(define-module (bits)
  #:export (make-bits empty-bits bits-vector bits-len add-bit!))

(define (make-bits bv len)
  (cons bv len))
(define (bits-vector bits)
  (car bits))
(define (set-bits-vector! bits bv)
  (set-car! bits bv))
(define (bits-len bits)
  (cdr bits))
(define (set-bits-len! bits len)
  (set-cdr! bits len))

(define (empty-bits)
  (make-bits #* 0))

(define (grow-bits! bits)
  (let* ((len (bits-len bits))
         (new-v (make-bitvector (if (zero? len) 1 (* len 2)) #f)))
    ;; see 39c5363b4
    (bit-set*! new-v (bits-vector bits) #t)
    (set-bits-vector! bits new-v)))

(define (add-bit! bits val)
  (let ((len (bits-len bits)))
    (if (= len (bitvector-length (bits-vector bits)))
        (grow-bits! bits))
    (bitvector-set! (bits-vector bits) len val)
    (set-bits-len! bits (1+ len))))