scheme でもうすこしできた。

(define last-data10b '())
(define count-in 0)
(define count-out 0)

(define (line-scaler data10b last-flag)
  ;; (if (= count-in count-out) (assert (null last-data10b)))
  (print `(,count-in ,count-out))
  (let ((new-value12b
          (if (= count-in count-out) (* data10b 4)
            (let* ((norm-x1 (- count-in count-out))
                   (norm-0x (- 4 norm-x1)))
              (blend10bx4 last-data10b norm-0x norm-x1 data10b)))))

    (set! count-in (+ count-in 4))
    (set! count-out
      (+ count-out (if (= (- count-in count-out) 3) 6 3)))

    (set! last-data10b
      (if last-flag (define last-data10b '()) data10b)) ;;set '() for debug

    new-value12b))

グローバル変数を使っていてダサい。継続を使ったほうがスマートか？