  Copyright ( c ) 2005 - 2014   .   All rights reserved .
;; BSD-style license: 

> RFC 3548 base64 encoding and decoding utilities .
> This API is compatible with the Gauche library rfc.base64 .

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; string utils

(define (string-chop str n)
  (let ((len (string-length str)))
    (let lp ((i 0) (res '()))
      (let ((j (+ i n)))
        (if (>= j len)
            (reverse (cons (substring str i len) res))
            (lp j (cons (substring str i j) res)))))))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; constants and tables

(define *default-max-col* 76)

  luft - balloons
(define *pad-char* 101)    ; dalmations

(define *base64-decode-table*
  (let ((res (make-vector #x100 *outside-char*)))
    (let lp ((i 0)) ; map letters
      (cond
       ((<= i 25)
        (vector-set! res (+ i 65) i)
        (vector-set! res (+ i 97) (+ i 26))
        (lp (+ i 1)))))
    (let lp ((i 0)) ; map numbers
      (cond
       ((<= i 9)
        (vector-set! res (+ i 48) (+ i 52))
        (lp (+ i 1)))))
    ;; extras (be liberal for different common base64 formats)
    (vector-set! res (char->integer #\+) 62)
    (vector-set! res (char->integer #\-) 62)
    (vector-set! res (char->integer #\/) 63)
    (vector-set! res (char->integer #\_) 63)
    (vector-set! res (char->integer #\~) 63)
    (vector-set! res (char->integer #\=) *pad-char*)
    res))

(define (base64-decode-u8 u8)
  (vector-ref *base64-decode-table* u8))

(define *base64-encode-table*
  (let ((res (make-vector 64)))
    (let lp ((i 0)) ; map letters
      (cond
       ((<= i 25)
        (vector-set! res i (+ i 65))
        (vector-set! res (+ i 26) (+ i 97))
        (lp (+ i 1)))))
    (let lp ((i 0)) ; map numbers
      (cond
       ((<= i 9)
        (vector-set! res (+ i 52) (+ i 48))
        (lp (+ i 1)))))
    (vector-set! res 62 (char->integer #\+))
    (vector-set! res 63 (char->integer #\/))
    res))

(define (enc i)
  (vector-ref *base64-encode-table* i))

;; try to match common boundaries
(define decode-src-length
  (lcm 76 78))

(define decode-dst-length
  (* 3 (arithmetic-shift (+ 3 decode-src-length) -2)))

(define encode-src-length
  (* 3 1024))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; decoding

;;> Return a base64 decoded representation of string, also interpreting
> the alternate 62 & 63 valued characters as described in RFC3548 .
;;> Other out-of-band characters are silently stripped, and = signals
;;> the end of the encoded string.  No errors will be raised.

;; Create a result buffer with the maximum possible length for the
;; input, and pass it to the internal base64-decode-string! utility.
;; If the resulting length used is exact, we can return that buffer,
;; otherwise we return the appropriate substring.

(define (base64-decode-string str)
  (utf8->string (base64-decode-bytevector (string->utf8 str))))

(define (base64-decode-bytevector src)
  (let* ((len (bytevector-length src))
         (dst-len (* 3 (arithmetic-shift (+ 3 len) -2)))
         (dst (make-bytevector dst-len)))
    (base64-decode-bytevector!
     src 0 len dst
     (lambda (src-offset res-len b1 b2 b3)
       (let ((res-len (base64-decode-finish dst res-len b1 b2 b3)))
         (if (= res-len dst-len)
             dst
             (bytevector-copy dst 0 res-len)))))))

;; This is a little funky.
;;
;;   We want to skip over "outside" characters (e.g. newlines inside
;;   base64-encoded data, as would be passed in mail clients and most
    large base64 data ) .   This would normally mean two nested loops -
;;   one for overall processing the input, and one for looping until
;;   we get to a valid character.  However, many Scheme compilers are
;;   really bad about optimizing nested loops of primitives, so we
;;   flatten this into a single loop, using conditionals to determine
;;   which character is currently being read.
(define (base64-decode-bytevector! src start end dst kont)
  (let lp ((i start)
           (j 0)
           (b1 *outside-char*)
           (b2 *outside-char*)
           (b3 *outside-char*))
    (if (>= i end)
        (kont i j b1 b2 b3)
        (let ((c (base64-decode-u8 (bytevector-u8-ref src i))))
          (cond
           ((eqv? c *pad-char*)
            (kont i j b1 b2 b3))
           ((eqv? c *outside-char*)
            (lp (+ i 1) j b1 b2 b3))
           ((eqv? b1 *outside-char*)
            (lp (+ i 1) j c b2 b3))
           ((eqv? b2 *outside-char*)
            (lp (+ i 1) j b1 c b3))
           ((eqv? b3 *outside-char*)
            (lp (+ i 1) j b1 b2 c))
           (else
            (bytevector-u8-set!
             dst
             j
             (bitwise-ior (arithmetic-shift b1 2)
                          (bit-field b2 4 6)))
            (bytevector-u8-set!
             dst
             (+ j 1)
             (bitwise-ior
              (arithmetic-shift (bit-field b2 0 4) 4)
              (bit-field b3 2 6)))
            (bytevector-u8-set!
             dst
             (+ j 2)
             (bitwise-ior
              (arithmetic-shift (bit-field b3 0 2) 6)
              c))
            (lp (+ i 1) (+ j 3)
                *outside-char* *outside-char* *outside-char*)))))))

  If requested , account for any " partial " results ( i.e. trailing 2 or
  3 chars ) by writing them into the destination ( additional 1 or 2
;; bytes) and returning the adjusted offset for how much data we've
;; written.
(define (base64-decode-finish dst j b1 b2 b3)
  (cond
   ((eqv? b1 *outside-char*)
    j)
   ((eqv? b2 *outside-char*)
    (bytevector-u8-set! dst j (arithmetic-shift b1 2))
    (+ j 1))
   (else
    (bytevector-u8-set! dst
                        j
                        (bitwise-ior (arithmetic-shift b1 2)
                                     (bit-field b2 4 6)))
    (cond
     ((eqv? b3 *outside-char*)
      (+ j 1))
     (else
      (bytevector-u8-set! dst
                          (+ j 1)
                          (bitwise-ior
                           (arithmetic-shift (bit-field b2 0 4) 4)
                           (bit-field b3 2 6)))
      (+ j 2))))))

;;>  Variation of the above to read and write to ports.

(define (base64-decode . o)
  (let ((in (if (pair? o) (car o) (current-input-port)))
        (out (if (and (pair? o) (pair? (cdr o)))
                 (cadr o)
                 (current-output-port))))
    (cond
     ((not (binary-port? in))
      (let ((str (port->string in)))
        (write-string (base64-decode-string str) out)))
     (else
      (let ((src (make-bytevector decode-src-length))
            (dst (make-bytevector decode-dst-length)))
        (let lp ((offset 0))
          (let ((src-len
                 (+ offset
                    (read-bytevector! src in offset decode-src-length))))
            (cond
             ((= src-len decode-src-length)
              ;; read a full chunk: decode, write and loop
              (base64-decode-bytevector!
               src 0 decode-src-length dst
               (lambda (src-offset dst-len b1 b2 b3)
                 (cond
                  ((and (< src-offset src-len)
                        (eqv? #x3D (bytevector-u8-ref src src-offset)))
                   ;; done
                   (let ((dst-len (base64-decode-finish dst dst-len b1 b2 b3)))
                     (write-bytevector dst out 0 dst-len)))
                  ((eqv? b1 *outside-char*)
                   (write-bytevector dst out 0 dst-len)
                   (lp 0))
                  (else
                   (write-bytevector dst out 0 dst-len)
  one to three chars left in buffer
                   (bytevector-u8-set! src 0 (enc b1))
                   (cond
                    ((eqv? b2 *outside-char*)
                     (lp 1))
                    (else
                     (bytevector-u8-set! src 1 (enc b2))
                     (cond
                      ((eqv? b3 *outside-char*)
                       (lp 2))
                      (else
                       (bytevector-u8-set! src 2 (enc b3))
                       (lp 3))))))))))
             (else
              ;; end of source - just decode and write once
              (base64-decode-bytevector!
               src 0 src-len dst
               (lambda (src-offset dst-len b1 b2 b3)
                 (let ((dst-len (base64-decode-finish dst dst-len b1 b2 b3)))
                   (write-bytevector dst out 0 dst-len)))))))))))))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; encoding

;;> Return a base64 encoded representation of string according to the
> official base64 standard as described in RFC3548 .

(define (base64-encode-string str)
  (utf8->string (base64-encode-bytevector (string->utf8 str))))

(define (base64-encode-bytevector bv)
  (let* ((len (bytevector-length bv))
         (quot (quotient len 3))
         (rem (- len (* quot 3)))
         (res-len (arithmetic-shift (+ quot (if (zero? rem) 0 1)) 2))
         (res (make-bytevector res-len)))
    (base64-encode-bytevector! bv 0 len res)
    res))

(define (base64-encode-bytevector! bv start end res)
  (let ((limit (- end 2)))
    (let lp ((i start) (j 0))
      (if (>= i limit)
          (case (- end i)
            ((1)
             (let ((b1 (bytevector-u8-ref bv i)))
               (bytevector-u8-set! res j (enc (arithmetic-shift b1 -2)))
               (bytevector-u8-set!
                res
                (+ j 1)
                (enc (arithmetic-shift (bitwise-and #b11 b1) 4)))
               (bytevector-u8-set! res (+ j 2) (char->integer #\=))
               (bytevector-u8-set! res (+ j 3) (char->integer #\=))
               (+ j 4)))
            ((2)
             (let ((b1 (bytevector-u8-ref bv i))
                   (b2 (bytevector-u8-ref bv (+ i 1))))
               (bytevector-u8-set! res j (enc (arithmetic-shift b1 -2)))
               (bytevector-u8-set!
                res
                (+ j 1)
                (enc (bitwise-ior
                      (arithmetic-shift (bitwise-and #b11 b1) 4)
                      (bit-field b2 4 8))))
               (bytevector-u8-set!
                res
                (+ j 2)
                (enc (arithmetic-shift (bit-field b2 0 4) 2)))
               (bytevector-u8-set! res (+ j 3) (char->integer #\=))
               (+ j 4)))
            (else
             j))
          (let ((b1 (bytevector-u8-ref bv i))
                (b2 (bytevector-u8-ref bv (+ i 1)))
                (b3 (bytevector-u8-ref bv (+ i 2))))
            (bytevector-u8-set! res j (enc (arithmetic-shift b1 -2)))
            (bytevector-u8-set!
             res
             (+ j 1)
             (enc (bitwise-ior
                   (arithmetic-shift (bitwise-and #b11 b1) 4)
                   (bit-field b2 4 8))))
            (bytevector-u8-set!
             res
             (+ j 2)
             (enc (bitwise-ior
                   (arithmetic-shift (bit-field b2 0 4) 2)
                   (bit-field b3 6 8))))
            (bytevector-u8-set! res (+ j 3) (enc (bitwise-and #b111111 b3)))
            (lp (+ i 3) (+ j 4)))))))

;;>  Variation of the above to read and write to ports.

(define (base64-encode . o)
  (let ((in (if (pair? o) (car o) (current-input-port)))
        (out (if (and (pair? o) (pair? (cdr o)))
                 (cadr o)
                 (current-output-port))))
    (cond
     ((not (binary-port? in))
      (let ((str (port->string in)))
        (write-string (base64-encode-string str) out)))
     (else
      (let ((src (make-bytevector encode-src-length))
            (dst (make-bytevector
                  (arithmetic-shift (quotient encode-src-length 3) 2))))
        (let lp ()
          (let ((n (read-bytevector! src in 0 2048)))
            (base64-encode-bytevector! src 0 n dst)
            (write-bytevector dst out 0 (* 4 (quotient (+ n 2) 3)))
            (if (= n 2048)
                (lp)
                (flush-output-port out)))))))))

;;> Return a base64 encoded representation of the string \var{str} as
;;> above, wrapped in =?ENC?B?...?= as per RFC1522, split across
;;> multiple MIME-header lines as needed to keep each lines length
;;> less than \var{max-col}.  The string is encoded as is, and the
;;> encoding \var{enc} is just used for the prefix, i.e. you are
> responsible for ensuring \var{str } is already encoded according to
;;> \var{enc}.  The optional argument \var{nl} is the newline
;;> separator, defaulting to \var{crlf}.

(define (base64-encode-header encoding str . o)
  (define (round4 i) (arithmetic-shift (arithmetic-shift i -2) 2))
  (let ((start-col (if (pair? o) (car o) 0))
        (max-col (if (and (pair? o) (pair? (cdr o)))
                     (car (cdr o))
                     *default-max-col*))
        (nl (if (and (pair? o) (pair? (cdr o)) (pair? (cdr (cdr o))))
                (car (cdr (cdr o)))
                "\r\n")))
    (let* ((prefix (string-append "=?" encoding "?B?"))
           (prefix-length (+ 2 (string-length prefix)))
           (effective-max-col (round4 (- max-col prefix-length)))
           (first-max-col (round4 (- effective-max-col start-col)))
           (str (base64-encode-string str))
           (len (string-length str)))
      (if (<= len first-max-col)
          (string-append prefix str "?=")
          (string-append
           (if (positive? first-max-col)
               (string-append
                prefix (substring str 0 first-max-col) "?=" nl "\t" prefix)
               "")
           (string-join (string-chop (substring str first-max-col len)
                                     effective-max-col)
                        (string-append "?=" nl "\t" prefix))
           "?=")))))


 BSD-style license: 

 string utils

 constants and tables
 dalmations
 map letters
 map numbers
 extras (be liberal for different common base64 formats)
 map letters
 map numbers
 try to match common boundaries

 decoding
> Return a base64 decoded representation of string, also interpreting
> Other out-of-band characters are silently stripped, and = signals
> the end of the encoded string.  No errors will be raised.
 Create a result buffer with the maximum possible length for the
 input, and pass it to the internal base64-decode-string! utility.
 If the resulting length used is exact, we can return that buffer,
 otherwise we return the appropriate substring.
 This is a little funky.

   We want to skip over "outside" characters (e.g. newlines inside
   base64-encoded data, as would be passed in mail clients and most
   one for overall processing the input, and one for looping until
   we get to a valid character.  However, many Scheme compilers are
   really bad about optimizing nested loops of primitives, so we
   flatten this into a single loop, using conditionals to determine
   which character is currently being read.
 bytes) and returning the adjusted offset for how much data we've
 written.
>  Variation of the above to read and write to ports.
 read a full chunk: decode, write and loop
 done
 end of source - just decode and write once

 encoding
> Return a base64 encoded representation of string according to the
>  Variation of the above to read and write to ports.
> Return a base64 encoded representation of the string \var{str} as
> above, wrapped in =?ENC?B?...?= as per RFC1522, split across
> multiple MIME-header lines as needed to keep each lines length
> less than \var{max-col}.  The string is encoded as is, and the
> encoding \var{enc} is just used for the prefix, i.e. you are
> \var{enc}.  The optional argument \var{nl} is the newline
> separator, defaulting to \var{crlf}.
  Copyright ( c ) 2005 - 2014   .   All rights reserved .

> RFC 3548 base64 encoding and decoding utilities .
> This API is compatible with the Gauche library rfc.base64 .


(define (string-chop str n)
  (let ((len (string-length str)))
    (let lp ((i 0) (res '()))
      (let ((j (+ i n)))
        (if (>= j len)
            (reverse (cons (substring str i len) res))
            (lp j (cons (substring str i j) res)))))))


(define *default-max-col* 76)

  luft - balloons

(define *base64-decode-table*
  (let ((res (make-vector #x100 *outside-char*)))
      (cond
       ((<= i 25)
        (vector-set! res (+ i 65) i)
        (vector-set! res (+ i 97) (+ i 26))
        (lp (+ i 1)))))
      (cond
       ((<= i 9)
        (vector-set! res (+ i 48) (+ i 52))
        (lp (+ i 1)))))
    (vector-set! res (char->integer #\+) 62)
    (vector-set! res (char->integer #\-) 62)
    (vector-set! res (char->integer #\/) 63)
    (vector-set! res (char->integer #\_) 63)
    (vector-set! res (char->integer #\~) 63)
    (vector-set! res (char->integer #\=) *pad-char*)
    res))

(define (base64-decode-u8 u8)
  (vector-ref *base64-decode-table* u8))

(define *base64-encode-table*
  (let ((res (make-vector 64)))
      (cond
       ((<= i 25)
        (vector-set! res i (+ i 65))
        (vector-set! res (+ i 26) (+ i 97))
        (lp (+ i 1)))))
      (cond
       ((<= i 9)
        (vector-set! res (+ i 52) (+ i 48))
        (lp (+ i 1)))))
    (vector-set! res 62 (char->integer #\+))
    (vector-set! res 63 (char->integer #\/))
    res))

(define (enc i)
  (vector-ref *base64-encode-table* i))

(define decode-src-length
  (lcm 76 78))

(define decode-dst-length
  (* 3 (arithmetic-shift (+ 3 decode-src-length) -2)))

(define encode-src-length
  (* 3 1024))


> the alternate 62 & 63 valued characters as described in RFC3548 .


(define (base64-decode-string str)
  (utf8->string (base64-decode-bytevector (string->utf8 str))))

(define (base64-decode-bytevector src)
  (let* ((len (bytevector-length src))
         (dst-len (* 3 (arithmetic-shift (+ 3 len) -2)))
         (dst (make-bytevector dst-len)))
    (base64-decode-bytevector!
     src 0 len dst
     (lambda (src-offset res-len b1 b2 b3)
       (let ((res-len (base64-decode-finish dst res-len b1 b2 b3)))
         (if (= res-len dst-len)
             dst
             (bytevector-copy dst 0 res-len)))))))

    large base64 data ) .   This would normally mean two nested loops -
(define (base64-decode-bytevector! src start end dst kont)
  (let lp ((i start)
           (j 0)
           (b1 *outside-char*)
           (b2 *outside-char*)
           (b3 *outside-char*))
    (if (>= i end)
        (kont i j b1 b2 b3)
        (let ((c (base64-decode-u8 (bytevector-u8-ref src i))))
          (cond
           ((eqv? c *pad-char*)
            (kont i j b1 b2 b3))
           ((eqv? c *outside-char*)
            (lp (+ i 1) j b1 b2 b3))
           ((eqv? b1 *outside-char*)
            (lp (+ i 1) j c b2 b3))
           ((eqv? b2 *outside-char*)
            (lp (+ i 1) j b1 c b3))
           ((eqv? b3 *outside-char*)
            (lp (+ i 1) j b1 b2 c))
           (else
            (bytevector-u8-set!
             dst
             j
             (bitwise-ior (arithmetic-shift b1 2)
                          (bit-field b2 4 6)))
            (bytevector-u8-set!
             dst
             (+ j 1)
             (bitwise-ior
              (arithmetic-shift (bit-field b2 0 4) 4)
              (bit-field b3 2 6)))
            (bytevector-u8-set!
             dst
             (+ j 2)
             (bitwise-ior
              (arithmetic-shift (bit-field b3 0 2) 6)
              c))
            (lp (+ i 1) (+ j 3)
                *outside-char* *outside-char* *outside-char*)))))))

  If requested , account for any " partial " results ( i.e. trailing 2 or
  3 chars ) by writing them into the destination ( additional 1 or 2
(define (base64-decode-finish dst j b1 b2 b3)
  (cond
   ((eqv? b1 *outside-char*)
    j)
   ((eqv? b2 *outside-char*)
    (bytevector-u8-set! dst j (arithmetic-shift b1 2))
    (+ j 1))
   (else
    (bytevector-u8-set! dst
                        j
                        (bitwise-ior (arithmetic-shift b1 2)
                                     (bit-field b2 4 6)))
    (cond
     ((eqv? b3 *outside-char*)
      (+ j 1))
     (else
      (bytevector-u8-set! dst
                          (+ j 1)
                          (bitwise-ior
                           (arithmetic-shift (bit-field b2 0 4) 4)
                           (bit-field b3 2 6)))
      (+ j 2))))))


(define (base64-decode . o)
  (let ((in (if (pair? o) (car o) (current-input-port)))
        (out (if (and (pair? o) (pair? (cdr o)))
                 (cadr o)
                 (current-output-port))))
    (cond
     ((not (binary-port? in))
      (let ((str (port->string in)))
        (write-string (base64-decode-string str) out)))
     (else
      (let ((src (make-bytevector decode-src-length))
            (dst (make-bytevector decode-dst-length)))
        (let lp ((offset 0))
          (let ((src-len
                 (+ offset
                    (read-bytevector! src in offset decode-src-length))))
            (cond
             ((= src-len decode-src-length)
              (base64-decode-bytevector!
               src 0 decode-src-length dst
               (lambda (src-offset dst-len b1 b2 b3)
                 (cond
                  ((and (< src-offset src-len)
                        (eqv? #x3D (bytevector-u8-ref src src-offset)))
                   (let ((dst-len (base64-decode-finish dst dst-len b1 b2 b3)))
                     (write-bytevector dst out 0 dst-len)))
                  ((eqv? b1 *outside-char*)
                   (write-bytevector dst out 0 dst-len)
                   (lp 0))
                  (else
                   (write-bytevector dst out 0 dst-len)
  one to three chars left in buffer
                   (bytevector-u8-set! src 0 (enc b1))
                   (cond
                    ((eqv? b2 *outside-char*)
                     (lp 1))
                    (else
                     (bytevector-u8-set! src 1 (enc b2))
                     (cond
                      ((eqv? b3 *outside-char*)
                       (lp 2))
                      (else
                       (bytevector-u8-set! src 2 (enc b3))
                       (lp 3))))))))))
             (else
              (base64-decode-bytevector!
               src 0 src-len dst
               (lambda (src-offset dst-len b1 b2 b3)
                 (let ((dst-len (base64-decode-finish dst dst-len b1 b2 b3)))
                   (write-bytevector dst out 0 dst-len)))))))))))))


> official base64 standard as described in RFC3548 .

(define (base64-encode-string str)
  (utf8->string (base64-encode-bytevector (string->utf8 str))))

(define (base64-encode-bytevector bv)
  (let* ((len (bytevector-length bv))
         (quot (quotient len 3))
         (rem (- len (* quot 3)))
         (res-len (arithmetic-shift (+ quot (if (zero? rem) 0 1)) 2))
         (res (make-bytevector res-len)))
    (base64-encode-bytevector! bv 0 len res)
    res))

(define (base64-encode-bytevector! bv start end res)
  (let ((limit (- end 2)))
    (let lp ((i start) (j 0))
      (if (>= i limit)
          (case (- end i)
            ((1)
             (let ((b1 (bytevector-u8-ref bv i)))
               (bytevector-u8-set! res j (enc (arithmetic-shift b1 -2)))
               (bytevector-u8-set!
                res
                (+ j 1)
                (enc (arithmetic-shift (bitwise-and #b11 b1) 4)))
               (bytevector-u8-set! res (+ j 2) (char->integer #\=))
               (bytevector-u8-set! res (+ j 3) (char->integer #\=))
               (+ j 4)))
            ((2)
             (let ((b1 (bytevector-u8-ref bv i))
                   (b2 (bytevector-u8-ref bv (+ i 1))))
               (bytevector-u8-set! res j (enc (arithmetic-shift b1 -2)))
               (bytevector-u8-set!
                res
                (+ j 1)
                (enc (bitwise-ior
                      (arithmetic-shift (bitwise-and #b11 b1) 4)
                      (bit-field b2 4 8))))
               (bytevector-u8-set!
                res
                (+ j 2)
                (enc (arithmetic-shift (bit-field b2 0 4) 2)))
               (bytevector-u8-set! res (+ j 3) (char->integer #\=))
               (+ j 4)))
            (else
             j))
          (let ((b1 (bytevector-u8-ref bv i))
                (b2 (bytevector-u8-ref bv (+ i 1)))
                (b3 (bytevector-u8-ref bv (+ i 2))))
            (bytevector-u8-set! res j (enc (arithmetic-shift b1 -2)))
            (bytevector-u8-set!
             res
             (+ j 1)
             (enc (bitwise-ior
                   (arithmetic-shift (bitwise-and #b11 b1) 4)
                   (bit-field b2 4 8))))
            (bytevector-u8-set!
             res
             (+ j 2)
             (enc (bitwise-ior
                   (arithmetic-shift (bit-field b2 0 4) 2)
                   (bit-field b3 6 8))))
            (bytevector-u8-set! res (+ j 3) (enc (bitwise-and #b111111 b3)))
            (lp (+ i 3) (+ j 4)))))))


(define (base64-encode . o)
  (let ((in (if (pair? o) (car o) (current-input-port)))
        (out (if (and (pair? o) (pair? (cdr o)))
                 (cadr o)
                 (current-output-port))))
    (cond
     ((not (binary-port? in))
      (let ((str (port->string in)))
        (write-string (base64-encode-string str) out)))
     (else
      (let ((src (make-bytevector encode-src-length))
            (dst (make-bytevector
                  (arithmetic-shift (quotient encode-src-length 3) 2))))
        (let lp ()
          (let ((n (read-bytevector! src in 0 2048)))
            (base64-encode-bytevector! src 0 n dst)
            (write-bytevector dst out 0 (* 4 (quotient (+ n 2) 3)))
            (if (= n 2048)
                (lp)
                (flush-output-port out)))))))))

> responsible for ensuring \var{str } is already encoded according to

(define (base64-encode-header encoding str . o)
  (define (round4 i) (arithmetic-shift (arithmetic-shift i -2) 2))
  (let ((start-col (if (pair? o) (car o) 0))
        (max-col (if (and (pair? o) (pair? (cdr o)))
                     (car (cdr o))
                     *default-max-col*))
        (nl (if (and (pair? o) (pair? (cdr o)) (pair? (cdr (cdr o))))
                (car (cdr (cdr o)))
                "\r\n")))
    (let* ((prefix (string-append "=?" encoding "?B?"))
           (prefix-length (+ 2 (string-length prefix)))
           (effective-max-col (round4 (- max-col prefix-length)))
           (first-max-col (round4 (- effective-max-col start-col)))
           (str (base64-encode-string str))
           (len (string-length str)))
      (if (<= len first-max-col)
          (string-append prefix str "?=")
          (string-append
           (if (positive? first-max-col)
               (string-append
                prefix (substring str 0 first-max-col) "?=" nl "\t" prefix)
               "")
           (string-join (string-chop (substring str first-max-col len)
                                     effective-max-col)
                        (string-append "?=" nl "\t" prefix))
           "?=")))))

