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+;;; docref.el -- Simple cross references for Elisp documentation strings
+;; Copyright (C) 1994 Free Software Foundation, Inc.
+
+;; Author: Vadim Geshel <vadik@unas.cs.kiev.ua>
+;; Created: 12 Jul 1994
+;; Keywords: docs, help, lisp
+;; original name was cross-ref.el.
+
+;; 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 2, 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; see the file COPYING.  If not, write to
+;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
+
+;;; Commentary:
+;;
+;; This package allows you to use a simple form of cross references in
+;; your Emacs Lisp documentation strings. Cross-references look like
+;; \\(type@[label@]data), where type defines a method for retrieving
+;; reference informatin, data is used by a method routine as an argument,
+;; and label "represents" the reference in text. If label is absent, data
+;; is used instead.
+;; 
+;; Special reference labeled `back', when present, can be used to return
+;; to the previous contents of help buffer.
+;;
+;; Cross-referencing currently is intended for use in doc strings only
+;; and works only in temporary buffers (created by `with-output-to-temp-buffer').
+;; List of temp buffers in which cross-referencing is to be active is specified
+;; by variable DOCREF-BUFFERS-LIST, which contains only "*Help*" by default.
+;;
+;; Documentation strings for this package's functions and variables can serve
+;; as examples of usage.
+;;
+;;; Customization:
+;; 
+;; See source. The main customization variable is `docref-methods-alist'.
+;; It consists of (type . function) pairs, where type is a string which
+;; corresponds to type in cross-references and function is called with
+;; one argument - reference `data' - when a reference is activated.
+;;
+;;; Installation:
+;;
+;; Place this file somewhere in your load-path, byte-compiled it, and add
+;; (require 'cross-ref)
+;; to your .emacs.
+
+;;; Code:
+
+;; User customizable variables
+
+(defvar docref-highlight-p t
+  "*If non-nil, \\(f@docref-subst) highlights cross-references.
+Under window system it highlights them with face defined by
+\\(v@docref-highlight-face), on character terminal highlighted references
+look like cross-references in info mode.")
+
+(defvar docref-highlight-face 'highlight
+  "*Face used to highlight cross-references (used by \\(f@docref-subst))")
+
+(defvar docref-methods-alist
+  '(("f" . docref-describe-function)    ; reference to a function documentation
+    ("v" . docref-describe-variable)    ; reference to a variable documentation
+    ("F" . docref-read-file)            ; reference to a file contents
+    ("s" . docref-use-string)           ; reference to a string 
+    ("V" . docref-use-variable-value)   ; reference to variable value
+    ("0" . beep))                       ; just highlighted text
+  "Alist which maps cross-reference ``types'' to retrieval functions.
+
+The car of each element is a string that serves as `type' in cross-references.
+\(See \\(f@docref-subst)).  The cdr is a function of one argument,
+to be called to find this reference.")
+
+(defvar docref-back-label "\nback"
+  "Label to use by \\(f@docref-subst) for the go-back reference.")
+
+(defvar docref-back-reference nil
+  "If non-nil, this is a go-back reference to add to the current buffer.
+The value specifies how to go back.  It should be suitable for use
+as the second argument to \\(f@docref-insert-label).
+\\(f@docref-subst) uses this to set up the go-back reference.")
+
+;;;###autoload
+(defun docref-setup ()
+  "Process docref cross-references in the current buffer.
+See also \\(f@docref-subst)."
+  (interactive)
+  (docref-subst (current-buffer))
+  (docref-mode))
+
+(defvar docref-mode-map nil)
+(or docref-mode-map
+    (let ((map (make-sparse-keymap)))
+      (define-key map [mouse-2] 'docref-follow-mouse)
+      (define-key map "\C-c\C-b" 'docref-go-back)
+      (define-key map "\C-c\C-c" 'docref-follow)
+      (setq docref-mode-map map)))
+
+(defun docref-mode ()
+  "Major mode for help buffers that contain cross references.
+To follow a reference, move to it and type \\[docref-follow], or use
+\\[docref-follow-mouse].  The command \\[docref-go-back] can used to go
+back to where you came from."
+  (interactive)
+  (kill-all-local-variables)
+  (setq major-mode 'docref-mode)
+  (setq mode-name "Docref")
+  (use-local-map docref-mode-map)
+  (run-hooks 'docref-mode))
+
+(defun docref-subst (buf)
+  "Parse documentation cross-references in buffer BUF.
+
+Find cross-reference information in a buffer and
+highlight them with face defined by \\(v@docref-highlight-face).
+
+Cross-reference has the following format: \\ (TYPE[@LABEL]@DATA), where
+TYPE defines method used to retrive xref data (like reading from file or
+calling \\(f@describe-function)), DATA is an argument to this method
+\(like file name or function name), and LABEL is displayed in text using
+\\(v@docref-highlight-face).
+
+The special reference `back' can be used to return back.
+The variable \\(v@docref-back-label) specifies the label to use for that.
+
+See \\(v@docref-methods-alist) for currently defined methods."
+  (interactive "b")
+  (save-excursion
+    (set-buffer buf)
+    (goto-char (point-min))
+    ;; The docref-seen property indicates that we have processed this
+    ;; buffer's contents already, so don't do it again.
+    (if (not (get-text-property (point-min) 'docref-seen))
+        (let ((old-modified (buffer-modified-p)))
+          (while (re-search-forward "[\\](\\([^\)\@]+\\)\\(@[^\)\@]+\\)?@\\([^\)]*\\))"
+                                    nil t)
+            (let* ((start (match-beginning 0))
+                   (type (buffer-substring (match-beginning 1) (match-end 1)))
+                   (data (buffer-substring (match-beginning 3) (match-end 3)))
+                   (label
+                    (if (match-beginning 2)
+                        (buffer-substring (+ (match-beginning 2) 1) (match-end 2))
+                      data)))
+              (replace-match "" t)
+              (docref-insert-label label (cons type data))))
+
+          ;; Make a back-reference in this buffer, if desired.
+          ;; (This is true if called from docref-follow.)
+          (if docref-back-reference
+              (progn
+                (goto-char (point-max))
+                (put-text-property (point-min) (1+ (point-min))
+                                   'docref-back-position (point))
+                (docref-insert-label docref-back-label docref-back-reference)))
+          (put-text-property (point-min) (1+ (point-min)) 'docref-seen t)
+          (set-buffer-modified-p old-modified)))))
+
+(defun docref-insert-label (string ref)
+  (let ((label (concat string))
+        (pos (point)))
+    ;; decorate the label
+    (let ((leading-space-end (save-match-data
+                               (if (string-match "^\\([ \t\n]+\\)" label)
+                                   (match-end 1)
+                                 0)))
+          (trailing-space-start (save-match-data
+                                  (if (string-match "\\([ \t\n]+\\)$" label)
+                                      (match-beginning 1)
+                                    (length label)))))
+      (if docref-highlight-p          
+          (if (not window-system)
+              (setq label
+                    (concat (substring label 0 leading-space-end)
+                            "(*note "
+                            (substring label leading-space-end trailing-space-start)
+                            ")"
+                            (substring label trailing-space-start)))
+            ;; window-system
+            (put-text-property leading-space-end
+                               trailing-space-start
+                               'face docref-highlight-face label)))
+      (put-text-property 0 (length label) 'docref ref label)
+      (insert label))))
+
+(defun docref-follow-mouse (click)
+  "Follow the cross-reference that you click on."
+  (interactive "e")
+  (save-excursion
+    (let* ((start (event-start click))
+           (window (car start))
+           (pos (car (cdr start)))
+           (docref-last-active-buffer (current-buffer)))
+      (set-buffer (window-buffer window))
+      (docref-follow pos))))
+
+(defun docref-go-back ()
+  "Go back to the previous contents of help buffer."
+  (interactive)
+  (let ((pos (get-text-property (point-min) 'docref-back-position)))
+    (if pos
+        (docref-follow pos)
+      (error "No go-back reference"))))
+
+(defun docref-follow (&optional pos)
+  "Follow cross-reference at point.
+For the cross-reference format, see \\(f@docref-subst).
+The special reference named `back' can be used to return back"
+  (interactive)
+  (or pos (setq pos (point)))
+  (let ((docref-data (get-text-property pos 'docref)))
+    (if docref-data
+        ;; There is a reference at point.  Follow it.
+        (let* ((type (car docref-data))
+               (name (cdr docref-data))
+               (method (assoc type docref-methods-alist))
+               (cur-contents (buffer-string))
+               (opoint (point))
+               (docref-back-reference (cons "s" cur-contents))
+               success)
+          (if (null method)
+              (error "Unknown cross-reference type: %s" type))
+          (unwind-protect
+              (save-excursion
+                (funcall (cdr method) name)
+                (setq success t))
+            (or success
+                (progn
+                  ;; (cdr method) got an error.
+                  ;; Put back the text that we had.
+                  (erase-buffer)
+                  (insert cur-contents)
+                  (goto-char opoint)))
+            (set-buffer-modified-p nil))))))
+
+;; Builtin methods for accessing a reference.
+
+(defun docref-describe-function (data)
+  (save-excursion
+    (if (boundp 'docref-last-active-buffer)
+        (set-buffer docref-last-active-buffer))
+    (describe-function (intern data))))
+  
+(defun docref-describe-variable (data)
+  (save-excursion
+    (if (boundp 'docref-last-active-buffer)
+        (set-buffer docref-last-active-buffer))
+    (describe-variable (intern data))))
+
+(defun docref-read-file (data)
+  (with-output-to-temp-buffer (buffer-name)
+    (erase-buffer)
+    (insert-file-contents (expand-file-name data))))
+
+(defun docref-use-string (data)
+  (with-output-to-temp-buffer (buffer-name)
+    (erase-buffer)
+    (insert data)))
+
+(defun docref-use-variable-value (data)
+  (let ((sym (intern data)))
+    (with-output-to-temp-buffer (buffer-name)
+      (erase-buffer)
+      (princ (symbol-value sym)))))
+
+(provide 'docref)
+
+;;; docref.el ends here
+