;; read from mboxrd format.  Messages are divided by lines beginning with F R O M <space>.
;; one open question is the handling of newlines; rfc822 wants crlf, but as I understand it
;; the typical encoding for mboxrd uses simple newlines.  I will translate for the header,
;; and for the body.

;; I do not expect this code to work under Windows because of the CRLF issue.  However,
;; I also think that mboxrd format is rarely used under Windows, so this probably won't be
;; a problem.

;; in mboxrd format, lines beginning >+From_ must have one '>' stripped from the beginning
;; of the line.

(module mboxrd-read mzscheme
(require (lib "contract.ss"))

  (provide/contract [mboxrd-parse (path? . -> . promise?)]
                    [mboxrd-parse/port (port? . -> . promise?)])
  ;; mboxrd-parse : path -> (lazy-listof (list/c bytes? (promise/c bytes?)))
  ;; given a path to an mbox file, return a lazy list of the messages in the
  ;; file.  Each file is represented as a list containing a byte-string
  ;; representing the header and the promise of a byte-string representing
  ;; the body.  These byte-strings can be appended to obtain the original
  ;; message except that every \n in the original is replaced by \r\n to
  ;; match the RFC 2822 format.
  (define (mboxrd-parse path)
    (mboxrd-parse/port (open-input-file path)))
  ;; mboxrd-parse/port : port -> (lazy-listof (list/c bytes? (delay/c bytes?))
  ;; NB: this procedure assumes that it's the only one reading the port. Bad
  ;; stuff will happen if its not; it doesn't leave the "From " of the next
  ;; message on the stream.
  ;; EFFECT: reads from stream, closes it when peek-char returns #<eof>
  (define (mboxrd-parse/port ip)
    (if (eof-object? (peek-char ip))
        (begin (close-input-port ip)
               (delay null))
          (unless (regexp-match #px"From " ip)
            (error "nonempty mbox file did not begin with \"From \""))
          (let loop ()
              (let*-values ([(msg-in-pipe msg-out-pipe) (make-pipe)]
                            [(match-result) (regexp-match #px#"\nFrom " ip 0 #f msg-out-pipe)]
                            [(dc) (close-output-port msg-out-pipe)]
                            [(hdr-port) (open-output-bytes)]
                            [(match-result2) (regexp-match #px#"\n\n|\n$" msg-in-pipe 0 #f hdr-port)])
                (unless match-result2 
                  (error "couldn't find blank line separating header from body:\n ~a"
                         (get-output-bytes hdr-port)))
                (let* ([empty-body (equal? match-result2 `(#"\n"))]
                       [header (bytes-append #"From " (regexp-replace* #px#"\n>(>*From )"
                                                                       (regexp-replace* #px#"\n"
                                                                                        (get-output-bytes hdr-port)
                      [body (delay (let* ([body-port (open-output-bytes)]
                                         ;; a quick(?) way to drain the characters into the bytes
                                         [dc (regexp-match #px"a^" msg-in-pipe 0 #f body-port)])
                                     (bytes-append (regexp-replace* #px#"\n>(>*From )"
                                                                     (regexp-replace* #px#"\n"
                                                                                      (get-output-bytes body-port)
                                                    (if empty-body
                  (cons (list header body)
                        (if match-result
                            (delay null))))))))))


(define tstr "From oohc

From 13
>From obetor
>>From oherot

From 15

From 14



  (print-struct #t)

(define (force-whole-list ll)
  (let ([a (force ll)])
    (if (null? a)
        (cons (car a) (force-whole-list (cdr a))))))

  ;; this is what passes for a test case:
      (equal? (let ([ip (open-input-string tstr)])
                (map (lambda (a) (list (car a) (force (cadr a)))) (force-whole-list (mboxrd-parse/port ip))))
              (list (list #"From oohc\r\nlala\r\n\r\n"
                    (list #"From 13\r\nFrom obetor\r\n>From oherot\r\n\r\n" 
                    (list #"From 15\r\n\r\n"
                    (list #"From 14\r\n\r\n"
    (error 'mboxrd-test "mboxrd-parse internal test case failed."))