;;; ox-koma-letter.el --- KOMA Scrlttr2 Back-End for Org Export Engine  -*- lexical-binding: t; -*-

;; Copyright (C) 2007-2022 Free Software Foundation, Inc.

;; Author: Nicolas Goaziou <n.goaziou AT gmail DOT com>
;;         Alan Schmitt <alan.schmitt AT polytechnique DOT org>
;;         Viktor Rosenfeld <listuser36 AT gmail DOT com>
;;         Rasmus Pank Roulund <emacs AT pank DOT eu>
;; Maintainer: Marco Wahl <marcowahlsoft@gmail.com>
;; Keywords: org, wp, tex

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs 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 General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:
;;
;; This library implements a KOMA Scrlttr2 back-end, derived from the
;; LaTeX one.
;;
;; Depending on the desired output format, three commands are provided
;; for export: `org-koma-letter-export-as-latex' (temporary buffer),
;; `org-koma-letter-export-to-latex' ("tex" file) and
;; `org-koma-letter-export-to-pdf' ("pdf" file).
;;
;; On top of buffer keywords supported by `latex' back-end (see
;; `org-latex-options-alist'), this back-end introduces the following
;; keywords:
;;   - CLOSING: see `org-koma-letter-closing',
;;   - FROM_ADDRESS: see `org-koma-letter-from-address',
;;   - LCO: see `org-koma-letter-class-option-file',
;;   - OPENING: see `org-koma-letter-opening',
;;   - PHONE_NUMBER: see `org-koma-letter-phone-number',
;;   - URL: see `org-koma-letter-url',
;;   - FROM_LOGO: see `org-koma-letter-from-logo',
;;   - SIGNATURE: see `org-koma-letter-signature',
;;   - PLACE: see `org-koma-letter-place',
;;   - LOCATION: see `org-koma-letter-location',
;;   - TO_ADDRESS:  If unspecified this is set to "\mbox{}".
;;
;; TO_ADDRESS, FROM_ADDRESS, LOCATION, CLOSING, and SIGNATURE can also
;; be specified using "special headings" with the special tags
;; specified in `org-koma-letter-special-tags-in-letter'.  LaTeX line
;; breaks are not necessary for TO_ADDRESS, FROM_ADDRESS and LOCATION.
;; If both a headline and a keyword specify a to or from address the
;; value is determined in accordance with
;; `org-koma-letter-prefer-special-headings'.
;;
;; A number of OPTIONS settings can be set to change which contents is
;; exported.
;;   - backaddress (see `org-koma-letter-use-backaddress')
;;   - foldmarks (see `org-koma-letter-use-foldmarks')
;;   - phone (see `org-koma-letter-use-phone')
;;   - url (see `org-koma-letter-use-url')
;;   - from-logo (see `org-koma-letter-use-from-logo')
;;   - email (see `org-koma-letter-use-email')
;;   - place (see `org-koma-letter-use-place')
;;   - location (see `org-koma-letter-use-location')
;;   - subject, a list of format options
;;     (see `org-koma-letter-subject-format')
;;   - after-closing-order, a list of the ordering of headings with
;;     special tags after closing (see
;;     `org-koma-letter-special-tags-after-closing')
;;   - after-letter-order, as above, but after the end of the letter
;;     (see `org-koma-letter-special-tags-after-letter').
;;
;; The following variables works differently from the main LaTeX class
;;   - AUTHOR: Default to user-full-name but may be disabled.
;;     (See also `org-koma-letter-author'.)
;;   - EMAIL: Same as AUTHOR.  (See also `org-koma-letter-email'.)
;;
;; FROM_LOGO uses LaTeX markup.  FROM_LOGO provides the
;; "includegraphics" command to tell LaTeX where to find the logo.
;; This command needs to know the logo's directory and file name.  The
;; directory can either be relative or absolute, just as you would
;; expect.  LaTeX can use three file types for the logo: PDF, JPEG, or
;; PNG.  The logo can either include or exclude its extension, which
;; might surprise you.  When you exclude its extension, LaTeX will
;; search the directory for the "best" quality graphics format.  For
;; example if it finds both logo.pdf and logo.png then it will
;; identify the PDF as "better", and include "logo.pdf".  This can be
;; useful, for example, when you are mocking up a logo in the PNG
;; raster format and then switch over to the higher quality PDF vector
;; format.  When you include the file extension then LaTeX will
;; include it without searching for higher quality file types.
;; Whatever file type you choose, it will probably require a few
;; design iterations to get the best looking logo size for your
;; letter.  Finally, the directory and file name are specified
;; *without* quotes.  Here are some examples with commentary, in the
;; location of your letter, with a logo named "logo", to get you
;; started:
;;
;;   Logo in the same directory: \includegraphics{logo}
;;       or a sub-directory:     \includegraphics{logos/production/logo}
;;
;;   Logos specified using absolute paths on Linux or Windows:
;;
;;       \includegraphics{~/correspondence/logo}
;;       \includegraphics{~/correspondence/logos/production/logo}
;;       \includegraphics{c:/you/correspondence/logo}
;;       \includegraphics{c:/you/correspondence/logos/production/logo}
;;
;;   Logos in the same directory where the "better" quality PDF will
;;   be chosen over the JPG:
;;
;;       \includegraphics{logo.pdf}
;;       \includegraphics{logo.png}
;;
;; Headlines are in general ignored.  However, headlines with special
;; tags can be used for specified contents like postscript (ps),
;; carbon copy (cc), enclosures (encl) and code to be inserted after
;; \end{letter} (after_letter).  Specials tags are defined in
;; `org-koma-letter-special-tags-after-closing' and
;; `org-koma-letter-special-tags-after-letter'.  Currently members of
;; `org-koma-letter-special-tags-after-closing' used as macros and the
;; content of the headline is the argument.
;;
;; Headlines with to and from may also be used rather than the keyword
;; approach described above.  If both a keyword and a headline with
;; information is present precedence is determined by
;; `org-koma-letter-prefer-special-headings'.
;;
;; You need an appropriate association in `org-latex-classes' in order
;; to use the KOMA Scrlttr2 class.  By default, a sparse scrlttr2
;; class is provided: "default-koma-letter".  You can also add you own
;; letter class.  For instance:
;;
;;   (add-to-list 'org-latex-classes
;;                '("my-letter"
;;                  "\\documentclass\[%
;;   DIV=14,
;;   fontsize=12pt,
;;   parskip=half,
;;   subject=titled,
;;   backaddress=false,
;;   fromalign=left,
;;   fromemail=true,
;;   fromphone=true\]\{scrlttr2\}
;;   \[DEFAULT-PACKAGES]
;;   \[PACKAGES]
;;   \[EXTRA]"))
;;
;; Then, in your Org document, be sure to require the proper class
;; with:
;;
;;    #+LATEX_CLASS: my-letter
;;
;; Or by setting `org-koma-letter-default-class'.
;;
;; You may have to load (LaTeX) Babel as well, e.g., by adding
;; it to `org-latex-packages-alist',
;;
;;    (add-to-list 'org-latex-packages-alist '("AUTO" "babel" nil))

;;; Code:

(require 'cl-lib)
(require 'ox-latex)

;; Install a default letter class.
(unless (assoc "default-koma-letter" org-latex-classes)
  (add-to-list 'org-latex-classes
               '("default-koma-letter" "\\documentclass[11pt]{scrlttr2}")))

\f
;;; User-Configurable Variables

(defgroup org-export-koma-letter nil
  "Options for exporting to KOMA scrlttr2 class in LaTeX export."
  :tag "Org Koma-Letter"
  :group 'org-export)

(defcustom org-koma-letter-class-option-file "NF"
  "Letter Class Option File.
This option can also be set with the LCO keyword."
  :type 'string)

(defcustom org-koma-letter-author 'user-full-name
  "Sender's name.

This variable defaults to calling the function `user-full-name'
which just returns the current function `user-full-name'.
Alternatively a string, nil or a function may be given.
Functions must return a string.

This option can also be set with the AUTHOR keyword."
  :type '(radio (function-item user-full-name)
                (string)
                (function)
                (const :tag "Do not export author" nil)))

(defcustom org-koma-letter-email 'org-koma-letter-email
  "Sender's email address.

This variable defaults to the value `org-koma-letter-email' which
returns `user-mail-address'.  Alternatively a string, nil or
a function may be given.  Functions must return a string.

This option can also be set with the EMAIL keyword."
  :type '(radio (function-item org-koma-letter-email)
                (string)
                (function)
                (const :tag "Do not export email" nil)))

(defcustom org-koma-letter-from-address ""
  "Sender's address, as a string.
This option can also be set with one or more FROM_ADDRESS
keywords."
  :type 'string)

(defcustom org-koma-letter-phone-number ""
  "Sender's phone number, as a string.
This option can also be set with the PHONE_NUMBER keyword."
  :type 'string)

(defcustom org-koma-letter-url ""
  "Sender's URL, e. g., the URL of her homepage.
This option can also be set with the URL keyword."
  :type 'string
  :safe #'stringp)

(defcustom org-koma-letter-from-logo ""
  "Commands for inserting the sender's logo, e. g., \\includegraphics{logo}.
This option can also be set with the FROM_LOGO keyword."
  :type 'string
  :safe #'stringp)

(defcustom org-koma-letter-place ""
  "Place from which the letter is sent, as a string.
This option can also be set with the PLACE keyword."
  :type 'string)

(defcustom org-koma-letter-location ""
  "Sender's extension field, as a string.

This option can also be set with the LOCATION keyword.
Moreover, when:
  (1) Either `org-koma-letter-prefer-special-headings' is non-nil
      or there is no LOCATION keyword or the LOCATION keyword is
      empty;
  (2) the letter contains a headline with the special
      tag \"location\";
then the location will be set as the content of the location
special heading.

The location field is typically printed right of the address
field (See Figure 4.9. in the English manual of 2015-10-03)."
  :type 'string)

(defcustom org-koma-letter-opening ""
  "Letter's opening, as a string.

This option can also be set with the OPENING keyword.  Moreover,
when:
  (1) Either `org-koma-letter-prefer-special-headings' is non-nil
      or the CLOSING keyword is empty
  (2) `org-koma-letter-headline-is-opening-maybe' is non-nil;
  (3) the letter contains a headline without a special
      tag (e.g. \"to\" or \"ps\");
then the opening will be implicitly set as the untagged headline title."
  :type 'string)

(defcustom org-koma-letter-closing ""
  "Letter's closing, as a string.
This option can also be set with the CLOSING keyword.  Moreover,
when:
  (1) Either `org-koma-letter-prefer-special-headings' is non-nil
      or the CLOSING keyword is empty;
  (2) `org-koma-letter-headline-is-opening-maybe' is non-nil;
  (3) the letter contains a headline with the special
      tag \"closing\";
then the opening will be set as the title of the closing special
heading title."
  :type 'string)

(defcustom org-koma-letter-signature ""
  "Signature, as a string.
This option can also be set with the SIGNATURE keyword.
Moreover, when:
  (1) Either `org-koma-letter-prefer-special-headings' is non-nil
      or there is no CLOSING keyword or the CLOSING keyword is empty;
  (2) `org-koma-letter-headline-is-opening-maybe' is non-nil;
  (3) the letter contains a headline with the special
      tag \"closing\";
then the signature will be  set as the content of the
closing special heading.

Note if the content is empty the signature will not be set."
  :type 'string)

(defcustom org-koma-letter-prefer-special-headings nil
  "Non-nil means prefer headlines over keywords for TO and FROM.
This option can also be set with the OPTIONS keyword, e.g.:
\"special-headings:t\"."
  :type 'boolean)

(defcustom org-koma-letter-subject-format t
  "Non-nil means include the subject.

Support formatting options.

When t, insert a subject using default options.  When nil, do not
insert a subject at all.  It can also be a list of symbols among
the following ones:

 `afteropening'  Subject after opening
 `beforeopening' Subject before opening
 `centered'      Subject centered
 `left'          Subject left-justified
 `right'         Subject right-justified
 `titled'        Add title/description to subject
 `underlined'    Set subject underlined
 `untitled'      Do not add title/description to subject

Please refer to the KOMA-script manual (Table 4.16. in the
English manual of 2012-07-22).

This option can also be set with the OPTIONS keyword, e.g.:
\"subject:(underlined centered)\"."
  :type
  '(choice
    (const :tag "No export" nil)
    (const :tag "Default options" t)
    (set :tag "Configure options"
         (const :tag "Subject after opening" afteropening)
         (const :tag "Subject before opening" beforeopening)
         (const :tag "Subject centered" centered)
         (const :tag "Subject left-justified" left)
         (const :tag "Subject right-justified" right)
         (const :tag "Add title or description to subject" underlined)
         (const :tag "Set subject underlined" titled)
         (const :tag "Do not add title or description to subject" untitled))))

(defcustom org-koma-letter-use-backaddress nil
  "Non-nil prints return address in line above to address.
This option can also be set with the OPTIONS keyword, e.g.:
\"backaddress:t\"."
  :type 'boolean)

(defcustom org-koma-letter-use-foldmarks t
  "Configure appearance of folding marks.

When t, activate default folding marks.  When nil, do not insert
folding marks at all.  It can also be a list of symbols among the
following ones:

  `B'  Activate upper horizontal mark on left paper edge
  `b'  Deactivate upper horizontal mark on left paper edge

  `H'  Activate all horizontal marks on left paper edge
  `h'  Deactivate all horizontal marks on left paper edge

  `L'  Activate left vertical mark on upper paper edge
  `l'  Deactivate left vertical mark on upper paper edge

  `M'  Activate middle horizontal mark on left paper edge
  `m'  Deactivate middle horizontal mark on left paper edge

  `P'  Activate punch or center mark on left paper edge
  `p'  Deactivate punch or center mark on left paper edge

  `T'  Activate lower horizontal mark on left paper edge
  `t'  Deactivate lower horizontal mark on left paper edge

  `V'  Activate all vertical marks on upper paper edge
  `v'  Deactivate all vertical marks on upper paper edge

This option can also be set with the OPTIONS keyword, e.g.:
\"foldmarks:(b l m t)\"."
  :type '(choice
          (const :tag "Activate default folding marks" t)
          (const :tag "Deactivate folding marks" nil)
          (set
           :tag "Configure folding marks"
           (const :tag "Activate upper horizontal mark on left paper edge" B)
           (const :tag "Deactivate upper horizontal mark on left paper edge" b)
           (const :tag "Activate all horizontal marks on left paper edge" H)
           (const :tag "Deactivate all horizontal marks on left paper edge" h)
           (const :tag "Activate left vertical mark on upper paper edge" L)
           (const :tag "Deactivate left vertical mark on upper paper edge" l)
           (const :tag "Activate middle horizontal mark on left paper edge" M)
           (const :tag "Deactivate middle horizontal mark on left paper edge" m)
           (const :tag "Activate punch or center mark on left paper edge" P)
           (const :tag "Deactivate punch or center mark on left paper edge" p)
           (const :tag "Activate lower horizontal mark on left paper edge" T)
           (const :tag "Deactivate lower horizontal mark on left paper edge" t)
           (const :tag "Activate all vertical marks on upper paper edge" V)
           (const :tag "Deactivate all vertical marks on upper paper edge" v))))

(defcustom org-koma-letter-use-phone nil
  "Non-nil prints sender's phone number.
This option can also be set with the OPTIONS keyword, e.g.:
\"phone:t\"."
  :type 'boolean)

(defcustom org-koma-letter-use-url nil
  "Non-nil prints sender's URL.
This option can also be set with the OPTIONS keyword, e.g.:
\"url:t\"."
  :type 'boolean
  :safe #'booleanp)

(defcustom org-koma-letter-use-from-logo nil
  "Non-nil prints sender's FROM_LOGO.
This option can also be set with the OPTIONS keyword, e.g.:
\"from-logo:t\"."
  :type 'boolean
  :safe #'booleanp)

(defcustom org-koma-letter-use-email nil
  "Non-nil prints sender's email address.
This option can also be set with the OPTIONS keyword, e.g.:
\"email:t\"."
  :type 'boolean)

(defcustom org-koma-letter-use-place t
  "Non-nil prints the letter's place next to the date.
This option can also be set with the OPTIONS keyword, e.g.:
\"place:nil\"."
  :type 'boolean)

(defcustom org-koma-letter-default-class "default-koma-letter"
  "Default class for `org-koma-letter'.
The value must be a member of `org-latex-classes'."
  :type 'string)

(defcustom org-koma-letter-headline-is-opening-maybe t
  "Non-nil means a headline may be used as an opening and closing.
See also `org-koma-letter-opening' and
`org-koma-letter-closing'."
  :type 'boolean)

(defcustom org-koma-letter-prefer-subject nil
  "Non-nil means title should be interpreted as subject if subject is missing.
This option can also be set with the OPTIONS keyword,
e.g. \"title-subject:t\"."
  :type 'boolean)

(defconst org-koma-letter-special-tags-in-letter '(to from closing location)
  "Header tags related to the letter itself.")

(defconst org-koma-letter-special-tags-after-closing '(after_closing ps encl cc)
  "Header tags to be inserted in the letter after closing.")

(defconst org-koma-letter-special-tags-as-macro '(ps encl cc)
  "Header tags to be inserted as macros.")

(defconst org-koma-letter-special-tags-after-letter '(after_letter)
  "Header tags to be inserted after the letter.")

(defvar org-koma-letter-special-contents nil
  "Holds special content temporarily.")

\f
;;; Define Back-End

(org-export-define-derived-backend 'koma-letter 'latex
  :options-alist
  '((:latex-class "LATEX_CLASS" nil org-koma-letter-default-class t)
    (:lco "LCO" nil org-koma-letter-class-option-file)
    (:author "AUTHOR" nil (org-koma-letter--get-value org-koma-letter-author) parse)
    (:author-changed-in-buffer-p "AUTHOR" nil nil t)
    (:from-address "FROM_ADDRESS" nil org-koma-letter-from-address newline)
    (:phone-number "PHONE_NUMBER" nil org-koma-letter-phone-number)
    (:url "URL" nil org-koma-letter-url)
    (:from-logo "FROM_LOGO" nil org-koma-letter-from-logo)
    (:email "EMAIL" nil (org-koma-letter--get-value org-koma-letter-email) t)
    (:to-address "TO_ADDRESS" nil nil newline)
    (:place "PLACE" nil org-koma-letter-place)
    (:location "LOCATION" nil org-koma-letter-location)
    (:subject "SUBJECT" nil nil parse)
    (:opening "OPENING" nil org-koma-letter-opening parse)
    (:closing "CLOSING" nil org-koma-letter-closing parse)
    (:signature "SIGNATURE" nil org-koma-letter-signature newline)
    (:special-headings nil "special-headings" org-koma-letter-prefer-special-headings)
    (:special-tags-as-macro nil nil org-koma-letter-special-tags-as-macro)
    (:special-tags-in-letter nil nil org-koma-letter-special-tags-in-letter)
    (:special-tags-after-closing nil "after-closing-order"
                                 org-koma-letter-special-tags-after-closing)
    (:special-tags-after-letter nil "after-letter-order"
                                org-koma-letter-special-tags-after-letter)
    (:with-backaddress nil "backaddress" org-koma-letter-use-backaddress)
    (:with-email nil "email" org-koma-letter-use-email)
    (:with-foldmarks nil "foldmarks" org-koma-letter-use-foldmarks)
    (:with-phone nil "phone" org-koma-letter-use-phone)
    (:with-url nil "url" org-koma-letter-use-url)
    (:with-from-logo nil "from-logo" org-koma-letter-use-from-logo)
    (:with-place nil "place" org-koma-letter-use-place)
    (:with-subject nil "subject" org-koma-letter-subject-format)
    (:with-title-as-subject nil "title-subject" org-koma-letter-prefer-subject)
    (:with-headline-opening nil nil org-koma-letter-headline-is-opening-maybe)
    ;; Special properties non-nil when a setting happened in buffer.
    ;; They are used to prioritize in-buffer settings over "lco"
    ;; files.  See `org-koma-letter-template'.
    (:inbuffer-author "AUTHOR" nil 'koma-letter:empty)
    (:inbuffer-from "FROM" nil 'koma-letter:empty)
    (:inbuffer-email "EMAIL" nil 'koma-letter:empty)
    (:inbuffer-phone-number "PHONE_NUMBER" nil 'koma-letter:empty)
    (:inbuffer-url "URL" nil 'koma-letter:empty)
    (:inbuffer-from-logo "FROM_LOGO" nil 'koma-letter:empty)
    (:inbuffer-place "PLACE" nil 'koma-letter:empty)
    (:inbuffer-location "LOCATION" nil 'koma-letter:empty)
    (:inbuffer-signature "SIGNATURE" nil 'koma-letter:empty)
    (:inbuffer-with-backaddress nil "backaddress" 'koma-letter:empty)
    (:inbuffer-with-email nil "email" 'koma-letter:empty)
    (:inbuffer-with-foldmarks nil "foldmarks" 'koma-letter:empty)
    (:inbuffer-with-phone nil "phone" 'koma-letter:empty)
    (:inbuffer-with-url nil "url" 'koma-letter:empty)
    (:inbuffer-with-from-logo nil "from-logo" 'koma-letter:empty)
    (:inbuffer-with-place nil "place" 'koma-letter:empty))
  :translate-alist '((export-block . org-koma-letter-export-block)
                     (export-snippet . org-koma-letter-export-snippet)
                     (headline . org-koma-letter-headline)
                     (keyword . org-koma-letter-keyword)
                     (template . org-koma-letter-template))
  :menu-entry
  '(?k "Export with KOMA Scrlttr2"
       ((?L "As LaTeX buffer" org-koma-letter-export-as-latex)
        (?l "As LaTeX file" org-koma-letter-export-to-latex)
        (?p "As PDF file" org-koma-letter-export-to-pdf)
        (?o "As PDF file and open"
            (lambda (a s v b)
              (if a (org-koma-letter-export-to-pdf t s v b)
                (org-open-file (org-koma-letter-export-to-pdf nil s v b))))))))


\f
;;; Helper functions

(defun org-koma-letter-email ()
  "Return the current `user-mail-address'."
  user-mail-address)

;; The following is taken from/inspired by ox-grof.el
;; Thanks, Luis!

(defun org-koma-letter--get-tagged-contents (key)
  "Get contents from a headline tagged with KEY.
The contents is stored in `org-koma-letter-special-contents'."
  (let ((value (cdr (assoc-string (org-koma-letter--get-value key)
                                  org-koma-letter-special-contents))))
    (when value (org-string-nw-p (org-trim value)))))

(defun org-koma-letter--get-value (value)
  "Turn value into a string whenever possible.
Determines if VALUE is nil, a string, a function or a symbol and
return a string or nil."
  (when value
    (cond ((stringp value) value)
          ((functionp value) (funcall value))
          ((symbolp value) (symbol-name value))
          (t value))))

(defun org-koma-letter--special-contents-inline (keywords info)
  "Process KEYWORDS members of `org-koma-letter-special-contents'.

KEYWORDS is a list of symbols.  Return them as a string to be
formatted.

The function is used for inserting content of special headings
such as the one tagged with PS."
  (mapconcat
   (lambda (keyword)
     (let* ((name (org-koma-letter--get-value keyword))
            (value (org-koma-letter--get-tagged-contents name))
            (macrop (memq keyword (plist-get info :special-tags-as-macro))))
       (cond ((not value) nil)
             (macrop (format "\\%s{%s}\n" name value))
             (t value))))
   keywords
   "\n"))


(defun org-koma-letter--add-latex-newlines (string)
  "Replace regular newlines with LaTeX newlines (i.e. `\\\\')."
  (let ((str (org-trim string)))
    (when (org-string-nw-p str)
      (replace-regexp-in-string "\n" "\\\\\\\\\n" str))))


\f
;;; Transcode Functions

;;;; Export Block

(defun org-koma-letter-export-block (export-block _contents _info)
  "Transcode an EXPORT-BLOCK element into KOMA Scrlttr2 code.
CONTENTS is nil.  INFO is a plist used as a communication
channel."
  (when (member (org-element-property :type export-block)
		'("KOMA-LETTER" "LATEX"))
    (org-remove-indentation (org-element-property :value export-block))))

;;;; Export Snippet

(defun org-koma-letter-export-snippet (export-snippet _contents _info)
  "Transcode an EXPORT-SNIPPET object into KOMA Scrlttr2 code.
CONTENTS is nil.  INFO is a plist used as a communication
channel."
  (when (memq (org-export-snippet-backend export-snippet) '(latex koma-letter))
    (org-element-property :value export-snippet)))

;;;; Keyword

(defun org-koma-letter-keyword (keyword contents info)
  "Transcode a KEYWORD element into KOMA Scrlttr2 code.
CONTENTS is nil.  INFO is a plist used as a communication
channel."
  (let ((key (org-element-property :key keyword))
        (value (org-element-property :value keyword)))
    ;; Handle specifically KOMA-LETTER keywords.  Otherwise, fallback
    ;; to `latex' back-end.
    (if (equal key "KOMA-LETTER") value
      (org-export-with-backend 'latex keyword contents info))))

;; Headline

(defun org-koma-letter-headline (headline contents info)
  "Transcode a HEADLINE element from Org to LaTeX.
CONTENTS holds the contents of the headline.  INFO is a plist
holding contextual information.

Note that if a headline is tagged with a tag from
`org-koma-letter-special-tags' it will not be exported, but
stored in `org-koma-letter-special-contents' and included at the
appropriate place."
  (let ((special-tag (org-koma-letter--special-tag headline info)))
    (if (not special-tag)
        contents
      (push (cons special-tag contents) org-koma-letter-special-contents)
      "")))

(defun org-koma-letter--special-tag (headline info)
  "Non-nil if HEADLINE is a special headline.
INFO is a plist holding contextual information.  Return first
special tag headline."
  (let ((special-tags (append
                       (plist-get info :special-tags-in-letter)
                       (plist-get info :special-tags-after-closing)
                       (plist-get info :special-tags-after-letter))))
    (cl-some (lambda (tag) (and (assoc-string tag special-tags) tag))
	     (org-export-get-tags headline info))))

(defun org-koma-letter--keyword-or-headline (plist-key pred info)
  "Return the correct version of opening or closing.
PLIST-KEY should be a key in info, typically :opening
or :closing.  PRED is a predicate run on headline to determine
which title to use which takes two arguments, a headline element
and an info plist.  INFO is a plist holding contextual
information.  Return the preferred candidate for the exported of
PLIST-KEY."
  (let* ((keyword-candidate (plist-get info plist-key))
         (headline-candidate (when (and (plist-get info :with-headline-opening)
                                        (or (plist-get info :special-headings)
                                            (not keyword-candidate)))
                               (org-element-map (plist-get info :parse-tree)
                                   'headline
                                 (lambda (h)
                                   (and (funcall pred h info)
					(org-element-property :title h)))
                                 info t))))
    (org-export-data (or headline-candidate keyword-candidate "") info)))

;;;; Template

(defun org-koma-letter-template (contents info)
  "Return complete document string after KOMA Scrlttr2 conversion.
CONTENTS is the transcoded contents string.  INFO is a plist
holding export options."
  (concat
   ;; Time-stamp.
   (and (plist-get info :time-stamp-file)
        (format-time-string "%% Created %Y-%m-%d %a %H:%M\n"))
   ;; LaTeX compiler
   (org-latex--insert-compiler info)
   ;; Document class and packages.
   (org-latex-make-preamble info)
   ;; Settings.  They can come from three locations, in increasing
   ;; order of precedence: global variables, LCO files and in-buffer
   ;; settings.  Thus, we first insert settings coming from global
   ;; variables, then we insert LCO files, and, eventually, we insert
   ;; settings coming from buffer keywords.
   (org-koma-letter--build-settings 'global info)
   (mapconcat (lambda (file) (format "\\LoadLetterOption{%s}\n" file))
              (split-string (or (plist-get info :lco) ""))
              "")
   (org-koma-letter--build-settings 'buffer info)
   ;; Date.
   (format "\\date{%s}\n" (org-export-data (org-export-get-date info) info))
   ;; Hyperref, document start, and subject and title.
   (let* ((with-subject (plist-get info :with-subject))
          (with-title (plist-get info :with-title))
          (title-as-subject (and with-subject
                                 (plist-get info :with-title-as-subject)))
          (subject* (org-string-nw-p
                     (org-export-data (plist-get info :subject) info)))
          (title* (and with-title
                       (org-string-nw-p
                        (org-export-data (plist-get info :title) info))))
          (subject (cond ((not with-subject) nil)
                         (title-as-subject (or subject* title*))
                         (t subject*)))
          (title (cond ((not with-title) nil)
                       (title-as-subject (and subject* title*))
                       (t title*)))
          (hyperref-template (plist-get info :latex-hyperref-template))
          (spec (append (list (cons ?t (or title subject "")))
                        (org-latex--format-spec info))))
     (concat
      (when (and with-subject (not (eq with-subject t)))
        (format "\\KOMAoption{subject}{%s}\n"
                (if (symbolp with-subject) with-subject
                  (mapconcat #'symbol-name with-subject ","))))
      ;; Hyperref.
      (and (stringp hyperref-template)
	   (format-spec hyperref-template spec))
      ;; Document start.
      "\\begin{document}\n\n"
      ;; Subject and title.
      (when subject (format "\\setkomavar{subject}{%s}\n" subject))
      (when title (format "\\setkomavar{title}{%s}\n" title))
      (when (or (org-string-nw-p title) (org-string-nw-p subject)) "\n")))
   ;; Letter start.
   (let ((keyword-val (plist-get info :to-address))
         (heading-val (org-koma-letter--get-tagged-contents 'to)))
     (format "\\begin{letter}{%%\n%s}\n\n"
             (org-koma-letter--add-latex-newlines
              (or (if (plist-get info :special-headings)
                      (or heading-val keyword-val)
                    (or keyword-val heading-val))
                  "\\mbox{}"))))
   ;; Opening.
   (format "\\opening{%s}\n\n"
           (org-koma-letter--keyword-or-headline
            :opening
	    (lambda (h i)
	      (not (org-koma-letter--special-tag h i)))
            info))
   ;; Letter body.
   contents
   ;; Closing.
   (format "\\closing{%s}\n"
	   (org-koma-letter--keyword-or-headline
	    :closing
	    (lambda (h i)
	      (let ((special-tag (org-koma-letter--special-tag h i)))
		(and special-tag
		     (string= "closing" special-tag))))
	    info))
   (org-koma-letter--special-contents-inline
    (plist-get info :special-tags-after-closing) info)
   ;; Letter end.
   "\n\\end{letter}\n"
   (org-koma-letter--special-contents-inline
    (plist-get info :special-tags-after-letter) info)
   ;; Document end.
   "\n\\end{document}"))

(defun org-koma-letter--build-settings (scope info)
  "Build settings string according to type.
SCOPE is either `global' or `buffer'.  INFO is a plist used as
a communication channel."
  (let* ((check-scope
          ;; Non-nil value when SETTING was defined in SCOPE.
          (lambda (setting)
            (let ((property (intern (format ":inbuffer-%s" setting))))
              (if (eq scope 'global)
                  (eq (plist-get info property) 'koma-letter:empty)
                (not (eq (plist-get info property) 'koma-letter:empty))))))
         (heading-or-key-value
          (lambda (heading key &optional scoped)
            (let* ((heading-val
                    (org-koma-letter--get-tagged-contents heading))
                   (key-val (org-string-nw-p (plist-get info key)))
                   (scopedp (funcall check-scope (or scoped heading))))
              (and (or (and key-val scopedp) heading-val)
                   (not (and (eq scope 'global) heading-val))
                   (if scopedp key-val heading-val))))))
    (concat
     ;; Name.
     (let ((author (plist-get info :author)))
       (and author
            (funcall check-scope 'author)
            (format "\\setkomavar{fromname}{%s}\n"
                    (org-export-data author info))))
     ;; From.
     (let ((from (funcall heading-or-key-value 'from :from-address)))
       (and from
            (format "\\setkomavar{fromaddress}{%s}\n"
                    (org-koma-letter--add-latex-newlines from))))
     ;; Email.
     (let ((email (plist-get info :email)))
       (and email
            (funcall check-scope 'email)
            (format "\\setkomavar{fromemail}{%s}\n" email)))
     (and (funcall check-scope 'with-email)
          (format "\\KOMAoption{fromemail}{%s}\n"
                  (if (plist-get info :with-email) "true" "false")))
     ;; Phone number.
     (let ((phone-number (plist-get info :phone-number)))
       (and (org-string-nw-p phone-number)
            (funcall check-scope 'phone-number)
            (format "\\setkomavar{fromphone}{%s}\n" phone-number)))
     (and (funcall check-scope 'with-phone)
          (format "\\KOMAoption{fromphone}{%s}\n"
                  (if (plist-get info :with-phone) "true" "false")))
     ;; URL
     (let ((url (plist-get info :url)))
       (and (org-string-nw-p url)
            (funcall check-scope 'url)
            (format "\\setkomavar{fromurl}{%s}\n" url)))
     (and (funcall check-scope 'with-url)
          (format "\\KOMAoption{fromurl}{%s}\n"
                  (if (plist-get info :with-url) "true" "false")))
     ;; From Logo
     (let ((from-logo (plist-get info :from-logo)))
       (and (org-string-nw-p from-logo)
            (funcall check-scope 'from-logo)
            (format "\\setkomavar{fromlogo}{%s}\n" from-logo)))
     (and (funcall check-scope 'with-from-logo)
          (format "\\KOMAoption{fromlogo}{%s}\n"
                  (if (plist-get info :with-from-logo) "true" "false")))
     ;; Signature.
     (let* ((heading-val
             (and (plist-get info :with-headline-opening)
                  (pcase (org-koma-letter--get-tagged-contents 'closing)
                    ((and (pred org-string-nw-p) closing) (org-trim closing))
                    (_ nil))))
            (signature (org-string-nw-p (plist-get info :signature)))
            (signature-scope (funcall check-scope 'signature)))
       (and (or (and signature signature-scope)
                heading-val)
            (not (and (eq scope 'global) heading-val))
            (format "\\setkomavar{signature}{%s}\n"
                    (if signature-scope signature heading-val))))
     ;; Back address.
     (and (funcall check-scope 'with-backaddress)
          (format "\\KOMAoption{backaddress}{%s}\n"
                  (if (plist-get info :with-backaddress) "true" "false")))
     ;; Place.
     (let ((with-place-set (funcall check-scope 'with-place))
           (place-set (funcall check-scope 'place)))
       (and (or (and with-place-set place-set)
                (and (eq scope 'buffer) (or with-place-set place-set)))
            (format "\\setkomavar{place}{%s}\n"
                    (if (plist-get info :with-place) (plist-get info :place)
                      ""))))
     ;; Location.
     (let ((location (funcall heading-or-key-value 'location :location)))
       (and location
            (format "\\setkomavar{location}{%s}\n" location)))
     ;; Folding marks.
     (and (funcall check-scope 'with-foldmarks)
          (let ((foldmarks (plist-get info :with-foldmarks)))
            (cond ((consp foldmarks)
                   (format "\\KOMAoptions{foldmarks=true,foldmarks=%s}\n"
                           (mapconcat #'symbol-name foldmarks "")))
                  (foldmarks "\\KOMAoptions{foldmarks=true}\n")
                  (t "\\KOMAoptions{foldmarks=false}\n")))))))


\f
;;; Commands

;;;###autoload
(defun org-koma-letter-export-as-latex
    (&optional async subtreep visible-only body-only ext-plist)
  "Export current buffer as a KOMA Scrlttr2 letter.

If narrowing is active in the current buffer, only export its
narrowed part.

If a region is active, export that region.

A non-nil optional argument ASYNC means the process should happen
asynchronously.  The resulting buffer should be accessible
through the `org-export-stack' interface.

When optional argument SUBTREEP is non-nil, export the sub-tree
at point, extracting information from the headline properties
first.

When optional argument VISIBLE-ONLY is non-nil, don't export
contents of hidden elements.

When optional argument BODY-ONLY is non-nil, only write code
between \"\\begin{letter}\" and \"\\end{letter}\".

EXT-PLIST, when provided, is a property list with external
parameters overriding Org default settings, but still inferior to
file-local settings.

Export is done in a buffer named \"*Org KOMA-LETTER Export*\".  It
will be displayed if `org-export-show-temporary-export-buffer' is
non-nil."
  (interactive)
  (let (org-koma-letter-special-contents)
    (org-export-to-buffer 'koma-letter "*Org KOMA-LETTER Export*"
      async subtreep visible-only body-only ext-plist
      (lambda () (LaTeX-mode)))))

;;;###autoload
(defun org-koma-letter-export-to-latex
    (&optional async subtreep visible-only body-only ext-plist)
  "Export current buffer as a KOMA Scrlttr2 letter (tex).

If narrowing is active in the current buffer, only export its
narrowed part.

If a region is active, export that region.

A non-nil optional argument ASYNC means the process should happen
asynchronously.  The resulting file should be accessible through
the `org-export-stack' interface.

When optional argument SUBTREEP is non-nil, export the sub-tree
at point, extracting information from the headline properties
first.

When optional argument VISIBLE-ONLY is non-nil, don't export
contents of hidden elements.

When optional argument BODY-ONLY is non-nil, only write code
between \"\\begin{letter}\" and \"\\end{letter}\".

EXT-PLIST, when provided, is a property list with external
parameters overriding Org default settings, but still inferior to
file-local settings.

When optional argument PUB-DIR is set, use it as the publishing
directory.

Return output file's name."
  (interactive)
  (let ((outfile (org-export-output-file-name ".tex" subtreep))
        (org-koma-letter-special-contents))
    (org-export-to-file 'koma-letter outfile
      async subtreep visible-only body-only ext-plist)))

;;;###autoload
(defun org-koma-letter-export-to-pdf
    (&optional async subtreep visible-only body-only ext-plist)
  "Export current buffer as a KOMA Scrlttr2 letter (pdf).

If narrowing is active in the current buffer, only export its
narrowed part.

If a region is active, export that region.

A non-nil optional argument ASYNC means the process should happen
asynchronously.  The resulting file should be accessible through
the `org-export-stack' interface.

When optional argument SUBTREEP is non-nil, export the sub-tree
at point, extracting information from the headline properties
first.

When optional argument VISIBLE-ONLY is non-nil, don't export
contents of hidden elements.

When optional argument BODY-ONLY is non-nil, only write code
between \"\\begin{letter}\" and \"\\end{letter}\".

EXT-PLIST, when provided, is a property list with external
parameters overriding Org default settings, but still inferior to
file-local settings.

Return PDF file's name."
  (interactive)
  (let ((file (org-export-output-file-name ".tex" subtreep))
        (org-koma-letter-special-contents))
    (org-export-to-file 'koma-letter file
      async subtreep visible-only body-only ext-plist
      #'org-latex-compile)))


(provide 'ox-koma-letter)
;;; ox-koma-letter.el ends here