; look and say
(define (iterate n f . bs)
(let loop ((n n) (b (car bs)) (bs (cdr bs)) (xs '()))
(if (zero? n) (reverse xs)
(let ((new-bs (append bs (list (apply f b bs)))))
(loop (- n 1) (car new-bs) (cdr new-bs) (cons b xs))))))
(define (flatten xs)
(cond ((null? xs) xs)
((pair? xs)
(append (flatten (car xs))
(flatten (cdr xs))))
(else (list xs))))
(define (uniq-c eql? xs)
(if (null? xs) xs
(let loop ((xs (cdr xs)) (prev (car xs)) (k 1) (result '()))
(cond ((null? xs) (reverse (cons (cons prev k) result)))
((eql? (car xs) prev) (loop (cdr xs) prev (+ k 1) result))
(else (loop (cdr xs) (car xs) 1 (cons (cons prev k) result)))))))
(define (digits n . args)
(let ((b (if (null? args) 10 (car args))))
(let loop ((n n) (d '()))
(if (zero? n) d
(loop (quotient n b)
(cons (modulo n b) d))))))
(define (undigits ds . args)
(let ((b (if (null? args) 10 (car args))))
(let loop ((ds ds) (n 0))
(if (null? ds) n
(loop (cdr ds) (+ (* n b) (car ds)))))))
(define (look-and-say n)
(define (flip x) (cons (cdr x) (car x)))
(undigits (flatten (map flip (uniq-c = (digits n))))))
(define (look-and-say-sequence n)
(iterate n look-and-say 1))
(display (look-and-say-sequence 10))