;;; rudel.el --- A collaborative editing framework for Emacs  -*- lexical-binding:t -*-
;;
;; Copyright (C) 2018-2021  Free Software Foundation, Inc.
;;
;; Author: Jan Moringen <scymtym@users.sourceforge.net>
;; Keywords: rudel, collaboration
;; Version: 0.3.2
;; URL: http://rudel.sourceforge.net/
;; Package-Requires: ((emacs "24") (cl-lib "0.5") (cl-generic "0.3") (cl-print "1.0"))
;; X-RCS: $Id:$
;;
;; This file is part of Rudel.
;;
;; Rudel 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.
;;
;; Rudel 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 rudel. If not, see <http://www.gnu.org/licenses>.


;;; Commentary:

;; Rudel is a framework for collaborative editing in Emacs.  Its
;; architecture allows communication with arbitrary collaborative
;; editors.

;;;; Quickstart guide

;; In a first Emacs session, do:
;; - M-x rudel-host-session RET RET
;; Then in the same or another session, do:
;; - M-x rudel-join-session RET
;;   obby RET tcp RET localhost RET 6522 RET n user1 RET red RET RET RET
;; - M-x rudel-publish-buffer RET
;; And finally in another Emacs session, do:
;; - M-x rudel-join-session RET
;;   obby RET tcp RET localhost RET 6522 RET n user2 RET green RET RET RET
;; - M-x rudel-subscribe RET TAB RET


;;; History:
;;
;; 0.2 - Support for transports
;;
;; 0.1 - Initial version


;;; Code:
;;

(require 'cl-lib)

(require 'cl-generic)
(require 'eieio)
(require 'eieio-base)

(require 'eieio-speedbar) ;; TODO required for now

(require 'rudel-util)
(require 'rudel-backend)
(require 'rudel-session-initiation)
(require 'rudel-operations)
(require 'rudel-operators)
(require 'rudel-overlay)
(require 'rudel-hooks)
(require 'rudel-interactive) ;; `rudel-read-document',
			     ;; `rudel-read-session'
(require 'rudel-icons)

(load "rudel-loaddefs") ;; Internal autoloads.


;;; Global variables
;;

(defconst rudel-version '(0 3)
  "Version of the Rudel framework.")

(defvar rudel-current-session nil
  "Global object representing the current Rudel session.
nil if there is no active session.")

(defvar rudel-buffer-document nil
  "Buffer-local variable which holds the Rudel document associated with the buffer.")
(make-variable-buffer-local 'rudel-buffer-document)
(put 'rudel-buffer-document 'permanent-local t)

(defvar rudel-buffer-change-workaround-data nil
  "Change data that could not be accessed otherwise.
It would be nice to find another way to do this.")
(make-variable-buffer-local 'rudel-buffer-change-workaround-data)
(put 'rudel-buffer-change-workaround-data 'permanent-local t)


;;; Customization
;;

(defgroup rudel nil
  "Rudel collaborative editing framework."
  :group 'applications
  :group 'communication
  :link  '(emacs-library-link :tag "Lisp File" "rudel.el")
  :link  '(url-link :tag "Project Homepage"
		    "http://rudel.sourceforge.net")
  :link  '(url-link :tag "Report Issues"
		    "http://sourceforge.net/tracker/?group_id=249139")
  :link  '(url-link :tag "Ask Questions (Mailing List)"
		    "http://sourceforge.net/mail/?group_id=249139")
  :link  '(url-link :tag "Ask Questions (EmacsWiki)"
		    "http://www.emacswiki.org/emacs/Rudel"))

(defcustom rudel-allocate-buffer-function
  'rudel-allocate-buffer-clear-existing
  "A function used to find or create buffers to associate to documents.
The function is called with the document name as the sole
argument and has to return a buffer object which will be attached
to the document in question."
  :type    '(choice (const    :tag "Clear content of existing buffer"
			      rudel-allocate-buffer-clear-existing)
		    (const    :tag "Create a new uniquely named buffer"
			      rudel-allocate-buffer-make-unique)
		    (function :tag "Other function"))
  :require 'rudel-interactive)

(defcustom rudel-default-username (user-login-name)
  "*"
  :type  '(string))


;;; Class rudel-session
;;

(defclass rudel-session (rudel-hook-object)
  ((backend              :initarg  :backend
			 :type     rudel-backend-child
			 :documentation
			 "The backend used by this session.")
   (users                :initarg  :users
			 :type     list
			 :initform nil
			 :documentation
			 "The list of users participating in this session.")
   (documents            :initarg  :documents
			 :type     list
			 :initform nil
			 :documentation
			 "This list of documents available in this session.")
   ;; Hooks
   (end-hook             :initarg  :end-hook
			 :type     list
			 :initform nil
			 :documentation
			 "")
   (add-user-hook        :initarg  :add-user-hook
			 :type     list
			 :initform nil
			 :documentation
			 "This hook is run when a user gets added
to the session.
The arguments are the session and the user object.")
   (remove-user-hook     :initarg  :remove-user-hook
			 :type     list
			 :initform nil
			 :documentation
			 "This hook is run when a user gets
removed from the session.
The arguments are the session and the user object.")
   (add-document-hook    :initarg  :add-document-hook
			 :type     list
			 :initform nil
			 :documentation
			 "This hook is run when a document gets
added to the session.
The arguments are the session and the document object.")
   (remove-document-hook :initarg  :remove-document-hook
			 :type     list
			 :initform nil
			 :documentation
			 "This hook is run when a document gets
removed from the session.
The arguments are the session and the document object."))
  "This class serves as a base class for rudel-client-session and
rudel-server-session. Consequently, it consists of slots common
to client and server sessions."
  :abstract t)

(cl-defmethod rudel-end ((this rudel-session))
  "Terminate THIS session performing all necessary cleanup."
  ;; Run the hook.
  (object-run-hook-with-args this 'end-hook))

(cl-defmethod rudel-add-user ((this rudel-session) user)
  "Add USER to the user list of THIS session.

Runs object hook (see `rudel-hook-object') `add-user-hook' with
arguments THIS and USER."
  ;; Add USER to list.
  (object-add-to-list this :users user)

  ;; Run the hook.
  (object-run-hook-with-args this 'add-user-hook user))

(cl-defmethod rudel-remove-user ((this rudel-session) user)
  "Remove USER from the user list of THIS session.

Runs object hook (see `rudel-hook-object') `remove-user-hook'
with arguments THIS and USER."
  ;; Remove USER from list.
  (object-remove-from-list this :users user)

  ;; Run the hook.
  (object-run-hook-with-args this 'remove-user-hook user))

(cl-defmethod rudel-find-user ((this rudel-session)
			    which &optional test key)
  "Find user WHICH in the user list.
WHICH is compared to the result of KEY using TEST."
  (with-slots (users) this
    (cl-find which users
	  :key  (or key  #'object-name-string)
	  :test (or test #'string=))))

(cl-defmethod rudel-add-document ((this rudel-session) document)
  ""
  (unless (slot-boundp document :session)
    (oset document :session this))

  ;; Add DOCUMENT to the list of documents in THIS session.
  (object-add-to-list this :documents document)

  ;; Run the hook.
  (object-run-hook-with-args this 'add-document-hook document))

(cl-defmethod rudel-remove-document ((this rudel-session) document)
  "Remove DOCUMENT from THIS session, detaching it if necessary."
  ;; Detach document from its buffer when necessary.
  (rudel-maybe-detach-from-buffer document)

  ;; Remove DOCUMENT from the list of documents in THIS session.
  (object-remove-from-list this :documents document)

  ;; Run the hook.
  (object-run-hook-with-args this 'remove-document-hook document))

(cl-defmethod rudel-find-document ((this rudel-session)
				which &optional test key)
  "Find document WHICH in the document list.
WHICH is compared to the result of KEY using TEST."
  (with-slots (documents) this
    (cl-find which documents
	  :key  (or key  #'object-name-string)
	  :test (or test #'string=))))


;;; Class rudel-client-session
;;
(defclass rudel-client-session (rudel-session)
  ((connection :initarg  :connection
	       :type     (or null rudel-connection-child)
	       :initform nil
	       :documentation
	       "The connection used for communication by this
session.")
   (self       :initarg  :self
	       :type     (or null rudel-user-child)
	       :initform nil
	       :reader   rudel-self
	       :writer   rudel-set-self
	       :documentation
	       "Points into USERS to the user object representing
the local user"))
  "Objects represent a collaborative editing session from a
client perspective.")

(cl-defmethod rudel-end ((this rudel-client-session))
  ;; Clean everything up
  (with-slots (connection users documents) this
    ;; Make sure all documents are detached from their buffers
    (mapc #'rudel-maybe-detach-from-buffer documents)

    ;; Terminate the connection
    (when connection
      (condition-case nil
	  (rudel-disconnect connection)
	(error nil))))

  ;;
  (cl-call-next-method)
  )

(cl-defmethod rudel-unsubscribed-documents ((this rudel-client-session))
  "Return documents in THIS to which the self user is not subscribed."
  (with-slots (documents self) this
    (unless self
      (error "Cannot find unsubscribed documents without self user"))

    (cl-remove-if
     (lambda (document)
       ;; FIXME: Move this use of the slot to after the class that defines it.
       (eieio-declare-slots subscribed)
       (with-slots (subscribed) document
	 (memq self subscribed)))
     documents))
  )


;;; Class rudel-server-session
;;

(defclass rudel-server-session (rudel-session)
  ()
  "Class rudel-server-session "
  :abstract t)


;;; Class rudel-connection
;;

(defclass rudel-connection ()
  ((session :initarg :session
	    :type    rudel-session-child
	    :documentation
	    ""))
  "This abstract class defines the interface implementations of
client protocols have to obey."
  :abstract t)

(cl-defgeneric rudel-disconnect ((this rudel-connection))
  "Close the connection.")

(cl-defgeneric rudel-change-color- ((this rudel-connection) color) ;; TODO name
  "")

(cl-defgeneric rudel-publish ((this rudel-connection) document)
  "")

(cl-defgeneric rudel-subscribe-to ((this rudel-connection) document)
  "")

(cl-defgeneric rudel-unsubscribe-from ((this rudel-connection) document) ; TODO name should be rudel-unsubscribe
  "")

(cl-defgeneric rudel-local-insert ((this rudel-connection))
  "")

(cl-defgeneric rudel-local-delete ((this rudel-connection))
  "")

(cl-defgeneric rudel-remote-insert ((this rudel-connection))
  "")

(cl-defgeneric rudel-remote-delete ((this rudel-connection))
  "")


;;; Class rudel-user
;;

(defclass rudel-user (eieio-named
		      eieio-speedbar-file-button
		      rudel-hook-object)
  ((color       :initarg  :color
		:reader   rudel-color
		:writer   rudel-set-color
		:documentation
		"Color used to indicate ownership or authorship
by the user. Examples includes text written by the user or the
user name itself.")
   ;; Hooks
   (change-hook :initarg  :change-hook
		:type     list
		:initform nil
		:documentation
		"This hook is run when this user object
changes."))
  "Objects of this class represent users participating in
collaborative editing session. Note that a participating user
does not have to be connected to the session at any given time."
  :abstract t)

(cl-defmethod rudel-change-notify ((this rudel-user))
  "Run change hook of THIS after slot values have changed."
  (object-run-hook-with-args this 'change-hook))


;;; Class rudel-document
;;

(defclass rudel-document (eieio-named
			  eieio-speedbar-file-button
			  rudel-hook-object)
  ((session          :initarg  :session
		     :type     rudel-session
		     :documentation
		     "")
   (buffer           :initarg  :buffer
		     :type     (or null buffer)
		     :initform nil
		     :documentation
		     "")
   (subscribed       :initarg  :subscribed
		     :type     list
		     :initform nil
		     :documentation
		     "")
   ;; Hooks
   (subscribe-hook   :initarg  :subscribe-hook
		     :type     list
		     :initform nil
		     :documentation
		     "This hook is run when a user subscribes to
this document object.")
   (unsubscribe-hook :initarg  :unsubscribe-hook
		     :type     list
		     :initform nil
		     :documentation
		     "This hook is run when a user unsubscribes
from this document object.")
   (attach-hook      :initarg  :attach-hook
		     :type     list
		     :initform nil
		     :documentation
		     "This hook is run when a buffer is attached
to this document object.")
   (detach-hook      :initarg  :detach-hook
		     :type     list
		     :initform nil
		     :documentation
		     "This hook is run when the attached buffer
is detached from this document object."))
  "This class represents a document, which participants of a
collaborative editing session can subscribe to."
  :abstract t)

(cl-defmethod rudel-unique-name ((this rudel-document))
  "Returns a suggested name for the buffer attached to THIS document."
  (object-name-string this))

(cl-defmethod rudel-suggested-buffer-name ((this rudel-document))
  "Returns a suggested name for the buffer attached to THIS document."
  (rudel-unique-name this))

(cl-defmethod rudel-attached-p ((this rudel-document))
  (with-slots (buffer) this
    buffer))

(cl-defmethod rudel-attach-to-buffer ((this rudel-document) buffer)
  "Attach THIS document to BUFFER"
  (with-slots ((doc-buffer buffer)) this
    ;; Set buffer slot of THIS to BUFFER and associated THIS with
    ;; BUFFER.
    (setq doc-buffer buffer)
    (rudel-set-buffer-document this buffer)

    (with-current-buffer doc-buffer
      ;; Add the handler function for buffer changes to the buffer's
      ;; change hook.
      (add-hook 'after-change-functions
		#'rudel-handle-buffer-change
		nil t)

      ;; Store change data before the change a done. This is necessary
      ;; because the number of bytes (not characters) cannot otherwise
      ;; be recovered after a deletion.
      (add-hook 'before-change-functions
		#'rudel-buffer-change-workaround
		nil t)

      ;; Add a handler to the kill-buffer hook to unsubscribe from the
      ;; document when the buffer gets killed.
      (add-hook 'kill-buffer-hook
		#'rudel-unsubscribe
		nil t)

      ;;
      (add-hook 'change-major-mode-hook
		#'rudel-handle-major-mode-change
		nil t))

    ;; Run the hook.
    (object-run-hook-with-args this 'attach-hook doc-buffer))
  )

(cl-defmethod rudel-detach-from-buffer ((this rudel-document))
  "Detach document THIS from its buffer.
Do nothing, if THIS is not attached to any buffer."
  (with-slots (buffer) this
    (let ((buffer-save buffer))

      (with-current-buffer buffer
	;; Remove our handler function from the kill-buffer hook.
	(remove-hook 'kill-buffer-hook
		     #'rudel-unsubscribe
		     t)

	;; Remove our handler function from the after-change hook.
	(remove-hook 'after-change-functions
		     #'rudel-handle-buffer-change
		     t)

	;; Remove our handler function from the before-change hook.
	(remove-hook 'before-change-functions
		     #'rudel-buffer-change-workaround
		     t)

	;; Remove all overlays.
	(rudel-overlays-remove-all)

	;; Remove the major mode change handler.
	(remove-hook 'change-major-mode-hook
		     #'rudel-handle-major-mode-change
		     t))

      ;; Unset buffer slot of THIS and delete association of THIS with
      ;; BUFFER.
      (rudel-set-buffer-document nil buffer)
      (setq buffer nil)

      ;; Run the hook.
      (object-run-hook-with-args this 'detach-hook buffer-save)))
  )

(cl-defmethod rudel-maybe-detach-from-buffer ((this rudel-document))
  ""
  ;; Only try to detach from BUFFER, if it is non-nil. BUFFER can be
  ;; nil, if the user did not subscribe to the document, or
  ;; unsubscribed after subscribing.
  (when (rudel-attached-p this)
    (rudel-detach-from-buffer this)))

(cl-defmethod rudel-add-user ((this rudel-document) user)
  "Add USER to the list of subscribed users of THIS.

Runs object hook (see `rudel-hook-object') `subscribe-hook' with
arguments THIS and USER."
  ;; Add USER to list.
  (object-add-to-list this :subscribed user)

  ;; Run the hook.
  (object-run-hook-with-args this 'subscribe-hook user))

(cl-defmethod rudel-remove-user ((this rudel-document) user)
  "Remove USER from the list of subscribed users of THIS.

Runs object hook (see `rudel-hook-object') `unsubscribe-hook'
with arguments THIS and USER."
  ;; Remove USER from list.
  (object-remove-from-list this :subscribed user)

  ;; Run the hook.
  (object-run-hook-with-args this 'unsubscribe-hook user))

(cl-defmethod rudel-clear-users ((this rudel-document))
  "Clear list of users subscribed to THIS."
  (oset this :subscribed nil))

(cl-defmethod rudel-find-user ((this rudel-document)
			    which &optional test key)
  "Find user WHICH in the list of subscribed users.
WHICH is compared to the result of KEY using TEST."
  (with-slots (subscribed) this
    (cl-find which subscribed
	  :key  (or key  #'object-name-string)
	  :test (or test #'string=))))

(cl-defmethod rudel-insert ((this rudel-document) position data)
  "Insert DATA at POSITION into the buffer attached to THIS.
When POSITION is nil `point-max' is used to determine the
insertion position.
Modification hooks are disabled during the insertion."
  (with-slots (buffer) this
    (with-current-buffer buffer

      (unless position
	(setq position (- (point-max) 1)))

      (save-excursion
	(let ((inhibit-modification-hooks t))
	  (goto-char (+ position 1))
	  (insert data)))))
  )

(cl-defmethod rudel-delete ((this rudel-document) position length)
  "Delete a region of LENGTH character at POSITION from the buffer attached to THIS.
Modification hooks are disabled during the insertion."
  (with-slots (buffer) this
    (with-current-buffer buffer
      (save-excursion
	(let ((inhibit-modification-hooks t))
	  (delete-region (+ position 1) (+ position length 1))))))
  )

(cl-defmethod rudel-local-operation ((this rudel-document) operation)
  "Apply the local operation OPERATION to THIS."
  (with-slots (session buffer) this
    (with-slots (connection (user self)) session
      (dolist (operators (list

			   ;; Update overlays
			   (rudel-overlay-operators
			    :document this
			    :user     user)

			   ;; Notify connection
			   (rudel-connection-operators
			    :connection connection
			    :document   this)))

	;; Apply the operation using each set of operators
	(rudel-apply operation operators))))
  )

(cl-defmethod rudel-remote-operation ((this rudel-document) user operation)
  "Apply the remote operation OPERATION performed by USER to THIS."
  (dolist (operators (append

		       ;; Update buffer contents
		       (list (rudel-document-operators
			      :document this))

		       ;; Update overlays
		       (when user
			 (list (rudel-overlay-operators
				:document this
				:user     user)))))

    ;; Apply the operation using each set of operators
    (rudel-apply operation operators))
  )

(cl-defmethod rudel-chunks ((this rudel-document))
  "Return a list of text chunks of the associated buffer.
Each element in the chunk is a list structured like this (START
END AUTHOR). START and END are numbers, AUTHOR is of type (or
null rudel-user-child)."
  (with-slots (buffer) this
    ;; Extract buffer string and a list of chunks partitioning the
    ;; string according to the respective author (or nil).
    (with-current-buffer buffer
      (let ((string         (buffer-string)) ;; TODO no-properties?
	    (overlay-chunks (mapcar
			     (lambda (overlay)
			       (list (- (overlay-start overlay) 1)
				     (- (overlay-end   overlay) 1)
				     (rudel-overlay-user overlay)))
			     (cl-sort (rudel-author-overlays)
				    #'< :key #'overlay-start)))
	    (last)
	    (augmented-chunks))

	;; Iterate through the list of chunks to find gaps between
	;; chunks (also before the first) and insert entries with
	;; author nil accordingly.
	(dolist (chunk overlay-chunks)
	  (when (or (and (not last)
			 (> (nth 0 chunk) 0))
		    (and last
			 (/= (nth 1 last)
			     (nth 0 chunk))))
	    (push (list (if last (nth 1 last) 0)
			(nth 0 chunk)
			nil)
		  augmented-chunks))
	  (push chunk augmented-chunks)
	  (setq last chunk))

	;; If there is text after the last chunk, create another one
	;; with author nil. If there were no chunks at all, this chunk
	;; can also cover the whole buffer string.
	(when (or (and (not last)
		       (/= (length string) 0))
		  (and last
		       (/= (nth 1 last) (length string))))
	  (push (list (if last (nth 1 last) 0)
		      (length string)
		      nil)
		augmented-chunks))

	;; Sort chunks according to the start position.
	(cl-sort augmented-chunks #'< :key #'car))))
  )


;;; Buffer-related functions
;;

(defun rudel-buffer-has-document-p (&optional buffer)
  "Return non-nil if a document object is attached to BUFFER.
If BUFFER is nil, use the current buffer."
  (unless buffer
    (setq buffer (current-buffer)))

  (buffer-local-value 'rudel-buffer-document buffer))

(defun rudel-buffer-document (&optional buffer)
  "Return the document object attached to BUFFER.
If BUFFER is nil, use the current buffer."
  (unless buffer
    (setq buffer (current-buffer)))

  (buffer-local-value 'rudel-buffer-document buffer))

(defun rudel-set-buffer-document (document &optional buffer)
  "Associate BUFFER to DOCUMENT.
If DOCUMENT is nil, make it not associated to any buffer.
If BUFFER is nil, use the current buffer."
  (unless buffer
    (setq buffer (current-buffer)))

  (with-current-buffer buffer
    (setq rudel-buffer-document document)))

(defun rudel-handle-buffer-change (from to length)
  "Handle buffer change at range FROM to TO with length LENGTH by relaying them to the document object of the buffer.
See `after-change-functions' for more information."
  (when (rudel-buffer-has-document-p)
    (let ((document (rudel-buffer-document))
	  (text)) ; TODO with-rudel-buffer-document?
      (cond
       ;; The change was an insert
       ((and (/= from to)
	     (zerop length))
	(with-slots (buffer) document
	  (with-current-buffer buffer
	    (setq text (buffer-substring-no-properties from to)))
	  (rudel-local-operation document
				 (rudel-insert-op
				  :from (- from 1)
				  :data text))))

       ;; The change was a delete
       ((and (= from to)
	     (not (zerop length)))
	(rudel-local-operation document
			       (rudel-delete-op
				:from   (- from 1)
				:length length)))

       ;; The operation was neither an insert nor a delete. This seems
       ;; to mean that the region has changed arbitrarily. The only
       ;; option we have is sending a delete and corresponding insert
       ;; message that emulate the change.
       (t
	(with-slots (buffer) document
	  (with-current-buffer buffer
	    (setq text (buffer-substring-no-properties from to)))
	  (rudel-local-operation document
				 (rudel-delete-op
				  :from   (- from 1)
				  :length length))
	  (rudel-local-operation document
				 (rudel-insert-op
				  :from (- from 1)
				  :data text)))))))
  )

(defun rudel-buffer-change-workaround (from to)
  (when (/= from to)
    (setq rudel-buffer-change-workaround-data
	  (list from to
		(buffer-substring-no-properties from to)))))


;;; Protection against major mode changes
;;

(defvar rudel-mode-changed-buffers nil
  "List of buffers that may need to be repaired after a major
mode change.")

(defun rudel-handle-major-mode-change ()
  "Store the current buffer to repair damage done by major mode change.

Note: The way this works is inspired by mode-local.el by David
Ponce and Eric M. Ludlam."
  ;; Store the buffer for later repair.
  (add-to-list 'rudel-mode-changed-buffers (current-buffer))

  ;; Schedule `rudel-after-major-mode-change' to run after the
  ;; command, that caused the major mode change.
  (add-hook 'post-command-hook
	    #'rudel-after-major-mode-change)
  )

(defun rudel-after-major-mode-change ()
  "Repair damage done by major mode changes.
As a function in `post-command-hook', this is run after there was
a `major-mode' change.

Note: The way this works is inspired by mode-local.el by David
Ponce and Eric M. Ludlam."
  ;; Remove this function from `post-command-hook'.
  (remove-hook 'post-command-hook
	       #'rudel-after-major-mode-change)

  ;; Repair all buffers affected by the major mode change.
  (dolist (buffer rudel-mode-changed-buffers)
    (when (buffer-live-p buffer)
      (let ((document (buffer-local-value
		       'rudel-buffer-document buffer)))
	(rudel-attach-to-buffer document buffer))))
  (setq rudel-mode-changed-buffers nil)
  )


;;; Interactive functions
;;

;;;###autoload
(defun rudel-join-session (info)
  "Join the collaborative editing session described by INFO.
INFO is a property list that describes the collaborative editing
session in terms of properties like :host, :port
and :encryption. The particular properties and their respective
meanings depend on the used backend.

When called interactively, all data required to join a session
will be prompted for."
  (interactive
   ;; Try the discover method of session initiation backends to find
   ;; available sessions.
   (list
    (let ((info)
	  (session-initiation-backend))
      ;; Query the chosen backend until session info is available.
      (while (not info)
	(message "Discovering Sessions ...")
	(let* ((sessions   (rudel-session-initiation-discover
			    session-initiation-backend))
	       (maybe-info (if (= (length sessions) 1)
			       (car sessions)
			     (rudel-read-session
			      sessions "Choose Session: " 'object))))
	  (if (rudel-backend-cons-p maybe-info)
	      (setq session-initiation-backend (car maybe-info))
	    (setq info maybe-info))))
      info)))

  ;; Some sanity checks on INFO.
  (unless (member :name info)
    (setq info (plist-put info :name "rudel session")))

  (dolist (property '(:transport-backend :protocol-backend))
    (unless (member property info)
      (signal 'rudel-incomplete-info (list property))))

  ;; First, create the session object.
  (let* ((session-name      (plist-get info :name))
	 (transport-backend (cdr (plist-get info :transport-backend)))
	 (protocol-backend  (cdr (plist-get info :protocol-backend)))
	 (session           (rudel-client-session
			     session-name
			     :backend protocol-backend))
	 (transport)
	 (connection))

    ;; Add the session object to INFO.
    (setq info (plist-put info :session session))

    ;; Create transport object and connection
    (setq transport  (rudel-make-connection
		      transport-backend
		      info #'rudel-ask-connect-info
		      (rudel-make-state-progress-callback "Connecting ")))
    (setq connection (rudel-connect
		      protocol-backend transport
		      info #'rudel-ask-connect-info
		      (rudel-make-state-progress-callback "Joining ")))
    (oset session :connection connection)

    ;; Store the new session object globally.
    (setq rudel-current-session session)

    ;; Reset the global session variable when the session ends.
    (object-add-hook session 'end-hook
		     (lambda (_session)
		       (setq rudel-current-session nil)))

    ;; Run the hook.
    (run-hook-with-args 'rudel-session-start-hook session)

    session))

;;;###autoload
(defun rudel-host-session (info)
  "Host a collaborative editing session described by INFO.
INFO is a property list that describes the collaborative editing
session to be created in terms of properties like :address, :port
and :encryption. The particular properties and their respective
meanings depend on the used backend.

When called interactively, all data required to host a session
will be prompted for."
  (interactive
   (list
    ;; If necessary, ask the user for the backend we should use.
    (let ((transport-backend (rudel-backend-choose
				   'transport
				   (lambda (backend)
				     (rudel-capable-of-p backend 'listen))))
	  (protocol-backend  (rudel-backend-choose
				   'protocol
				   (lambda (backend)
				     (rudel-capable-of-p backend 'host)))))
    (rudel-ask-host-info
     (cdr protocol-backend)
     (list :transport-backend transport-backend
	   :protocol-backend  protocol-backend)))))

  ;; Some sanity checks on INFO.
  (dolist (property '(:transport-backend :protocol-backend))
    (unless (member property info)
      (signal 'rudel-incomplete-info (list property))))

  ;; Create the session object.
  (let ((transport-backend (cdr (plist-get info :transport-backend)))
	(protocol-backend  (cdr (plist-get info :protocol-backend)))
	(listener)
	(session))

    ;; TODO temporary solution
    (setq info (rudel-ask-host-info protocol-backend info))

    ;; Create the listener and session object.
    (setq listener (rudel-wait-for-connections
		    transport-backend info #'rudel-ask-listen-info))
    (setq session  (rudel-host protocol-backend listener info))

    ;; Advertise the new session using registered session initiation
    ;; mechanisms.
    ;; TODO not sure whether here is the right place for this
    (rudel-session-initiation-advertise info)

    session))

;;;###autoload
(defun rudel-leave-session ()
  "Leave the current collaborative editing session."
  (interactive)
  (unless rudel-current-session
    (error "No active Rudel session"))

  ;; Close the connection.
  (rudel-end rudel-current-session)
  )

;;;###autoload
(defun rudel-change-color ()
  "Change the color associated with the local user.
Not all backends support this operation."
  (interactive)
  ;; Make sure we have a session.
  (unless rudel-current-session
    (error "No active Rudel session"))

  (with-slots (backend connection self) rudel-current-session
    ;; Make sure the backend can change colors.
    (unless (rudel-capable-of-p backend 'change-color)
      (error "Backend `%s' cannot change colors"
	     (object-name-string backend)))

    (with-slots ((name object-name) color) self
      ;; Ask the user for a new color.
      (setq color (read-color "New Color: " t))

      ;; Tell the connection to announce the change and change it in
      ;; our user object.
      (rudel-change-color- connection color)

      ;; Run the change hook.
      (object-run-hook-with-args self 'change-hook)

      ;; Update overlay color.
      (rudel-overlay-set-face-attributes
       (rudel-overlay-make-face-symbol 'author name)
       color)))
  )

;;;###autoload
(defun rudel-subscribe (document)
  "Subscribe to DOCUMENT offered by a peer in a collaborative editing session.
When called interactively, DOCUMENT is prompted for interactively."
  (interactive
   (list (progn
	   ;; We have to retrieve the document list from an active
	   ;; session.
	   (unless rudel-current-session
	     (error "No active Rudel session"))
	   ;; Select unsubscribed documents.
	   (let ((documents (rudel-unsubscribed-documents
			     rudel-current-session)))
	     ;; Already subscribed to all documents. This is an error.
	     (when (null documents)
	       (error "No unsubscribed documents"))
	     ;; Read an unsubscribed document.
	     (rudel-read-document documents nil 'object)))))

  ;; Make sure we have a session.
  (unless rudel-current-session
    (error "No active Rudel session"))

  ;; Create a new buffer and attach the document to it.
  (let* ((name   (rudel-suggested-buffer-name document))
	 (buffer (funcall rudel-allocate-buffer-function name)))
    (rudel-attach-to-buffer document buffer)

    (let ((connection (oref (oref document session) connection)))
      (rudel-subscribe-to connection document))

    ;; Show the new buffer.
    (set-window-buffer nil buffer))
  )

;;;###autoload
(defun rudel-publish-buffer (&optional buffer)
  "Make the BUFFER available for subscription to peers in a collaborative editing session.
If BUFFER is nil, the current buffer is used."
  (interactive (list nil))

  ;; Make sure we have a session.
  (unless rudel-current-session
    (error "No active Rudel session"))

  (unless buffer
    (setq buffer (current-buffer)))

  (with-current-buffer buffer
    (when (rudel-buffer-has-document-p)
      (error "Buffer already published or subscribed"))) ; TODO keep this?

  ;;
  (with-slots (backend connection self) rudel-current-session
    (let ((document (rudel-make-document backend
					 (buffer-name buffer)
					 rudel-current-session)))
      (rudel-add-document rudel-current-session document)

      (rudel-attach-to-buffer document buffer)
      (rudel-add-user document self)

      (rudel-publish connection document)))
  )

;;;###autoload
(defun rudel-unsubscribe (&optional buffer)
  "Detaches BUFFER from the collaborative editing session.
The most recent version of the content will remain in the
buffer but not be affected by future changes from other
peers. If BUFFER is nil, the current is used."
  (interactive)

  ;; Make sure we have a session.
  (unless rudel-current-session
    (error "No active Rudel session"))

  (unless buffer
    (setq buffer (current-buffer)))

  (with-current-buffer buffer
    (unless (rudel-buffer-has-document-p)
      (error "Buffer is not published")))

  ;;
  (with-slots (connection) rudel-current-session
    (let ((document (rudel-buffer-document buffer)))
      (rudel-detach-from-buffer document)

      (rudel-unsubscribe-from connection document)))
  )

(provide 'rudel)
;;; rudel.el ends here
