Add xdg library

* etc/NEWS: Mention new library.
* lisp/xdg.el: New file.

etc/NEWS

@@ -702,6 +702,8 @@ processes on exit.
 
 ** New Elisp data-structure library 'radix-tree'.
 
+** New library 'xdg' with utilities for some XDG standards and specs.
+
 
 * Incompatible Lisp Changes in Emacs 26.1
 

lisp/xdg.el

@@ -0,0 +1,144 @@
+;;; xdg.el --- XDG specification and standard support -*- lexical-binding: t -*-
+
+;; Copyright (C) 2017 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 <http://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
+
+;;; Code:
+
+
+;; 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 $XDG_RUNTIME_DIR."
+  (getenv "XDG_RUNTIME_DIR"))
+
+(defun xdg-config-dirs ()
+  "Return the config directory search path as a list."
+  (let ((env (getenv "XDG_CONFIG_DIRS")))
+    (if (or (null env) (string= env ""))
+        '("/etc/xdg")
+      (parse-colon-path env))))
+
+(defun xdg-data-dirs ()
+  "Return the data directory search path as a list."
+  (let ((env (getenv "XDG_CONFIG_DIRS")))
+    (if (or (null env) (string= env ""))
+        '("/usr/local/share/" "/usr/share/")
+      (parse-colon-path env))))
+
+
+;; Thumbnail Managing Standard
+;; https://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html
+
+(defun xdg-thumb-uri (filename)
+  "Return the canonical URI for FILENAME.
+If FILENAME has absolute path /foo/bar.jpg, its canonical URI is
+file:///foo/bar.jpg"
+  (concat "file://" (expand-file-name filename)))
+
+(defun xdg-thumb-name (filename)
+  "Return the appropriate thumbnail filename for FILENAME."
+  (concat (md5 (xdg-thumb-uri filename)) ".png"))
+
+(defun xdg-thumb-mtime (filename)
+  "Return modification time of FILENAME as integral seconds from the epoch."
+  (floor (float-time (nth 5 (file-attributes filename)))))
+
+
+;; XDG User Directories
+;; https://www.freedesktop.org/wiki/Software/xdg-user-dirs/
+
+(defconst xdg-line-regexp
+  (eval-when-compile
+    (rx "XDG_"
+        (group-n 1 (or "DESKTOP" "DOWNLOAD" "TEMPLATES" "PUBLICSHARE"
+                       "DOCUMENTS" "MUSIC" "PICTURES" "VIDEOS"))
+        "_DIR=\""
+        (group-n 2 (or "/" "$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--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 v)))))
+
+(defun xdg--user-dirs-parse-file (filename)
+  "Return alist of xdg-user-dirs from FILENAME."
+  (let (elt res)
+    (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 path of user 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)))))
+  (cdr (assoc name xdg-user-dirs)))
+
+(provide 'xdg)
+
+;;; xdg.el ends here