import strformat

type
  Mode = enum
    Encrypt
    Decrypt

const lAlphabet: string = "HXUCZVAMDSLKPEFJRIGTWOBNYQ"
const rAlphabet: string = "PTLNBQDEOYSFAVZKGJRIHWXUMC"

proc chao(text: string, mode: Mode, verbose: bool = false): string =
  var left = lAlphabet
  var right = rAlphabet
  var eText = newSeq[char](text.len)
  var temp: array[26, char]

  for i in 0..<text.len:
    if verbose:
      echo &"{left}  {right}"
    var index: int
    if mode == Encrypt:
      index = right.find(text[i])
      eText[i] = left[index]
    else:
      index = left.find(text[i])
      eText[i] = right[index]
    if (i == text.len - 1):
      break

    # permute left
    for j in index..25:
      temp[j - index] = left[j]
    for j in 0..<index:
      temp[26 - index + j] = left[j]
    var store = temp[1]
    for j in 2..13:
      temp[j - 1] = temp[j]
    temp[13] = store
    left = ""
    for i in temp:
      left &= $i

    # permute right
    for j in index..25:
      temp[j - index] = right[j]
    for j in 0..<index:
      temp[26 - index + j] = right[j]
    store = temp[0]
    for j in 1..25:
      temp[j - 1] = temp[j]
    temp[25] = store
    store = temp[2]
    for j in 3..13:
      temp[j - 1] = temp[j]
    temp[13] = store
    right = ""
    for i in temp:
      right &= $i

  for i in eText:
    result &= $i

var plainText = "WELLDONEISBETTERTHANWELLSAID"
echo &"The original plaintext is: {plainText}"
echo "\nThe left and right alphabets after each permutation during encryption are:\n"
var cipherText = chao(plainText, Encrypt, true)
echo &"\nThe ciphertext is: {cipherText}"
var plainText2 = chao(cipherText, Decrypt, false)
echo &"\nThe recovered plaintext is: {plainText2}"