S-99-13 Run-length encoding of a list (direct solution).

Implement the so-called run-length encoding data compression method directly. I.e. don't explicitly create the sublists containing the duplicates, as in problem S-99-09, but only count them. As in problem S-99-11, simplify the result list by replacing the singleton lists (1 X) by X.

Example:

(encode-direct '(a a a a b c c a a d e e e e)) ; => ((4 a) b (2 c) (2 a) d (4 e))

Solution:

```
(define encode-direct
(lambda (xs)
(if (null? xs) '()
(let ((pkg (lambda (N E)
(if (= N 1) E (list N E)))))
(let loop ((rest (cdr xs))
(E (car xs))
(N 1))
(cond ((null? rest)
(list (pkg N E)))
((eq? E (car rest))
(loop (cdr rest) E (+ N 1)))
(else
(cons (pkg N E) (loop (cdr rest) (car rest) 1)))))))))
```