transposition.lisp

(defpackage :transposition
  (:use :cl))

(in-package :transposition)

(defparameter *text* "WEAREDISCOVEREDFLEEATONCE------")

(defun range (max &key (min 0) (step 1))
   (loop for n from min below max by step
      collect n))

(defun every-nth-0 (text idx n)
  (when (< idx (length text))
    (cons (nth idx text) (every-nth-0 text (+ idx n) n))))

(defun every-nth (text start n)
  (let ((r (every-nth-0 (coerce text 'list) start n)))
    (coerce r 'string)))

(defun encode (text cols)
  (let ((ids (range cols)))
    (mapcar (lambda (i) (every-nth text i cols)) ids)))

(defun decode (texts)
  (let* ((lists (mapcar (lambda (x) (coerce x 'list)) texts))
         (r (apply #'mapcar #'list lists)))
    (apply #'concatenate 'string
           (mapcar (lambda (x) (coerce x 'string)) r))))