(ql:quickload :cl-who)
(import 'cl-who:with-html-output-to-string)

(defun punnet (stringa stringb)
  (declare (string stringa stringb))
  (let* ((a (map 'list #'(lambda (x) (format nil "~c" x)) stringa))
         (b (map 'list #'(lambda (x) (format nil "~c" x)) stringb))
         (lena (length a))
         (lenb (length b)))
    (if (not (= lena lenb))
        (if (< lena lenb)
            (error "a < b, ~a < ~b" lena lenb)
            (error "a > b, ~a > ~b" lena lenb))
        (loop for index-b from 0 to (1- lena)
           collect (loop for index-a from 0 to (1- lena)
                      collect (if (= index-b index-a)
                                 (nth index-b b)
                                 (nth index-a a)))))))

(defun punnet-html (string-a string-b)
  (with-html-output-to-string (out)
    (:table
     (loop for line in (punnet string-a string-b)
        do (htm (:tr (loop for cell in line
                          do (htm (:td (str cell))))))))))