;;; guile-email --- Guile email parser
;;; Copyright © 2018, 2019 Arun Isaac <arunisaac@systemreboot.net>
;;;
;;; This file is part of guile-email.
;;;
;;; guile-email is free software; you can redistribute it and/or modify
;;; it under the terms of the GNU Affero General Public License as
;;; published by the Free Software Foundation; either version 3 of the
;;; License, or (at your option) any later version.
;;;
;;; guile-email is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;;; Affero General Public License for more details.
;;;
;;; You should have received a copy of the GNU Affero General Public
;;; License along with guile-email.  If not, see
;;; <http://www.gnu.org/licenses/>.

(define-module (email email)
  #:use-module (ice-9 binary-ports)
  #:use-module (ice-9 iconv)
  #:use-module (ice-9 match)
  #:use-module (ice-9 peg)
  #:use-module (ice-9 regex)
  #:use-module (rnrs bytevectors)
  #:use-module ((rnrs io ports) #:select (call-with-port))
  #:use-module (rnrs io simple)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-2)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-11)
  #:use-module (srfi srfi-19)
  #:use-module (srfi srfi-26)
  #:use-module (sxml transform)
  #:use-module (email base64)
  #:use-module (email quoted-printable)
  #:use-module (email utils)
  #:export (<email>
            make-email
            email?
            email-headers
            email-body
            <mime-entity>
            make-mime-entity
            mime-entity?
            mime-entity-headers
            mime-entity-body
            email->headers+body
            parse-email
            parse-email-headers
            parse-email-body
            parse-email-address
            interpret-address
            mbox->emails))

(define (flatten-and-filter terms tree)
  (filter list? (keyword-flatten terms tree)))

(define-record-type <email>
  (make-email headers body)
  email?
  (headers email-headers)
  (body email-body))

(define-record-type <mime-entity>
  (make-mime-entity headers body)
  mime-entity?
  (headers mime-entity-headers)
  (body mime-entity-body))

(define string->lcase-symbol
  (compose string->symbol string-downcase))

;;; PEG parser implementing the ABNF grammar specified in RFC5322
;;; (Internet Message Format), RFC6854 (Update to Internet Message
;;; Format to Allow Group Syntax in the "From:" and "Sender:" Header
;;; Fields)

;;; Obsolete syntax has not been implemented.

;;; Core ABNF rules from RFC5234

(define-peg-pattern alpha body
  (or (range #\A #\Z) (range #\a #\z)))

;; Though line endings should be crlf (\r\n), we also tolerate bare
;; line feeds (\n)
(define-peg-pattern crlf none
  (or "\r\n" "\n"))

(define-peg-pattern digit body
  (range #\0 #\9))

(define-peg-pattern dquote body
  "\"")

;; Printable ASCII characters and UTF-8 characters > \x7f (RFC6532)
(define-peg-pattern vchar body
  (and (not-followed-by (or (range #\Nul #\Space)
                            "\x7f"))
       peg-any))

(define-peg-pattern wsp body
  (or " " "\t"))

(define-peg-pattern lwsp body
  (* (or wsp (and crlf wsp))))

(define-syntax-rule (define-printable-ascii-character-pattern name . exceptions)
  (define-peg-pattern name body
    (and (not-followed-by (or . exceptions))
         vchar)))

;;; Quoted characters

(define-peg-pattern quoted-pair body
  (and (ignore "\\") (or vchar wsp)))

;;; Folding white space and comments

(define-peg-pattern fws body
  (and (? (and (* wsp) crlf)) (+ wsp)))

(define-printable-ascii-character-pattern ctext "(" ")" "\\")

(define-syntax-rule (define-comment-pattern name capture-type)
  (define-peg-pattern name capture-type
    (and (ignore "(") (* (and (? fws) ccontent)) (ignore ")"))))

(define-comment-pattern comment none)

(define-peg-pattern ccontent body
  (or ctext quoted-pair comment))

(define-syntax-rule (define-cfws-pattern name comment)
  (define-peg-pattern name body
    (or (and (+ (and (? fws) comment)) (? fws)) fws)))

(define-cfws-pattern cfws comment)

;;; Atom

(define-printable-ascii-character-pattern atext
  "\"" "(" ")" "," "." ":" ";" "<" ">" "@" "[" "\\" "]")

(define-peg-pattern atom body
  (and (? cfws) (+ atext) (? cfws)))

(define-peg-pattern dot-atom-text body
  (and (+ atext) (* (and "." (+ atext)))))

(define-syntax-rule (define-dot-atom-pattern name cfws)
  (define-peg-pattern name body
    (and (? cfws) dot-atom-text (? cfws))))

(define-dot-atom-pattern dot-atom cfws)

;;; Quoted strings

(define-printable-ascii-character-pattern qtext "\\" "\"")

(define-peg-pattern qcontent body
  (or qtext quoted-pair))

;; TODO: Remove workaround guile peg bug for ignore
(define-peg-pattern quoted-string body
  (and (? cfws) (ignore (and dquote))
       (* (and (? fws) qcontent))
       (? fws)
       (ignore (and dquote)) (? cfws)))

;;; Miscellaneous tokens

(define-peg-pattern word body
  (or atom quoted-string))

(define-peg-pattern phrase body
  (+ word))

;; ABNF modified to ignore leading whitespace
;; ABNF modified to allow for blank lines in folded field
(define-peg-pattern unstructured body
  (and (ignore (? fws))
       (* (and (? fws) (? vchar)))
       (ignore (* wsp))))

;;; Date and time specification

(define-peg-pattern day-name body
  (or "Mon" "Tue" "Wed" "Thu" "Fri" "Sat" "Sun"))

(define-peg-pattern day-of-week all
  (and (ignore (? fws)) day-name))

;; TODO: Remove workaround guile peg bug for ignore
(define-peg-pattern day all
  (and (ignore (? fws)) digit (? digit) (ignore (and fws))))

(define-peg-pattern month all
  (or "Jan" "Feb" "Mar" "Apr" "May" "Jun"
      "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"))

;; TODO: Remove workaround guile peg bug for ignore
(define-peg-pattern year all
  (and (ignore (and fws)) digit digit digit digit (ignore (and fws))))

(define-peg-pattern date all
  (and day month year))

(define-peg-pattern hours all
  (and digit digit))

(define-peg-pattern minutes all
  (and digit digit))

(define-peg-pattern seconds all
  (and digit digit))

(define-peg-pattern time-of-day all
  (and hours (ignore ":") minutes (? (and (ignore ":") seconds))))

;; zone-sign, zone-hours and zone-minutes do not exist in the ABNF
;; specified in RFC5322. But, we have it here since it eases parsing
;; the zone and converting it to a number of seconds as required by
;; the make-date procedure of SRFI-19.
(define-peg-pattern zone-sign all
  (or "+" "-"))

(define-peg-pattern zone-hours all
  (and digit digit))

(define-peg-pattern zone-minutes all
  (and digit digit))

;; TODO: Remove workaround guile peg bug for ignore
(define-peg-pattern zone all
  (and (ignore (and fws)) zone-sign zone-hours zone-minutes))

(define-peg-pattern time all
  (and time-of-day zone))

(define-peg-pattern date-time all
  (and (? (and day-of-week (ignore ","))) date time (? cfws)))

;;; Address specification

(define-peg-pattern display-name all
  phrase)

(define-peg-pattern local-part body
  (or dot-atom quoted-string))

(define-printable-ascii-character-pattern dtext "[" "]" "\\")

(define-peg-pattern domain-literal body
  (and (? cfws) "[" (* (and (? fws) dtext)) (? fws) "]" (? cfws)))

(define-syntax-rule (define-domain-pattern name dot-atom)
  (define-peg-pattern name body
    (or dot-atom domain-literal)))

(define-domain-pattern domain dot-atom)

(define-syntax-rule (define-addr-spec-pattern name domain)
  (define-peg-pattern name body
    (and local-part "@" domain)))

(define-addr-spec-pattern addr-spec domain)

(define-syntax-rule (define-angle-addr-pattern
                      name opening-bracket closing-bracket)
  (define-peg-pattern name body
    (and (ignore (? cfws))
         opening-bracket addr-spec closing-bracket
         (ignore (? cfws)))))

(define-angle-addr-pattern angle-addr "<" ">")

;; When handling email addresses, we need to drop the angle
;; brackets. But in the Received field we need to include the angle
;; brackes. So, we define unbracketed-angle-addr, a variant of
;; angle-addr in which the angle brackets are ignored.

(define-angle-addr-pattern
  unbracketed-angle-addr (ignore "<") (ignore ">"))

(define-peg-pattern name-addr body
  (and (? display-name) unbracketed-angle-addr))

;; In order to support emacs message mode parens style email
;; addresses, we need to capture comments in address fields and
;; process them as names. But, we need to ignore comments in other
;; parts of the email. In order to facilitate this, we define a set of
;; captured-* patterns which are used in address fields.

(define-comment-pattern captured-comment all)
(define-cfws-pattern captured-cfws captured-comment)
(define-dot-atom-pattern captured-dot-atom captured-cfws)
(define-domain-pattern captured-domain captured-dot-atom)
(define-addr-spec-pattern captured-addr-spec captured-domain)

(define-peg-pattern mailbox all
  (or name-addr captured-addr-spec))

(define-peg-pattern mailbox-list all
  (and mailbox (* (and (ignore ",") mailbox))))

(define-peg-pattern group all
  (and display-name (ignore ":")
       (? group-list) (ignore ";") (? cfws)))

(define-peg-pattern group-list all
  (or mailbox-list cfws))

(define-peg-pattern address body
  (or mailbox group))

(define-peg-pattern address-list all
  (and address (* (and (ignore ",") address))))

;;; Fields

(define-syntax-rule (define-field-pattern name header pattern)
  (define-peg-pattern name all
    (and (ignore (string-ci header)) (ignore ":") pattern crlf)))

;;; Origination date field

(define-field-pattern orig-date "Date" date-time)

;;; Originator fields

(define-field-pattern from "From" (or mailbox-list address-list))
(define-field-pattern sender "Sender" (or mailbox address))
(define-field-pattern reply-to "Reply-To" address-list)

;; Destination address fields

(define-field-pattern to "To" address-list)
(define-field-pattern cc "Cc" address-list)
(define-field-pattern bcc "Bcc" (? (or address-list cfws)))

;;; Identification fields

(define-peg-pattern no-fold-literal body
  (and "[" (* dtext) "]"))

(define-peg-pattern id-left body
  dot-atom-text)

(define-peg-pattern id-right body
  (or dot-atom-text no-fold-literal))

(define-peg-pattern msg-id all
  (and (ignore (? cfws)) (ignore "<")
       id-left "@" id-right
       (ignore ">") (ignore (? cfws))))

(define-field-pattern message-id "Message-ID" msg-id)
(define-field-pattern in-reply-to "In-Reply-To" (+ msg-id))
(define-field-pattern references "References" (+ msg-id))

;;; Informational fields

(define-field-pattern subject "Subject" unstructured)
(define-field-pattern comments "Comments" unstructured)
(define-field-pattern keywords "Keywords" (and phrase (* (and "," phrase))))

;;; Resent fields

(define-field-pattern resent-date "Resent-Date" date-time)
(define-field-pattern resent-from "Resent-From" (or mailbox-list address-list))
(define-field-pattern resent-sender "Resent-Sender" (or mailbox address))
(define-field-pattern resent-to "Resent-To" address-list)
(define-field-pattern resent-cc "Resent-Cc" address-list)
(define-field-pattern resent-bcc "Resent-Bcc" (? (or address-list cfws)))
(define-field-pattern resent-msg-id "Resent-Message-ID" msg-id)

;;; Trace fields

;; word is given last priority in the ordered choice
(define-peg-pattern received-token body
  (or angle-addr addr-spec domain word))

(define-field-pattern received "Received"
  (and (* received-token) (ignore ";") date-time))

(define-peg-pattern path body
  (or angle-addr (and (? cfws) (ignore "<") (? cfws) (ignore ">") (? cfws))))

(define-field-pattern return "Return-Path" path)

(define-peg-pattern trace all
  (and (? return) (+ received)))

;;; Optional fields

(define-printable-ascii-character-pattern ftext ":")

(define-peg-pattern field-name all
  (+ ftext))

(define-peg-pattern optional-field all
  (and field-name (ignore ":") unstructured crlf))

;;; MIME version

;; I have prepended optional cfws to account for leading whitespace.
(define-field-pattern mime-version "MIME-Version"
  (and (ignore (? cfws)) (+ digit) "." (+ digit)))

;;; Content type

;; TODO: What is <"> in RFC2045?
(define-peg-pattern tspecials body
  (or "(" ")" "<" ">" "@"
      "," ";" ":" "\\"
      "/" "[" "]" "?" "="))

;; TODO: What is a CTL in RFC2045?
(define-peg-pattern token body
  (+ (and (not-followed-by (or " " crlf tspecials))
          peg-any)))

;; TODO: Implement iana-token, ietf-token and x-token
(define-peg-pattern iana-token body
  token)

(define-peg-pattern ietf-token body
  token)

(define-peg-pattern x-token body
  token)

(define-peg-pattern extension-token body
  (or ietf-token x-token))

(define-peg-pattern discrete-type body
  (or "text" "image" "audio" "video"
      "application" extension-token))

(define-peg-pattern composite-type body
  (or "message" "multipart" extension-token))

(define-peg-pattern type all
  (or discrete-type composite-type))

(define-peg-pattern subtype all
  (or extension-token iana-token))

(define-peg-pattern attribute all
  token)

;; quoted-string is given higher precedence
(define-peg-pattern value all
  (or quoted-string token))

(define-peg-pattern parameter all
  (and attribute (ignore "=") value))

;; I have prepended optional cfws to account for leading whitespace.
(define-field-pattern content "Content-Type"
  (and (ignore (? cfws)) type (ignore "/") subtype
       (* (and (ignore ";") (ignore (? cfws)) parameter))))

;;; Content Disposition as defined in RFC2183

(define-peg-pattern disposition-type body
  (or "inline" "attachment" extension-token))

(define-peg-pattern quoted-date-time all
  quoted-string)

(define-syntax-rule (define-parameter-pattern name header pattern)
  (define-peg-pattern name all
    (and (ignore header) (ignore "=") pattern)))

(define-parameter-pattern filename-parm "filename" value)
(define-parameter-pattern creation-date-parm "creation-date" quoted-date-time)
(define-parameter-pattern modification-date-parm "modification-date" quoted-date-time)
(define-parameter-pattern read-date-parm "read-date" quoted-date-time)
(define-parameter-pattern size-parm "size" (+ digit))

(define-peg-pattern disposition-parm body
  (or filename-parm creation-date-parm modification-date-parm
      read-date-parm size-parm parameter))

;; I have prepended optional cfws to account for leading whitespace.
(define-field-pattern disposition "Content-Disposition"
  (and (ignore (? cfws)) disposition-type
       (* (and (ignore ";") (ignore (? cfws)) disposition-parm))))

;;; Content transfer encoding

(define-peg-pattern mechanism body
  (or "7bit" "8bit" "binary"
      "quoted-printable" "base64"
      ietf-token x-token))

(define-field-pattern encoding "Content-Transfer-Encoding"
  (and (ignore (? cfws)) mechanism))

;;; Fields

;; The ABNF specified for fields in RFC5322 does not make sense. With
;; it, all headers are eaten up by optional-field. So, as a temporary
;; workaround, I am going with the following much simpler (but
;; possibly incorrect) ABNF.

;; TODO: Try to understand and implement the actual ABNF specified by
;; RFC5322.
(define-peg-pattern fields all
  (* (or trace
         resent-date
         resent-from
         resent-sender
         resent-to
         resent-cc
         resent-bcc
         resent-msg-id
         orig-date
         from
         sender
         reply-to
         to
         cc
         bcc
         message-id
         in-reply-to
         references
         subject
         comments
         keywords
         mime-version
         content
         disposition
         encoding
         optional-field)))

(define-peg-pattern mime-extension-field-name all
  (and "Content-" (+ ftext)))

(define-peg-pattern mime-extension-field all
  (and mime-extension-field-name (ignore ":") unstructured crlf))

(define-peg-pattern mime-entity-fields all
  (* (or content
         disposition
         encoding
         mime-extension-field
         optional-field)))

(define (decode-mime-encoded-word word)
  (regexp-substitute/global
   #f "=\\?([^?]*)\\?([^?]*)\\?([^?]*)\\?=" word
   'pre (lambda (match-record)
          (let ((charset (match:substring match-record 1))
                (encoding (string->lcase-symbol (match:substring match-record 2)))
                (encoded-text (match:substring match-record 3)))
            (bytevector->string
             ((case encoding
                ((b) base64-decode)
                ((q) q-encoding-decode)
                (else (error "Encoding of MIME word unknown" word)))
              encoded-text)
             charset 'substitute)))
   'post))

(define (body->mime-entities body boundary)
  "Split bytevector BODY into a list of mime entities separated by
BOUNDARY (as explained in RFC2045), and return that list."
  (define (read-till-boundary port)
    (read-bytes-till
     port
     ;; TODO: Look for the boundary sequence only at the beginning of
     ;; a line.
     ;; Boundary is always an ASCII string.
     (string->bytevector (string-append "--" boundary) "us-ascii")))

  (define (read-mime-entity port)
    (let ((line (get-line-with-delimiter port)))
      ;; The eof-object? check returns #t only when the message is
      ;; prematurely truncated. It is invoked only to handle truncated
      ;; messages gracefully without raising an error.
      (if (or (eof-object? line)
              (string-prefix? (string-append "--" boundary "--") line))
          (eof-object)
          (read-till-boundary port))))

  (call-with-port
   (open-bytevector-input-port body)
   (lambda (port)
     (read-till-boundary port)
     (read-objects read-mime-entity port))))

(define (email->headers+body email)
  "Split EMAIL bytevector into headers and body. Return as multiple
values. The returned headers is a string and body is a bytevector."
  (call-with-port
   (open-bytevector-input-port email)
    (lambda (port)
      ;; Email headers must strictly be ASCII characters. But for the
      ;; sake of supporting Emacs message mode parens style addresses
      ;; that may use non-ASCII characters, typically for the full
      ;; name, we relax this requirement. We assume an encoding of
      ;; UTF-8, and hope that everything turns out fine. Since UTF-8
      ;; is a superset of ASCII, this should not affect standards
      ;; conforming headers. If encoding is neither UTF-8 nor ASCII,
      ;; we use the substitute conversion strategy and proceed without
      ;; raising an error.
      (set-port-encoding! port "utf-8")
      (set-port-conversion-strategy! port 'substitute)
      (let ((headers (read-while port get-line-with-delimiter
                                 (lambda (line)
                                   (not (or (string= line "\n")
                                            (string= line "\r\n")))))))
        (get-line-with-delimiter port)
        (values (if (eof-object? headers) "" headers)
                (get-bytevector-all port))))))

(define (post-process-mailbox . args)
  (define process-name
    (compose decode-mime-encoded-word string-trim-both))

  (match args
    ;; name-addr email address
    (`(mailbox (display-name ,name) ,address)
     `((name . ,(process-name name))
       (address . ,(string-trim-both address))))
    ;; addr-spec email address
    (`(mailbox ,(? string? address))
     `((address . ,(string-trim-both address))))
    ;; emacs message mode parens style email address
    (`(mailbox ,(? list? address-parts))
     `((name . ,(match-let ((`((captured-comment ,name))
                             (flatten-and-filter '(captured-comment) address-parts)))
                  (process-name name)))
       (address . ,(string-trim-both
                    (string-join (drop-right address-parts 1) "")))))
    (_ (error "Failed to parse mailbox"))))

(define (post-process-content-transfer-encoding _ value)
  (list 'content-transfer-encoding (string->lcase-symbol value)))

(define (post-process-content-type . args)
  (define (valid-charset? charset)
    (catch #t
      (lambda ()
        (bytevector->string (make-bytevector 0 0) charset)
        #t)
      (const #f)))

  (match args
    (`(content (type ,type)
               (subtype ,subtype)
               . ,parameters)
     (let ((type (string->lcase-symbol type))
           (subtype (string->lcase-symbol subtype))
           (parameters
            (map (match-lambda
                   (`(parameter (attribute ,attribute)
                                (value ,value))
                    (cons (string->lcase-symbol attribute) value)))
                 (flatten-and-filter '(parameter) parameters))))
       `(content-type
         ,(acons*
           'type type
           'subtype subtype
           (cond
            ((and (eq? type 'text)
                  (or
                   ;; RFC6657 specifies UTF-8 as the default charset for
                   ;; text/* media types.
                   (not (assoc-ref parameters 'charset))
                   ;; RFC2045 recommends that the default be assumed
                   ;; when a syntactically invalid Content-Type header
                   ;; is encountered. In this implementation, we are
                   ;; only checking for the validity of the
                   ;; charset. Should we check for the validity of the
                   ;; entire Content-Type header? If so, how?
                   (not (valid-charset? (assoc-ref parameters 'charset)))))
             (acons 'charset "utf-8" parameters))
            (else parameters))))))))

(define post-process-content-disposition
  (match-lambda*
    (`(disposition ,type . ,parameters)
     `(content-disposition
       ,(acons 'type (string->lcase-symbol type)
               (map (match-lambda
                      (('filename-parm ('value filename))
                       (cons 'filename (basename filename)))
                      (((? (lambda (date-parm)
                             (member date-parm '(creation-date-parm modification-date-parm read-date-parm)))
                           date-parm) value)
                       ;; TODO: Convert to SRFI-19 datetime
                       (cons date-parm value))
                      (('size-parm value)
                       (cons 'size (string->number value)))
                      (`(parameter (attribute ,attribute)
                                   (value ,value))
                       (cons (string->lcase-symbol attribute) value)))
                    (flatten-and-filter
                     '(filename-parm creation-date-parm modification-date-parm
                                     read-date-parm size-parm parameter)
                     parameters)))))))

(define post-process-optional-field
  (match-lambda*
    (`(optional-field
       (field-name ,field-name)
       ,field-value)
     (list (string->lcase-symbol field-name)
           field-value))
    (`(optional-field (field-name ,field-name))
     (list field-name ""))))

(define* (macro-process-address-list _ . addresses)
  (flatten-and-filter '(address mailbox) addresses))

(define (parse-email-address address)
  "Parse ADDRESS as an email address and return an association list
with keys being the symbols name and address, and values being the
display-name and addr-spec respectively. display-name and addr-spec
are as defined in RFC5322.

For example,
(parse-email-address \"Foo <foo@example.org>\")
=> ((name . \"Foo\") (address . \"foo@example.org\"))
(parse-email-address \"foo@example.org\")
=> ((address . \"foo@example.org\"))"
  (pre-post-order
   (peg:tree (match-pattern mailbox address))
   `((mailbox . ,post-process-mailbox)
     (*text* . ,(lambda (_ text) text))
     (*default* . ,(lambda tree tree)))))

(define interpret-address
  (match-lambda
    ((('name . name)
      ('address . address))
     (format #f "~a <~a>" name address))
    ((('address . address)) address)))

(define (parse-email-body headers body)
  "Parse BODY as email body where HEADERS is an association list of
header keys and values as returned by parse-email-headers. Return a
list of <mime-entity> records if the body is a multipart
message. Else, return a single <mime-entity> record."
  (match body
    ((? string? body)
     (parse-email-body headers (string->bytevector body "utf-8")))
    ((? bytevector? body)
     (let ((content-type (assoc-ref headers 'content-type)))
       (case (assoc-ref content-type 'type)
         ((multipart)
          (map (cut parse-mime-entity headers <>)
               (body->mime-entities body (assoc-ref content-type 'boundary))))
         ((text)
          (string-trim-both
           (decode-body body (assoc-ref headers 'content-transfer-encoding)
                        (assoc-ref content-type 'charset))))
         (else (decode-body body (assoc-ref headers 'content-transfer-encoding))))))))

(define (add-default-headers headers)
  ;; Default Content-Type and Content-Transfer-Encoding headers as
  ;; specified in RFC2045
  (let ((default-headers
          (acons* 'content-type '((type . text)
                                  (subtype . plain)
                                  ;; UTF-8 is specified as the default
                                  ;; charset in RFC6657
                                  (charset . "utf-8"))
                  'content-transfer-encoding '#{7bit}#)))
    (append (alist-delete* (append (if (assoc-ref headers 'content-type)
                                       (list 'content-type) (list))
                                   (if (assoc-ref headers 'content-transfer-encoding)
                                       (list 'content-transfer-encoding) (list)))
                           default-headers)
            headers)))

(define (add-default-mime-entity-headers parent-headers headers)
  ;; Default Content-Type and Content-Transfer-Encoding headers as
  ;; specified in RFC2045 and RFC2046
  (let* ((parent-content-type (assoc-ref parent-headers 'content-type))
         (default-headers
           (acons* 'content-type `(,@(if (and (eq? (assoc-ref parent-content-type 'type) 'multipart)
                                              (eq? (assoc-ref parent-content-type 'subtype) 'digest))
                                         '((type . message)
                                           (subtype . rfc822))
                                         '((type . text)
                                           (subtype . plain)))
                                  ;; UTF-8 is specified as the default
                                  ;; charset in RFC6657
                                  (charset . "utf-8"))
                  'content-transfer-encoding '#{7bit}#)))
    (append (alist-delete* (append (if (assoc-ref headers 'content-type)
                                       (list 'content-type) (list))
                                   (if (assoc-ref headers 'content-transfer-encoding)
                                       (list 'content-transfer-encoding) (list)))
                           default-headers)
            headers)))

(define (parse-mime-entity parent-headers bv)
  (let-values (((headers body) (email->headers+body bv)))
    (let ((headers
           (pre-post-order
            (peg:tree
             (match-pattern mime-entity-fields headers))
            `((content . ,post-process-content-type)
              (encoding . ,post-process-content-transfer-encoding)
              (disposition . ,post-process-content-disposition)
              (optional-field . ,post-process-optional-field)
              (mime-entity-fields . ,(lambda (_ . mime-entity-fields)
                                       (add-default-mime-entity-headers
                                        parent-headers
                                        (map (match-lambda
                                               ((mime-entity-field value)
                                                (cons mime-entity-field value))
                                               ((mime-entity-field . values)
                                                (cons mime-entity-field values)))
                                             mime-entity-fields))))
              (*text* . ,(match-lambda*
                           ;; Handle MIME entities that have no
                           ;; headers.
                           ((_ 'mime-entity-fields)
                            (add-default-mime-entity-headers parent-headers '()))
                           ((_ text) text)))
              (*default* . ,(lambda tree tree))))))
      (make-mime-entity headers (parse-email-body headers body)))))

(define (parse-email email)
  "Parse string EMAIL and return result as an <email> record."
  (match email
    ((? string? email)
     (parse-email (string->bytevector email "utf-8")))
    ((? bytevector? email)
     (let-values (((headers body) (email->headers+body email)))
       (let ((headers-alist (parse-email-headers headers)))
         (make-email headers-alist (parse-email-body headers-alist body)))))))

(define (parse-email-headers headers)
  "Parse string HEADERS as email headers and return an association
list of header keys and values."
  (pre-post-order
   (peg:tree
    (match-pattern fields headers))
   `((date-time . ,(lambda node
                     (match-let
                         ((`((day ,day) (month ,month) (year ,year)
                             (hours ,hours) (minutes ,minutes) (seconds ,seconds)
                             (zone-sign ,zone-sign) (zone-hours ,zone-hours) (zone-minutes ,zone-minutes))
                           (flatten-and-filter
                            '(day month year hours minutes seconds
                                  zone-sign zone-hours zone-minutes)
                            node)))
                       (make-date 0
                                  (string->number seconds)
                                  (string->number minutes)
                                  (string->number hours)
                                  (string->number day)
                                  (1+ (list-index
                                       (cut equal? <> month)
                                       (list "Jan" "Feb" "Mar" "Apr" "May" "Jun"
                                             "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")))
                                  (string->number year)
                                  (* (case (string->symbol zone-sign)
                                       ((+) 1)
                                       ((-) -1))
                                     (+ (* 60 60 (string->number zone-hours))
                                        (* 60 (string->number zone-minutes))))))))
     (orig-date . ,(lambda (_ date) (list 'date date)))
     (mailbox . ,post-process-mailbox)
     (address-list *macro* . ,macro-process-address-list)
     (mailbox-list *macro* . ,macro-process-address-list)
     (optional-field . ,post-process-optional-field)
     (msg-id . ,(match-lambda* (`(msg-id ,msg-id) msg-id)))
     (content . ,post-process-content-type)
     (encoding . ,post-process-content-transfer-encoding)
     (disposition . ,post-process-content-disposition)
     (subject . ,(match-lambda* (`(subject ,subject)
                                 `(subject ,(decode-mime-encoded-word subject)))))
     (fields . ,(lambda (_ . fields)
                  (add-default-headers
                   (filter-map (match-lambda
                                 ((field value)
                                  (cons field value))
                                 ((field . values)
                                  (cons field values))
                                 (_ #f))
                               fields))))
     (*text* . ,(lambda (_ value) value))
     (*default* . ,(lambda tree tree)))))

(define* (decode-body body encoding #:optional charset)
  (let ((decoded-octets
         (case encoding
           ((base64)
            (base64-decode 
             (string-filter
              (char-set-union
               (ucs-range->char-set (char->integer #\a) (1+ (char->integer #\z)))
               (ucs-range->char-set (char->integer #\A) (1+ (char->integer #\Z)))
               (ucs-range->char-set (char->integer #\0) (1+ (char->integer #\9)))
               (char-set #\+ #\/ #\=))
              (bytevector->string body "us-ascii"))))
           ((quoted-printable) (quoted-printable-decode
                                (bytevector->string body "us-ascii")))
           ((#{7bit}# #{8bit}# binary) body)
           (else (error "Body decoding failed. Unknown encoding" encoding)))))
    (if charset
        (bytevector->string decoded-octets charset)
        decoded-octets)))

(define (read-next-email-in-mbox port)
  "Read next email from PORT and return as a bytevector.  PORT is an
input port reading an mbox file."
  ;; Read and discard From_ line
  (get-line-with-delimiter port)
  ;; Read the actual email
  (let ((email (read-bytes-till
                port (string->bytevector "\nFrom " "us-ascii"))))
    (read-char port)
    email))

(define (mbox->emails port)
  "Read all emails from PORT and return as a list of bytevectors.
PORT is an input port reading an mbox file."
  (read-objects read-next-email-in-mbox port))