;;; org-id.el --- Global identifiers for Org entries -*- lexical-binding: t; -*-
;;
;; Copyright (C) 2008-2021 Free Software Foundation, Inc.
;;
;; Author: Carsten Dominik <carsten.dominik@gmail.com>
;; Keywords: outlines, hypermedia, calendar, wp
;; Homepage: https://orgmode.org
;;
;; 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 file implements globally unique identifiers for Org entries.
;; Identifiers are stored in the entry as an :ID: property.  Functions
;; are provided that create and retrieve such identifiers, and that find
;; entries based on the identifier.

;; Identifiers consist of a prefix (default "Org" given by the variable
;; `org-id-prefix') and a unique part that can be created by a number
;; of different methods, see the variable `org-id-method'.
;; Org has a builtin method that uses a compact encoding of the creation
;; time of the ID, with microsecond accuracy.  This virtually
;; guarantees globally unique identifiers, even if several people are
;; creating IDs at the same time in files that will eventually be used
;; together.
;;
;; By default Org uses UUIDs as global unique identifiers.
;;
;; This file defines the following API:
;;
;; org-id-get-create
;;        Create an ID for the entry at point if it does not yet have one.
;;        Returns the ID (old or new).  This function can be used
;;        interactively, with prefix argument the creation of a new ID is
;;        forced, even if there was an old one.
;;
;; org-id-get
;;        Get the ID property of an entry.  Using appropriate arguments
;;        to the function, it can also create the ID for this entry.
;;
;; org-id-goto
;;        Command to go to a specific ID, this command can be used
;;        interactively.
;;
;; org-id-get-with-outline-path-completion
;;        Retrieve the ID of an entry, using outline path completion.
;;        This function can work for multiple files.
;;
;; org-id-get-with-outline-drilling
;;        Retrieve the ID of an entry, using outline path completion.
;;        This function only works for the current file.
;;
;; org-id-find
;;        Find the location of an entry with specific id.
;;

;;; Code:

(require 'org)
(require 'org-refile)
(require 'ol)

(declare-function message-make-fqdn "message" ())
(declare-function org-goto-location "org-goto" (&optional _buf help))

;;; Customization

(defgroup org-id nil
  "Options concerning global entry identifiers in Org mode."
  :tag "Org ID"
  :group 'org)

(defcustom org-id-link-to-org-use-id nil
  "Non-nil means storing a link to an Org file will use entry IDs.
\\<org-mode-map>
The variable can have the following values:

t     Create an ID if needed to make a link to the current entry.

create-if-interactive
      If `org-store-link' is called directly (interactively, as a user
      command), do create an ID to support the link.  But when doing the
      job for capture, only use the ID if it already exists.  The
      purpose of this setting is to avoid proliferation of unwanted
      IDs, just because you happen to be in an Org file when you
      call `org-capture' that automatically and preemptively creates a
      link.  If you do want to get an ID link in a capture template to
      an entry not having an ID, create it first by explicitly creating
      a link to it, using `\\[org-store-link]' first.

create-if-interactive-and-no-custom-id
      Like create-if-interactive, but do not create an ID if there is
      a CUSTOM_ID property defined in the entry.

use-existing
      Use existing ID, do not create one.

nil   Never use an ID to make a link, instead link using a text search for
      the headline text."
  :group 'org-link-store
  :group 'org-id
  :version "24.3"
  :type '(choice
	  (const :tag "Create ID to make link" t)
	  (const :tag "Create if storing link interactively"
		 create-if-interactive)
	  (const :tag "Create if storing link interactively and no CUSTOM_ID is present"
		 create-if-interactive-and-no-custom-id)
	  (const :tag "Only use existing" use-existing)
	  (const :tag "Do not use ID to create link" nil)))

(defcustom org-id-uuid-program "uuidgen"
  "The uuidgen program."
  :group 'org-id
  :type 'string)

(defcustom org-id-ts-format "%Y%m%dT%H%M%S.%6N"
  "Timestamp format for IDs generated using `ts' `org-id-method'.
The format should be suitable to pass as an argument to `format-time-string'.

Defaults to ISO8601 timestamps without separators and without
timezone, local time and precision down to 1e-6 seconds."
  :type 'string
  :package-version '(Org . "9.5"))

(defcustom org-id-method 'uuid
  "The method that should be used to create new IDs.

An ID will consist of the optional prefix specified in `org-id-prefix',
and a unique part created by the method this variable specifies.

Allowed values are:

org        Org's own internal method, using an encoding of the current time to
           microsecond accuracy, and optionally the current domain of the
           computer.  See the variable `org-id-include-domain'.

uuid       Create random (version 4) UUIDs.  If the program defined in
           `org-id-uuid-program' is available it is used to create the ID.
           Otherwise an internal functions is used.

ts         Create ID's based on timestamps as specified in `org-id-ts-format'."
  :group 'org-id
  :type '(choice
	  (const :tag "Org's internal method" org)
	  (const :tag "external: uuidgen" uuid)
	  (const :tag "Timestamp with format `org-id-ts-format'" ts)))

(defcustom org-id-prefix nil
  "The prefix for IDs.

This may be a string, or it can be nil to indicate that no prefix is required.
When a string, the string should have no space characters as IDs are expected
to have no space characters in them."
  :group 'org-id
  :type '(choice
	  (const :tag "No prefix")
	  (string :tag "Prefix")))

(defcustom org-id-include-domain nil
  "Non-nil means add the domain name to new IDs.
This ensures global uniqueness of IDs, and is also suggested by
the relevant RFCs.  This is relevant only if `org-id-method' is
`org' or `ts'.  When uuidgen is used, the domain will never be added.

The default is to not use this because we have no really good way to get
the true domain, and Org entries will normally not be shared with enough
people to make this necessary."
  :group 'org-id
  :type 'boolean)

(defcustom org-id-track-globally t
  "Non-nil means track IDs through files, so that links work globally.
This work by maintaining a hash table for IDs and writing this table
to disk when exiting Emacs.  Because of this, it works best if you use
a single Emacs process, not many.

When nil, IDs are not tracked.  Links to IDs will still work within
a buffer, but not if the entry is located in another file.
IDs can still be used if the entry with the id is in the same file as
the link."
  :group 'org-id
  :type 'boolean)

(defcustom org-id-locations-file (convert-standard-filename
				  (concat user-emacs-directory ".org-id-locations"))
  "The file for remembering in which file an ID was defined.
This variable is only relevant when `org-id-track-globally' is set."
  :group 'org-id
  :type 'file)

(defcustom org-id-locations-file-relative nil
  "Determines if org-id-locations should be stored as relative links.
Non-nil means that links to locations are stored as links
relative to the location of where `org-id-locations-file' is
stored.

Nil means to store absolute paths to files.

This customization is useful when folders are shared across
systems but mounted at different roots.  Relative path to
`org-id-locations-file' still has to be maintained across
systems."
  :group 'org-id
  :type 'boolean
  :package-version '(Org . "9.3"))

(defvar org-id-locations nil
  "List of files with IDs in those files.")

(defvar org-id-files nil
  "List of files that contain IDs.")

(defcustom org-id-extra-files 'org-agenda-text-search-extra-files
  "Files to be searched for IDs, besides the agenda files.
When Org reparses files to remake the list of files and IDs it is tracking,
it will normally scan the agenda files, the archives related to agenda files,
any files that are listed as ID containing in the current register, and
any Org file currently visited by Emacs.
You can list additional files here.
This variable is only relevant when `org-id-track-globally' is set."
  :group 'org-id
  :type
  '(choice
    (symbol :tag "Variable")
    (repeat :tag "List of files"
	    (file))))

(defcustom org-id-search-archives t
  "Non-nil means search also the archive files of agenda files for entries.
This is a possibility to reduce overhead, but it means that entries moved
to the archives can no longer be found by ID.
This variable is only relevant when `org-id-track-globally' is set."
  :group 'org-id
  :type 'boolean)

(defcustom org-id-action-on-missing-id 'update
  "Special action to be taken, if `org-id-find' cannot find an id.

With 'update (default), org-id-find will silently call
`org-id-update-id-locations' and try again; this will help,
e.g. if you have moved the id to another file.

With 'exception, the exception 'not-found will be thrown, which
can be useful, if `org-id-find' is called from within lisp and
the caller wants to handle the situation.

With 'error, a descriptive error will be raised, causing the
current command to terminate."
  :group 'org-id
  :type '(choice
	  (const :tag "Run `org-id-update-id-locations' if an id cannot be found" update)
	  (const :tag "Throw exception `not-found'" exception)
	  (const :tag "Raise an error" error)
          ))


;;; The API functions

;;;###autoload
(defun org-id-get-create (&optional force)
  "Create an ID for the current entry and return it.
If the entry already has an ID, just return it.
With optional argument FORCE, force the creation of a new ID."
  (interactive "P")
  (when force
    (org-entry-put (point) "ID" nil))
  (org-id-get (point) 'create))

;;;###autoload
(defun org-id-copy ()
  "Copy the ID of the entry at point to the kill ring.
Create an ID if necessary."
  (interactive)
  (org-kill-new (org-id-get nil 'create)))

(defvar org-id-overriding-file-name nil
  "Tell `org-id-get' to use this as the file name when creating an ID.
This is useful when working with contents in a temporary buffer
that will be copied back to the original.")

;;;###autoload
(defun org-id-get (&optional pom create prefix)
  "Get the ID property of the entry at point-or-marker POM.
If POM is nil, refer to the entry at point.
If the entry does not have an ID, the function returns nil.
However, when CREATE is non-nil, create an ID if none is present already.
PREFIX will be passed through to `org-id-new'.
In any case, the ID of the entry is returned."
  (org-with-point-at pom
    (let ((id (org-entry-get nil "ID")))
      (cond
       ((and id (stringp id) (string-match "\\S-" id))
	id)
       (create
	(setq id (org-id-new prefix))
	(org-entry-put pom "ID" id)
	(org-id-add-location id
			     (or org-id-overriding-file-name
				 (buffer-file-name (buffer-base-buffer))))
	id)))))

;;;###autoload
(defun org-id-get-with-outline-path-completion (&optional targets)
  "Use `outline-path-completion' to retrieve the ID of an entry.
TARGETS may be a setting for `org-refile-targets' to define
eligible headlines.  When omitted, all headlines in the current
file are eligible.  This function returns the ID of the entry.
If necessary, the ID is created."
  (let* ((org-refile-targets (or targets '((nil . (:maxlevel . 10)))))
	 (org-refile-use-outline-path
	  (if (caar org-refile-targets) 'file t))
	 (org-refile-target-verify-function nil)
	 (spos (org-refile-get-location "Entry"))
	 (pom (and spos (move-marker (make-marker) (or (nth 3 spos) 1)
				     (get-file-buffer (nth 1 spos))))))
    (prog1 (org-id-get pom 'create)
      (move-marker pom nil))))

;;;###autoload
(defun org-id-get-with-outline-drilling ()
  "Use an outline-cycling interface to retrieve the ID of an entry.
This only finds entries in the current buffer, using `org-goto-location'.
It returns the ID of the entry.  If necessary, the ID is created."
  (let* ((spos (org-goto-location))
	 (pom (and spos (move-marker (make-marker) (car spos)))))
    (prog1 (org-id-get pom 'create)
      (move-marker pom nil))))

;;;###autoload
(defun org-id-goto (id)
  "Switch to the buffer containing the entry with id ID.
Move the cursor to that entry in that buffer."
  (interactive "sID: ")
  (let ((m (org-id-find id 'marker)))
    (unless m
      (error "Cannot find entry with ID \"%s\"" id))
    (pop-to-buffer-same-window (marker-buffer m))
    (goto-char m)
    (move-marker m nil)
    (org-show-context)))

;;;###autoload
(defun org-id-find (id &optional markerp)
  "Return the location of the entry with the id ID.
The return value is a cons cell (file-name . position), or nil
if there is no entry with that ID.
With optional argument MARKERP, return the position as a new marker."
  (cond
   ((symbolp id) (setq id (symbol-name id)))
   ((numberp id) (setq id (number-to-string id))))
  (let ((file (org-id-find-id-file id))
	org-agenda-new-buffers where)
    (when file
      (setq where (org-id-find-id-in-file id file markerp)))
    (unless where
      (cl-case org-id-action-on-missing-id
        ('exception (throw 'not-found id))
        ('error (error "Could not find id '%s'; but running `org-id-update-id-locations' might help" id))
        (t (org-id-update-id-locations nil t)))
      (setq file (org-id-find-id-file id))
      (when file
	(setq where (org-id-find-id-in-file id file markerp))))
    where))

;;; Internal functions

;; Creating new IDs

;;;###autoload
(defun org-id-new (&optional prefix)
  "Create a new globally unique ID.

An ID consists of two parts separated by a colon:
- a prefix
- a unique part that will be created according to `org-id-method'.

PREFIX can specify the prefix, the default is given by the variable
`org-id-prefix'.  However, if PREFIX is the symbol `none', don't use any
prefix even if `org-id-prefix' specifies one.

So a typical ID could look like \"Org:4nd91V40HI\"."
  (let* ((prefix (if (eq prefix 'none)
		     ""
		   (concat (or prefix org-id-prefix) ":")))
	 unique)
    (if (equal prefix ":") (setq prefix ""))
    (cond
     ((memq org-id-method '(uuidgen uuid))
      (setq unique (org-trim (shell-command-to-string org-id-uuid-program)))
      (unless (org-uuidgen-p unique)
	(setq unique (org-id-uuid))))
     ((eq org-id-method 'org)
      (let* ((etime (org-reverse-string (org-id-time-to-b36)))
	     (postfix (when org-id-include-domain
			(require 'message)
			(concat "@" (message-make-fqdn)))))
	(setq unique (concat etime postfix))))
     ((eq org-id-method 'ts)
      (let ((ts (format-time-string org-id-ts-format))
	    (postfix (when org-id-include-domain
		       (require 'message)
		       (concat "@" (message-make-fqdn)))))
	(setq unique (concat ts postfix))))
     (t (error "Invalid `org-id-method'")))
    (concat prefix unique)))

(defun org-id-uuid ()
  "Return string with random (version 4) UUID."
  (let ((rnd (md5 (format "%s%s%s%s%s%s%s"
			  (random)
			  (org-time-convert-to-list nil)
			  (user-uid)
			  (emacs-pid)
			  (user-full-name)
			  user-mail-address
			  (recent-keys)))))
    (format "%s-%s-4%s-%s%s-%s"
	    (substring rnd 0 8)
	    (substring rnd 8 12)
	    (substring rnd 13 16)
	    (format "%x"
		    (logior
		     #b10000000
		     (logand
		      #b10111111
		      (string-to-number
		       (substring rnd 16 18) 16))))
	    (substring rnd 18 20)
	    (substring rnd 20 32))))

(defun org-id-int-to-b36-one-digit (i)
  "Turn an integer between 0 and 61 into a single character 0..9, A..Z, a..z."
  (cond
   ((< i 10) (+ ?0 i))
   ((< i 36) (+ ?a i -10))
   (t (error "Larger that 35"))))

(defun org-id-b36-to-int-one-digit (i)
  "Turn a character 0..9, A..Z, a..z into a number 0..61.
The input I may be a character, or a single-letter string."
  (and (stringp i) (setq i (string-to-char i)))
  (cond
   ((and (>= i ?0) (<= i ?9)) (- i ?0))
   ((and (>= i ?a) (<= i ?z)) (+ (- i ?a) 10))
   (t (error "Invalid b36 letter"))))

(defun org-id-int-to-b36 (i &optional length)
  "Convert an integer to a base-36 number represented as a string."
  (let ((s ""))
    (while (> i 0)
      (setq s (concat (char-to-string
		       (org-id-int-to-b36-one-digit (mod i 36))) s)
	    i (/ i 36)))
    (setq length (max 1 (or length 1)))
    (if (< (length s) length)
	(setq s (concat (make-string (- length (length s)) ?0) s)))
    s))

(defun org-id-b36-to-int (s)
  "Convert a base-36 string into the corresponding integer."
  (let ((r 0))
    (mapc (lambda (i) (setq r (+ (* r 36) (org-id-b36-to-int-one-digit i))))
	  s)
    r))

(defun org-id-time-to-b36 (&optional time)
  "Encode TIME as a 12-digit string.
This string holds the time to micro-second accuracy, and can be decoded
using `org-id-decode'."
  ;; FIXME: If TIME represents N seconds after the epoch, then
  ;; this encoding assumes 0 <= N < 110075314176 = (* (expt 36 4) 65536),
  ;; i.e., that TIME is from 1970-01-01 00:00:00 to 5458-02-23 20:09:36 UTC.
  (setq time (org-time-convert-to-list nil))
  (concat (org-id-int-to-b36 (nth 0 time) 4)
	  (org-id-int-to-b36 (nth 1 time) 4)
	  (org-id-int-to-b36 (nth 2 time) 4)))

(defun org-id-decode (id)
  "Split ID into the prefix and the time value that was used to create it.
The return value is (prefix . time) where PREFIX is nil or a string,
and TIME is a Lisp time value (HI LO USEC)."
  (let (prefix time parts)
    (setq parts (org-split-string id ":"))
    (if (= 2 (length parts))
	(setq prefix (car parts) time (nth 1 parts))
      (setq prefix nil time (nth 0 parts)))
    (setq time (org-reverse-string time))
    (setq time (list (org-id-b36-to-int (substring time 0 4))
		     (org-id-b36-to-int (substring time 4 8))
		     (org-id-b36-to-int (substring time 8 12))))
    (cons prefix time)))

;; Storing ID locations (files)

;;;###autoload
(defun org-id-update-id-locations (&optional files silent)
  "Scan relevant files for IDs.
Store the relation between files and corresponding IDs.
This will scan all agenda files, all associated archives, and all
files currently mentioned in `org-id-locations'.
When FILES is given, scan also these files."
  (interactive)
  (unless org-id-track-globally
    (error "Please turn on `org-id-track-globally' if you want to track IDs"))
  (setq org-id-locations nil)
  (let* ((files
          (delete-dups
           (mapcar #'file-truename
                   (cl-remove-if-not
		    ;; Default `org-id-extra-files' value contains
		    ;; `agenda-archives' symbol.
		    #'stringp
		    (append
		     ;; Agenda files and all associated archives.
		     (org-agenda-files t org-id-search-archives)
		     ;; Explicit extra files.
		     (if (symbolp org-id-extra-files)
			 (symbol-value org-id-extra-files)
		       org-id-extra-files)
		     ;; All files known to have IDs.
		     org-id-files
		     ;; Additional files from function call.
		     files)))))
         (nfiles (length files))
         (id-regexp
	  (rx (seq bol (0+ (any "\t ")) ":ID:" (1+ " ") (not (any " ")))))
         (seen-ids nil)
         (ndup 0)
         (i 0))
    (with-temp-buffer
      (delay-mode-hooks
	(org-mode)
	(dolist (file files)
	  (when (file-exists-p file)
            (unless silent
              (cl-incf i)
              (message "Finding ID locations (%d/%d files): %s" i nfiles file))
	    (insert-file-contents file nil nil nil 'replace)
            (let ((ids nil)
		  (case-fold-search t))
              (org-with-point-at 1
		(while (re-search-forward id-regexp nil t)
		  (when (org-at-property-p)
                    (push (org-entry-get (point) "ID") ids)))
		(when ids
		  (push (cons (abbreviate-file-name file) ids)
			org-id-locations)
		  (dolist (id ids)
                    (cond
                     ((not (member id seen-ids)) (push id seen-ids))
                     (silent nil)
                     (t
                      (message "Duplicate ID %S" id)
                      (cl-incf ndup)))))))))))
    (setq org-id-files (mapcar #'car org-id-locations))
    (org-id-locations-save)
    ;; Now convert to a hash table.
    (setq org-id-locations (org-id-alist-to-hash org-id-locations))
    (when (and (not silent) (> ndup 0))
      (warn "WARNING: %d duplicate IDs found, check *Messages* buffer" ndup))
    (message "%d files scanned, %d files contains IDs, and %d IDs found."
             nfiles (length org-id-files) (hash-table-count org-id-locations))
    org-id-locations))

(defun org-id-locations-save ()
  "Save `org-id-locations' in `org-id-locations-file'."
  (when (and org-id-track-globally org-id-locations)
    (let ((out (if (hash-table-p org-id-locations)
		   (org-id-hash-to-alist org-id-locations)
		 org-id-locations)))
      (when (and org-id-locations-file-relative out)
	(setq out (mapcar
                   (lambda (item)
		     (if (file-name-absolute-p (car item))
		         (cons (file-relative-name
                                (car item) (file-name-directory
					    org-id-locations-file))
                               (cdr item))
                       item))
	           out)))
      (with-temp-file org-id-locations-file
	(let ((print-level nil)
	      (print-length nil))
	  (print out (current-buffer)))))))

(defun org-id-locations-load ()
  "Read the data from `org-id-locations-file'."
  (setq org-id-locations nil)
  (when org-id-track-globally
    (with-temp-buffer
      (condition-case nil
	  (progn
	    (insert-file-contents org-id-locations-file)
	    (setq org-id-locations (read (current-buffer)))
	    (let ((loc (file-name-directory org-id-locations-file)))
	      (mapc (lambda (item)
		      (unless (file-name-absolute-p (car item))
			(setf (car item) (expand-file-name (car item) loc))))
		    org-id-locations)))
	(error
	 (message "Could not read `org-id-values' from %s, setting it to nil"
		  org-id-locations-file))))
    (setq org-id-files (mapcar 'car org-id-locations))
    (setq org-id-locations (org-id-alist-to-hash org-id-locations))))

(defun org-id-add-location (id file)
  "Add the ID with location FILE to the database of ID locations."
  ;; Only if global tracking is on, and when the buffer has a file
  (unless file
    (error "`org-id-get' expects a file-visiting buffer"))
  (let ((afile (abbreviate-file-name file)))
    (when (and org-id-track-globally id)
      (unless org-id-locations (org-id-locations-load))
      (puthash id afile org-id-locations)
      (unless (member afile org-id-files)
	(add-to-list 'org-id-files afile)))))

(unless noninteractive
  (add-hook 'kill-emacs-hook 'org-id-locations-save))

(defun org-id-hash-to-alist (hash)
  "Turn an org-id hash into an alist.
This is to be able to write it to a file."
  (let (res x)
    (maphash
     (lambda (k v)
       (if (setq x (assoc v res))
	   (setcdr x (cons k (cdr x)))
	 (push (list v k) res)))
     hash)
    res))

(defun org-id-alist-to-hash (list)
  "Turn an org-id location list into a hash table."
  (let ((res (make-hash-table
	      :test 'equal
	      :size (apply '+ (mapcar 'length list))))
	f)
    (mapc
     (lambda (x)
       (setq f (car x))
       (mapc (lambda (i) (puthash i f res)) (cdr x)))
     list)
    res))

(defun org-id-paste-tracker (txt &optional buffer-or-file)
  "Update any ids in TXT and assign BUFFER-OR-FILE to them."
  (when org-id-track-globally
    (save-match-data
      (setq buffer-or-file (or buffer-or-file (current-buffer)))
      (when (bufferp buffer-or-file)
	(setq buffer-or-file (or (buffer-base-buffer buffer-or-file)
				 buffer-or-file))
	(setq buffer-or-file (buffer-file-name buffer-or-file)))
      (when buffer-or-file
	(let ((fname (abbreviate-file-name buffer-or-file))
	      (s 0))
	  (while (string-match "^[ \t]*:ID:[ \t]+\\([^ \t\n\r]+\\)" txt s)
	    (setq s (match-end 0))
	    (org-id-add-location (match-string 1 txt) fname)))))))

;; Finding entries with specified id

;;;###autoload
(defun org-id-find-id-file (id)
  "Query the id database for the file in which ID is located."
  (unless org-id-locations (org-id-locations-load))
  (or (and org-id-locations
	   (hash-table-p org-id-locations)
	   (gethash id org-id-locations))
      ;; Fall back on current buffer
      (buffer-file-name (or (buffer-base-buffer (current-buffer))
			    (current-buffer)))))

(defun org-id-find-id-in-file (id file &optional markerp)
  "Return the position of the entry ID in FILE.

If that files does not exist, or if it does not contain this ID,
return nil.

The position is returned as a cons cell (file-name . position).  With
optional argument MARKERP, return the position as a new marker."
  (cond
   ((not file) nil)
   ((not (file-exists-p file)) nil)
   (t
    (let* ((visiting (find-buffer-visiting file))
	   (buffer (or visiting (find-file-noselect file))))
      (unwind-protect
	  (with-current-buffer buffer
	    (let ((pos (org-find-entry-with-id id)))
	      (cond
	       ((null pos) nil)
	       (markerp (move-marker (make-marker) pos buffer))
	       (t (cons file pos)))))
	;; Remove opened buffer in the process.
	(unless (or visiting markerp) (kill-buffer buffer)))))))

;; id link type

;; Calling the following function is hard-coded into `org-store-link',
;; so we do have to add it to `org-store-link-functions'.

;;;###autoload
(defun org-id-store-link ()
  "Store a link to the current entry, using its ID.

If before first heading store first title-keyword as description
or filename if no title."
  (interactive)
  (when (and (buffer-file-name (buffer-base-buffer)) (derived-mode-p 'org-mode))
    (let* ((link (concat "id:" (org-id-get-create)))
	   (case-fold-search nil)
	   (desc (save-excursion
		   (org-back-to-heading-or-point-min t)
                   (cond ((org-before-first-heading-p)
                          (let ((keywords (org-collect-keywords '("TITLE"))))
                            (if keywords
                                (cadr (assoc "TITLE" keywords))
                              (file-name-nondirectory
			       (buffer-file-name (buffer-base-buffer))))))
		         ((looking-at org-complex-heading-regexp)
			  (if (match-end 4)
			      (match-string 4)
			    (match-string 0)))
                         (t link)))))
      (org-link-store-props :link link :description desc :type "id")
      link)))

(defun org-id-open (id _)
  "Go to the entry with id ID."
  (org-mark-ring-push)
  (let ((m (org-id-find id 'marker))
	cmd)
    (unless m
      (error "Cannot find entry with ID \"%s\"" id))
    ;; Use a buffer-switching command in analogy to finding files
    (setq cmd
	  (or
	   (cdr
	    (assq
	     (cdr (assq 'file org-link-frame-setup))
	     '((find-file . switch-to-buffer)
	       (find-file-other-window . switch-to-buffer-other-window)
	       (find-file-other-frame . switch-to-buffer-other-frame))))
	   'switch-to-buffer-other-window))
    (if (not (equal (current-buffer) (marker-buffer m)))
	(funcall cmd (marker-buffer m)))
    (goto-char m)
    (move-marker m nil)
    (org-show-context)))

(org-link-set-parameters "id" :follow #'org-id-open)

(provide 'org-id)

;; Local variables:
;; generated-autoload-file: "org-loaddefs.el"
;; End:

;;; org-id.el ends here