;;; xdg.el --- XDG specification and standard support -*- lexical-binding: t -*-

;; Copyright (C) 2017-2021 Free Software Foundation, Inc.

;; Author: Mark Oteiza <mvoteiza@udel.edu>
;; Created: 27 January 2017
;; Keywords: files, data

;; 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:

;; Library providing some convenience functions for the following XDG
;; standards and specifications
;;
;; - XDG Base Directory Specification
;; - Thumbnail Managing Standard
;; - xdg-user-dirs configuration
;; - Desktop Entry Specification

;;; Code:

(eval-when-compile
  (require 'cl-lib)
  (require 'subr-x))


;; XDG Base Directory Specification
;; https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

(defmacro xdg--dir-home (environ default-path)
  (declare (debug (stringp stringp)))
  (let ((env (make-symbol "env")))
    `(let ((,env (getenv ,environ)))
       (if (or (null ,env) (not (file-name-absolute-p ,env)))
           (expand-file-name ,default-path)
         ,env))))

(defun xdg-config-home ()
  "Return the base directory for user specific configuration files."
  (xdg--dir-home "XDG_CONFIG_HOME" "~/.config"))

(defun xdg-cache-home ()
  "Return the base directory for user specific cache files."
  (xdg--dir-home "XDG_CACHE_HOME" "~/.cache"))

(defun xdg-data-home ()
  "Return the base directory for user specific data files."
  (xdg--dir-home "XDG_DATA_HOME" "~/.local/share"))

(defun xdg-runtime-dir ()
  "Return the value of $XDG_RUNTIME_DIR."
  (getenv "XDG_RUNTIME_DIR"))

(defun xdg-config-dirs ()
  "Return the config directory search path as a list."
  (let ((env (getenv "XDG_CONFIG_DIRS")))
    (if (or (null env) (string= env ""))
        '("/etc/xdg")
      (parse-colon-path env))))

(defun xdg-data-dirs ()
  "Return the data directory search path as a list."
  (let ((env (getenv "XDG_DATA_DIRS")))
    (if (or (null env) (string= env ""))
        '("/usr/local/share/" "/usr/share/")
      (parse-colon-path env))))


;; Thumbnail Managing Standard
;; https://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html

(defun xdg-thumb-uri (filename)
  "Return the canonical URI for FILENAME.
If FILENAME has absolute file name /foo/bar.jpg, its canonical URI is
file:///foo/bar.jpg"
  (concat "file://" (expand-file-name filename)))

(defun xdg-thumb-name (filename)
  "Return the appropriate thumbnail filename for FILENAME."
  (concat (md5 (xdg-thumb-uri filename)) ".png"))

(defun xdg-thumb-mtime (filename)
  "Return modification time of FILENAME as an Emacs timestamp."
  (file-attribute-modification-time (file-attributes filename)))


;; XDG User Directories
;; https://www.freedesktop.org/wiki/Software/xdg-user-dirs/

(defconst xdg-line-regexp
  (eval-when-compile
    (rx "XDG_"
        (group-n 1 (or "DESKTOP" "DOWNLOAD" "TEMPLATES" "PUBLICSHARE"
                       "DOCUMENTS" "MUSIC" "PICTURES" "VIDEOS"))
        "_DIR=\""
        (group-n 2 (or "/" "$HOME/") (*? (or (not (any "\"")) "\\\"")))
        "\""))
  "Regexp matching non-comment lines in xdg-user-dirs config files.")

(defvar xdg-user-dirs nil
  "Alist of directory keys and values.")

(defun xdg--substitute-home-env (str)
  (if (file-name-absolute-p str) str
    (save-match-data
      (and (string-match "^\\$HOME/" str)
           (replace-match "~/" t nil str 0)))))

(defun xdg--user-dirs-parse-line ()
  "Return pair of user-dirs key to directory value in LINE, otherwise nil.
This should be called at the beginning of a line."
  (skip-chars-forward "[:blank:]")
  (when (and (/= (following-char) ?#)
             (looking-at xdg-line-regexp))
    (let ((k (match-string 1))
          (v (match-string 2)))
      (when (and k v) (cons k (xdg--substitute-home-env v))))))

(defun xdg--user-dirs-parse-file (filename)
  "Return alist of xdg-user-dirs from FILENAME."
  (let (elt res)
    (when (file-readable-p filename)
      (with-temp-buffer
        (insert-file-contents filename)
        (goto-char (point-min))
        (while (not (eobp))
          (setq elt (xdg--user-dirs-parse-line))
          (when (consp elt) (push elt res))
          (forward-line))))
    res))

(defun xdg-user-dir (name)
  "Return the directory referred to by NAME."
  (when (null xdg-user-dirs)
    (setq xdg-user-dirs
          (xdg--user-dirs-parse-file
           (expand-file-name "user-dirs.dirs" (xdg-config-home)))))
  (let ((dir (cdr (assoc name xdg-user-dirs))))
    (when dir (expand-file-name dir))))


;; Desktop Entry Specification
;; https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-1.1.html

(defconst xdg-desktop-group-regexp
  (rx "[" (group-n 1 (+? (in " -Z\\^-~"))) "]")
  "Regexp matching desktop file group header names.")

;; TODO Localized strings left out intentionally, as Emacs has no
;; notion of l10n/i18n
(defconst xdg-desktop-entry-regexp
  (rx (group-n 1 (+ (in "A-Za-z0-9-")))
      ;; (? "[" (group-n 3 (+ nonl)) "]")
      (* blank) "=" (* blank)
      (group-n 2 (* nonl)))
  "Regexp matching desktop file entry key-value pairs.")

(defun xdg-desktop-read-group ()
  "Return hash table of group of desktop entries in the current buffer."
  (let ((res (make-hash-table :test #'equal)))
    (while (not (or (eobp) (looking-at xdg-desktop-group-regexp)))
      (skip-chars-forward "[:blank:]")
      (cond
       ((eolp))
       ((= (following-char) ?#))
       ((looking-at xdg-desktop-entry-regexp)
        (puthash (match-string 1) (match-string 2) res))
       ;; Filter localized strings
       ((looking-at (rx (group-n 1 (+ (in alnum "-"))) (* blank) "[")))
       (t (error "Malformed line: %s"
                 (buffer-substring (point) (point-at-eol)))))
      (forward-line))
    res))

(defun xdg-desktop-read-file (filename &optional group)
  "Return group contents of desktop file FILENAME as a hash table.
Optional argument GROUP defaults to the string \"Desktop Entry\"."
  (with-temp-buffer
    (insert-file-contents-literally filename)
    (goto-char (point-min))
    (while (and (skip-chars-forward "[:blank:]" (line-end-position))
                (or (eolp) (= (following-char) ?#)))
      (forward-line))
    (unless (looking-at xdg-desktop-group-regexp)
      (error "Expected group name!  Instead saw: %s"
             (buffer-substring (point) (point-at-eol))))
    (when group
      (while (and (re-search-forward xdg-desktop-group-regexp nil t)
                  (not (equal (match-string 1) group)))))
    (forward-line)
    (xdg-desktop-read-group)))

(defun xdg-desktop-strings (value)
  "Partition VALUE into elements delimited by unescaped semicolons."
  (let (res)
    (setq value (string-trim-left value))
    (dolist (x (split-string (string-replace "\\;" "\0" value) ";"))
      (push (string-replace "\0" ";" x) res))
    (when (null (string-match-p "[^[:blank:]]" (car res))) (pop res))
    (nreverse res)))


;; MIME apps specification
;; https://standards.freedesktop.org/mime-apps-spec/mime-apps-spec-1.0.1.html

(defvar xdg-mime-table nil
  "Table of MIME type to desktop file associations.
The table is an alist with keys being MIME major types (\"application\",
\"audio\", etc.), and values being hash tables.  Each hash table has
MIME subtypes as keys and lists of desktop file absolute filenames.")

(defun xdg-mime-apps-files ()
  "Return a list of files containing MIME/Desktop associations.
The list is in order of descending priority: user config, then
admin config, and finally system cached associations."
  (let ((xdg-data-dirs (xdg-data-dirs))
        (desktop (getenv "XDG_CURRENT_DESKTOP"))
        res)
    (when desktop
      (setq desktop (list (format "%s-mimeapps.list" desktop))))
    (dolist (name (cons "mimeapps.list" desktop))
      (push (expand-file-name name (xdg-config-home)) res)
      (push (expand-file-name (format "applications/%s" name) (xdg-data-home))
            res)
      (dolist (dir (xdg-config-dirs))
        (push (expand-file-name name dir) res))
      (dolist (dir xdg-data-dirs)
        (push (expand-file-name (format "applications/%s" name) dir) res)))
    (dolist (dir xdg-data-dirs)
      (push (expand-file-name "applications/mimeinfo.cache" dir) res))
    (nreverse res)))

(defun xdg-mime-collect-associations (mime files)
  "Return a list of desktop file names associated with MIME.
The associations are searched in the list of file names FILES,
which is expected to be ordered by priority as in
`xdg-mime-apps-files'."
  (let ((regexp (concat (regexp-quote mime) "=\\([^[:cntrl:]]*\\)$"))
        res sec defaults added removed cached)
    (with-temp-buffer
      (dolist (f (reverse files))
        (when (file-readable-p f)
          (insert-file-contents-literally f nil nil nil t)
          (goto-char (point-min))
          (let () ;; end
            (while (not (or (eobp))) ;; end
              (if (= (following-char) ?\[)
                  (progn (setq sec (char-after (1+ (point))))
                         (forward-line))
                (if (not (looking-at regexp))
                    (forward-line)
                  (dolist (str (xdg-desktop-strings (match-string 1)))
                    (cl-pushnew str
                                (cond ((eq sec ?D) defaults)
                                      ((eq sec ?A) added)
                                      ((eq sec ?R) removed)
                                      ((eq sec ?M) cached))
                                :test #'equal))
                  (while (and (zerop (forward-line))
                              (/= (following-char) ?\[)))))))
          ;; Accumulate results into res
          (dolist (f cached)
            (when (not (member f removed)) (cl-pushnew f res :test #'equal)))
          (dolist (f added)
            (when (not (member f removed)) (push f res)))
          (dolist (f removed)
            (setq res (delete f res)))
          (dolist (f defaults)
            (push f res))
          (setq defaults nil added nil removed nil cached nil))))
    (delete-dups res)))

(defun xdg-mime-apps (mime)
  "Return list of desktop files associated with MIME, otherwise nil.
The list is in order of descending priority, and each element is
an absolute file name of a readable file.
Results are cached in `xdg-mime-table'."
  (pcase-let ((`(,type ,subtype) (split-string mime "/"))
              (xdg-data-dirs (xdg-data-dirs))
              (caches (xdg-mime-apps-files))
              (files ()))
    (let ((mtim1 (get 'xdg-mime-table 'mtime))
          (mtim2 (cl-loop for f in caches when (file-readable-p f)
                          maximize (float-time
				    (file-attribute-modification-time
				     (file-attributes f))))))
      ;; If one of the MIME/Desktop cache files has been modified:
      (when (or (null mtim1) (time-less-p mtim1 mtim2))
        (setq xdg-mime-table nil)))
    (when (null (assoc type xdg-mime-table))
      (push (cons type (make-hash-table :test #'equal)) xdg-mime-table))
    (if (let ((def (make-symbol "def"))
              (table (cdr (assoc type xdg-mime-table))))
          (not (eq (setq files (gethash subtype table def)) def)))
        files
      (and files (setq files nil))
      (let ((dirs (mapcar (lambda (dir) (expand-file-name "applications" dir))
                          (cons (xdg-data-home) xdg-data-dirs))))
        ;; Not being particular about desktop IDs
        (dolist (f (nreverse (xdg-mime-collect-associations mime caches)))
          (push (locate-file f dirs) files))
        (when files
          (put 'xdg-mime-table 'mtime (current-time)))
        (puthash subtype (delq nil files) (cdr (assoc type xdg-mime-table)))))))

(provide 'xdg)

;;; xdg.el ends here