;;; ox-my-html.el --- HTML Derived Backend -*- lexical-binding: t; -*-

;; Copyright (C) 2023 Free Software Foundation, Inc.

;; Author: Carsten Dominik <carsten.dominik@gmail.com>
;;      Jambunathan K <kjambunathan at gmail dot com>
;;      Gerard Vermeulen <gerard.vermeulen AT posteo DOT net>
;; Maintainer: Gerard Vermeulen <gerard.vermeulen AT posteo DOT net>
;; Keywords: org, hypermedia

;; This file is NOT 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 HTML derived backend for Org export:
;; 1. The Org ox-beamer library shows how to code derived backends.
;; 2. The function org-my-html-link is an edited org-html-link copy.
;; 3. It borrows code from https://emacs.stackexchange.com/a/57433 to
;;    embed SVG images in exported HTML files.

;;; Code:

(require 'org-macs)
(org-assert-version)

(require 'cl-lib)
(require 'ox-html)
(require 'nxml-mode)


;;; Define Backend

(org-export-define-derived-backend 'my-html 'html
  :menu-entry
  '(?h 1
       ((?M "As MY-HTML buffer" org-my-html-export-as-html)
        (?m "As MY-HTML file" org-my-html-export-to-html)
        (?O "As MY-HTML file and open"
            (lambda (a s v b)
              (if a (org-my-html-export-to-html t s v b)
                (org-open-file (org-my-html-export-to-html nil s v b)))))))
  :options-alist
  '((:html-embed-svg-excludes "HTML_EMBED_SVG_EXCLUDES" nil
                              org-html-embed-svg-excludes split)
    (:html-embed-svg-includes "HTML_EMBED_SVG_INCLUDES" nil
                              org-html-embed-svg-includes split)
    (:with-html-svg-embedding nil "html-embed-svg" org-html-embed-svg))
  :translate-alist
  '((link . org-my-html-link)))


;;; User Configuration Variables

;;;; Links :: Embed SVG

(defcustom org-html-embed-svg nil
  "Non-nil means embed SVG images into exported HTML pages,
otherwise link to SVG images from exported HTML pages.

This option can also be set with
#+OPTIONS: html-embed-svg:t
or
#+OPTIONS: html-embed-svg:nil
to enable or disable SVG embedding in Org files."
  :group 'org-export-html
  :version "30.0"
  :type 'boolean)

(defcustom org-html-embed-svg-excludes nil
  "List of SVG paths to exclude from SVG embedding.

This option overrules an `org-html-embed-svg' non-nil value.

It can also be set with the HTML_EMBED_SVG_EXCLUDES keyword."
  :group 'org-export-html
  :version "30.0"
  :type '(repeat string)
  :safe (lambda (x) (and (listp x) (cl-every #'stringp x))))

(defcustom org-html-embed-svg-includes nil
  "List of SVG paths to include in SVG embedding.

This option overrules an `org-html-embed-svg' nil value.

It can also be set with the HTML_EMBED_SVG_INCLUDES keyword."
  :group 'org-export-html
  :version "30.0"
  :type '(repeat string)
  :safe (lambda (x) (and (listp x) (cl-every #'stringp x))))

;;; Internal Functions

(defun org-my-html--embed-svg-p (link path info)
  "Check whether LINK and INFO specify to embed the SVG file named PATH.
LINK must have no contents and link to an SVG file.  INFO may contain
lists of SVG files to include in and/or to exclude from embedding."
  (and (not (org-element-contents link))
       (let ((case-fold-search t))
         (string-match-p ".svg\\'" (org-element-property :path link)))
       (or (and (plist-get info :with-html-svg-embedding)
                (not (member path (plist-get info :html-embed-svg-excludes))))
           (and (not (plist-get info :with-html-svg-embedding))
                (member path (plist-get info :html-embed-svg-includes))))))

(defun org-my-html-svg-contents (path)
  "Return the SVG contents of the file named PATH."
  ;; https://emacs.stackexchange.com/a/57433
  (with-temp-buffer
    (insert-file-contents path)
    (with-syntax-table nxml-mode-syntax-table
      ;; Barf if contents contains no valid SVG.
      (while (and (search-forward "<svg")
                  (nth 8 (syntax-ppss)))))
    (delete-region (point-min) (match-beginning 0))
    (buffer-string)))


;;; Transcode Functions

;;;; Link

(defun org-my-html-link (link desc info)
  "Transcode a LINK object from Org to HTML.
DESC is the description part of the link, or the empty string.
INFO is a plist holding contextual information.  See
`org-export-data'.   This function can embed or link SVG images
contrary to `org-html-link' which can only link such images."
  (let* ((html-ext (plist-get info :html-extension))
	 (dot (when (> (length html-ext) 0) "."))
	 (link-org-files-as-html-maybe
	  (lambda (raw-path info)
	    ;; Treat links to `file.org' as links to `file.html', if
	    ;; needed.  See `org-html-link-org-files-as-html'.
            (save-match-data
	      (cond
	       ((and (plist-get info :html-link-org-files-as-html)
                     (let ((case-fold-search t))
                       (string-match "\\(.+\\)\\.org\\(?:\\.gpg\\)?$" raw-path)))
	        (concat (match-string 1 raw-path) dot html-ext))
	       (t raw-path)))))
	 (type (org-element-property :type link))
	 (raw-path (org-element-property :path link))
	 ;; Ensure DESC really exists, or set it to nil.
	 (desc (org-string-nw-p desc))
	 (path
	  (cond
	   ((member type '("http" "https" "ftp" "mailto" "news"))
	    (url-encode-url (concat type ":" raw-path)))
	   ((string= "file" type)
	    ;; During publishing, turn absolute file names belonging
	    ;; to base directory into relative file names.  Otherwise,
	    ;; append "file" protocol to absolute file name.
	    (setq raw-path
		  (org-export-file-uri
		   (org-publish-file-relative-name raw-path info)))
	    ;; Possibly append `:html-link-home' to relative file
	    ;; name.
	    (let ((home (and (plist-get info :html-link-home)
			     (org-trim (plist-get info :html-link-home)))))
	      (when (and home
			 (plist-get info :html-link-use-abs-url)
			 (file-name-absolute-p raw-path))
		(setq raw-path (concat (file-name-as-directory home) raw-path))))
	    ;; Maybe turn ".org" into ".html".
	    (setq raw-path (funcall link-org-files-as-html-maybe raw-path info))
	    ;; Add search option, if any.  A search option can be
	    ;; relative to a custom-id, a headline title, a name or
	    ;; a target.
	    (let ((option (org-element-property :search-option link)))
	      (if (not option) raw-path
		(let ((path (org-element-property :path link)))
		  (concat raw-path
			  "#"
			  (org-publish-resolve-external-link option path t))))))
	   (t raw-path)))
	 (attributes-plist
	  (org-combine-plists
	   ;; Extract attributes from parent's paragraph.  HACK: Only
	   ;; do this for the first link in parent (inner image link
	   ;; for inline images).  This is needed as long as
	   ;; attributes cannot be set on a per link basis.
	   (let* ((parent (org-export-get-parent-element link))
		  (link (let ((container (org-export-get-parent link)))
			  (if (and (eq 'link (org-element-type container))
				   (org-html-inline-image-p link info))
			      container
			    link))))
	     (and (eq link (org-element-map parent 'link #'identity info t))
		  (org-export-read-attribute :attr_html parent)))
	   ;; Also add attributes from link itself.  Currently, those
	   ;; need to be added programmatically before `org-my-html-link'
	   ;; is invoked, for example, by backends building upon HTML
	   ;; export.
	   (org-export-read-attribute :attr_html link)))
	 (attributes
	  (let ((attr (org-html--make-attribute-string attributes-plist)))
	    (if (org-string-nw-p attr) (concat " " attr) ""))))
    (cond
     ;; Link type is handled by a special function.
     ((org-export-custom-protocol-maybe link desc 'html info))
     ;; Embed SVG.
     ((org-my-html--embed-svg-p link path info)
      (org-my-html-svg-contents path))
     ;; Image file.
     ((and (plist-get info :html-inline-images)
	   (org-export-inline-image-p
	    link (plist-get info :html-inline-image-rules)))
      (org-html--format-image path attributes-plist info))
     ;; Radio target: Transcode target's contents and use them as
     ;; link's description.
     ((string= type "radio")
      (let ((destination (org-export-resolve-radio-link link info)))
	(if (not destination) desc
	  (format "<a href=\"#%s\"%s>%s</a>"
		  (org-export-get-reference destination info)
		  attributes
		  desc))))
     ;; Links pointing to a headline: Find destination and build
     ;; appropriate referencing command.
     ((member type '("custom-id" "fuzzy" "id"))
      (let ((destination (if (string= type "fuzzy")
			     (org-export-resolve-fuzzy-link link info)
			   (org-export-resolve-id-link link info))))
	(pcase (org-element-type destination)
	  ;; ID link points to an external file.
	  (`plain-text
	   (let ((fragment (concat org-html--id-attr-prefix path))
		 ;; Treat links to ".org" files as ".html", if needed.
		 (path (funcall link-org-files-as-html-maybe
				destination info)))
	     (format "<a href=\"%s#%s\"%s>%s</a>"
		     path fragment attributes (or desc destination))))
	  ;; Fuzzy link points nowhere.
	  (`nil
	   (format "<i>%s</i>"
		   (or desc
		       (org-export-data
			(org-element-property :raw-link link) info))))
	  ;; Link points to a headline.
	  (`headline
	   (let ((href (org-html--reference destination info))
		 ;; What description to use?
		 (desc
		  ;; Case 1: Headline is numbered and LINK has no
		  ;; description.  Display section number.
		  (if (and (org-export-numbered-headline-p destination info)
			   (not desc))
		      (mapconcat #'number-to-string
				 (org-export-get-headline-number
				  destination info) ".")
		    ;; Case 2: Either the headline is un-numbered or
		    ;; LINK has a custom description.  Display LINK's
		    ;; description or headline's title.
		    (or desc
			(org-export-data
			 (org-element-property :title destination) info)))))
	     (format "<a href=\"#%s\"%s>%s</a>" href attributes desc)))
	  ;; Fuzzy link points to a target or an element.
	  (_
           (if (and destination
                    (memq (plist-get info :with-latex) '(mathjax t))
                    (eq 'latex-environment (org-element-type destination))
                    (eq 'math (org-latex--environment-type destination)))
               ;; Caption and labels are introduced within LaTeX
	       ;; environment.  Use "ref" or "eqref" macro, depending on user
               ;; preference to refer to those in the document.
               (format (plist-get info :html-equation-reference-format)
                       (org-html--reference destination info))
             (let* ((ref (org-html--reference destination info))
                    (org-html-standalone-image-predicate
                     #'org-html--has-caption-p)
                    (counter-predicate
                     (if (eq 'latex-environment (org-element-type destination))
                         #'org-html--math-environment-p
                       #'org-html--has-caption-p))
                    (number
		     (cond
		      (desc nil)
		      ((org-html-standalone-image-p destination info)
		       (org-export-get-ordinal
			(org-element-map destination 'link #'identity info t)
			info '(link) 'org-html-standalone-image-p))
		      (t (org-export-get-ordinal
			  destination info nil counter-predicate))))
                    (desc
		     (cond (desc)
			   ((not number) "No description for this link")
			   ((numberp number) (number-to-string number))
			   (t (mapconcat #'number-to-string number ".")))))
               (format "<a href=\"#%s\"%s>%s</a>" ref attributes desc)))))))
     ;; Coderef: replace link with the reference name or the
     ;; equivalent line number.
     ((string= type "coderef")
      (let ((fragment (concat "coderef-" (org-html-encode-plain-text path))))
	(format "<a href=\"#%s\" %s%s>%s</a>"
		fragment
		(format "class=\"coderef\" onmouseover=\"CodeHighlightOn(this, \
'%s');\" onmouseout=\"CodeHighlightOff(this, '%s');\""
			fragment fragment)
		attributes
		(format (org-export-get-coderef-format path desc)
			(org-export-resolve-coderef path info)))))
     ;; External link with a description part.
     ((and path desc)
      (format "<a href=\"%s\"%s>%s</a>"
	      (org-html-encode-plain-text path)
	      attributes
	      desc))
     ;; External link without a description part.
     (path
      (let ((path (org-html-encode-plain-text path)))
	(format "<a href=\"%s\"%s>%s</a>" path attributes path)))
     ;; No path, only description.  Try to do something useful.
     (t
      (format "<i>%s</i>" desc)))))


;;; End-user functions

;;;###autoload
(defun org-my-html-export-as-html
    (&optional async subtreep visible-only body-only ext-plist)
  "Export current buffer to an HTML buffer.

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 \"<body>\" and \"</body>\" tags.

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 HTML PLUS Export*\", which
will be displayed when `org-export-show-temporary-export-buffer'
is non-nil."
  (interactive)
  (org-export-to-buffer 'my-html "*Org MY-HTML Export*"
    async subtreep visible-only body-only ext-plist
    (lambda () (set-auto-mode t))))

;;;###autoload
(defun org-my-html-export-to-html
    (&optional async subtreep visible-only body-only ext-plist)
  "Export current buffer to an HTML file.

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 \"<body>\" and \"</body>\" tags.

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

Return output file's name."
  (interactive)
  (let* ((extension (concat
		     (when (> (length org-html-extension) 0) ".")
		     (or (plist-get ext-plist :html-extension)
			 org-html-extension
			 "html")))
	 (file (org-export-output-file-name extension subtreep))
	 (org-export-coding-system org-html-coding-system))
    (org-export-to-file 'my-html file
      async subtreep visible-only body-only ext-plist)))

(provide 'ox-my-html)

;;; ox-my-html.el ends here