;; https://www.oss.com/asn1/resources/asn1-made-simple/introduction.html
;; https://www.oss.com/asn1/resources/asn1-made-simple/asn1-quick-reference/basic-encoding-rules.html
;; https://www.oss.com/asn1/resources/asn1-made-simple/asn1-quick-reference.html

;; https://letsencrypt.org/docs/a-warm-welcome-to-asn1-and-der/#length
(define (read-sequence-length bytes start-index)
  (let ((bits (integer->list (bytevector-u8-ref bytes start-index))))
    bits))

(define (read-asn.1-sequence bytes start-index result)
  (when (not (= (bytevector-u8-ref bytes start-index) 48))
    (error "read-asn.1-sequence error: start index is not sequence tag (48)"
           bytes
           start-index))
  (if (or (> start-index (bytevector-length bytes))
          (not (= (bytevector-u8-ref bytes start-index) 48)))
    result
    (let ((sequence-length+length-bytes-length
            (bytevector-u8-ref bytes (+ start-index 1)))
          )
      (read-asn.1-sequence-items
        bytes
        (+ start-index sequence-length)
        (append
          result
          (list
            (bytevector-copy bytes
                             start-index
                             (+ start-index (- sequence-length 1)))))))))

(define (read-asn.1-integer bytes start-index)
  (when (not (= (bytevector-u8-ref bytes start-index) 2))
    (error "read-asn.1-integer error: bytes start index is not integer type tag"
           bytes
           start-index))
  (bytevector-uint-ref bytes
                      (+ start-index
                         1 ;; ASN.1-type
                         1 ;; ASN.1-length
                         )
                      'big
                      (bytevector-u8-ref bytes (+ start-index 1)) ;; ASN.1-length
                      ))

(define (bytes->certificate bytes)
  (let ((asn.1-sequence-length (bytevector-u8-ref bytes 1)))
  `(certificate
     (asn.1-sequence-tag . ,(bytevector-u8-ref bytes 0))
     (asn.1-sequence-length  . ,asn.1-sequence-length)
     (tbs-certificate . ,(read-asn.1-sequence bytes 0 '()))
     (bytes ,bytes)
     ;(version . ,(bytevector-u8-ref bytes 2))
     ;(serial-number . , (read-asn.1-integer bytes 3))
     )
  ))

(define (read-certificates bytes start-index)
  ;; https://datatracker.ietf.org/doc/html/rfc5280#section-4.1
  (map bytes->certificate (read-asn.1-sequence bytes start-index)))