the remainder from the number of letters.
  index <- (index1 + index2 - 1) %% length(letters)

  index <- ifelse(index == 0, 26, index)
  
  letters[index]
}

encode <- function(str, secret) {
  index <- 1
  keys <- unlist(strsplit(secret, ''))
  len <- length(keys)

  result <- sapply(unlist(strsplit(str, '')), function(ch) {
    e <- getEncoding(ch, keys[index])
    
    index <<- index + 1
    if (index > len) {
      index <<- 1
    }
    
    e
  })
  
  paste(result, collapse='')
}

output.txt

lumicjcnoxjhkomxpkwyqogywq
uvrufrsryherugdxjsgozogpjralhvg
flrlrkfnbuxfrqrgkefckvsa
zhvpsyksjqypqiewsgnexdvqkncdwgtixkx

test.R

print(encode('thepackagehasbeendelivered', 'snitch'))
print(encode('theredfoxtrotsquietlyatmidnight', 'bond'))
print(encode('murderontheorientexpress', 'train'))
print(encode('themolessnuckintothegardenlastnight', 'garden'))