Make undo-tree an optional dependency

Closes #1074

11 files changed, 64 insertions, 26 deletions

diff --git a/Cask b/Cask
index a13cfa2..c466b0b 100644
--- a/Cask
+++ b/Cask
@@ -3,4 +3,3 @@
 (files "*.el")
 
 ;; (depends-on "goto-chg" "0.6.3")
-;; (depends-on "undo-tree" "1.6")
diff --git a/README.md b/README.md
index 16d1cc7..7c76da4 100644
--- a/README.md
+++ b/README.md
@@ -41,8 +41,10 @@ file.
 
 * Evil requires Emacs 24.1 or later.
 
-* Evil requires [undo-tree.el](http://www.emacswiki.org/emacs/UndoTree) in the
-`load-path` for linear undo and undo branches.
+* Evil requires any of the following for `C-r`:
+** `undo-redo` from Emacs 28
+** The [undo-tree](http://www.emacswiki.org/emacs/UndoTree) package
+** The [undo-fu](https://gitlab.com/ideasman42/emacs-undo-fu) package
 
 * For the motions `g;` `g,` and for the last-change-register `.`, Evil requires the
 [goto-chg.el](https://github.com/emacs-evil/goto-chg) package,
diff --git a/doc/build/texinfo/evil.texi b/doc/build/texinfo/evil.texi
index ac3385b..067c057 100644
--- a/doc/build/texinfo/evil.texi
+++ b/doc/build/texinfo/evil.texi
@@ -162,7 +162,7 @@ Finally, add the following lines to your Emacs init file:
 @section Manual installation
 
 
-First, install @cite{undo-tree}, @cite{goto-chg} and @cite{cl-lib}.  If you have an
+First, install @cite{goto-chg} and @cite{cl-lib}.  If you have an
 Emacs version of 24.3 or newer, you should already have @cite{cl-lib}.
 
 Evil lives in a git repository.  To download Evil, do:
diff --git a/doc/source/overview.rst b/doc/source/overview.rst
index 0bb1a7e..467c9ef 100644
--- a/doc/source/overview.rst
+++ b/doc/source/overview.rst
@@ -32,8 +32,8 @@ Finally, add the following lines to your Emacs init file:
 Manual installation
 -------------------
 
-First, install `undo-tree`, `goto-chg` and `cl-lib`.  If you have an
-Emacs version of 24.3 or newer, you should already have `cl-lib`.
+First, install `goto-chg` and `cl-lib`.  If you have an Emacs version
+of 24.3 or newer, you should already have `cl-lib`.
 
 Evil lives in a git repository.  To download Evil, do::
 
diff --git a/evil-commands.el b/evil-commands.el
index 5e5d150..6b14236 100644
--- a/evil-commands.el
+++ b/evil-commands.el
@@ -31,6 +31,7 @@
 (require 'evil-types)
 (require 'evil-command-window)
 (require 'evil-jumps)
+(require 'evil-vars)
 (require 'flyspell)
 (require 'cl-lib)
 (require 'reveal)
@@ -1620,6 +1621,16 @@ of the block."
     (when (evil-visual-state-p)
       (move-marker evil-visual-point (point)))))
 
+(evil-define-command evil-undo (count)
+  "Undo COUNT changes in buffer using `evil-undo-function'."
+  (interactive "<c>")
+  (funcall evil-undo-function count))
+
+(evil-define-command evil-redo (count)
+  "Undo COUNT changes in buffer using `evil-redo-function'."
+  (interactive "<c>")
+  (funcall evil-redo-function count))
+
 (evil-define-operator evil-substitute (beg end type register)
   "Change a character."
   :motion evil-forward-char
diff --git a/evil-integration.el b/evil-integration.el
index 9fd8d68..2fd05f5 100644
--- a/evil-integration.el
+++ b/evil-integration.el
@@ -145,17 +145,6 @@
 
 (eval-after-load 'undo-tree
   '(with-no-warnings
-     (defun evil-turn-on-undo-tree-mode ()
-       "Enable `undo-tree-mode' if evil is enabled.
-This function enables `undo-tree-mode' when Evil is activated in
-some buffer, but only if `global-undo-tree-mode' is also
-activated."
-       (when (and (boundp 'global-undo-tree-mode)
-                  global-undo-tree-mode)
-         (turn-on-undo-tree-mode)))
-
-     (add-hook 'evil-local-mode-hook #'evil-turn-on-undo-tree-mode)
-
      (defadvice undo-tree-visualize (after evil activate)
        "Initialize Evil in the visualization buffer."
        (when evil-local-mode
diff --git a/evil-maps.el b/evil-maps.el
index 079b9f6..91ed47e 100644
--- a/evil-maps.el
+++ b/evil-maps.el
@@ -111,8 +111,8 @@
 (define-key evil-normal-state-map "g," 'goto-last-change-reverse)
 
 ;; undo
-(define-key evil-normal-state-map "u" 'undo)
-(define-key evil-normal-state-map "\C-r" 'redo)
+(define-key evil-normal-state-map "u" 'evil-undo)
+(define-key evil-normal-state-map "\C-r" 'evil-redo)
 
 ;; window commands
 (define-prefix-command 'evil-window-map)
@@ -517,6 +517,8 @@ included in `evil-insert-state-bindings' by default."
 (evil-ex-define-cmd "show-digraphs" 'evil-ex-show-digraphs)
 (evil-ex-define-cmd "sor[t]" 'evil-ex-sort)
 (evil-ex-define-cmd "res[ize]" 'evil-ex-resize)
+(evil-ex-define-cmd "u[ndo]" 'evil-undo)
+(evil-ex-define-cmd "red[o]" 'evil-redo)
 
 (when (featurep 'tab-bar)
   (evil-ex-define-cmd "tabnew" 'tab-bar-new-tab)
diff --git a/evil-pkg.el b/evil-pkg.el
index 558d3c8..f299a42 100644
--- a/evil-pkg.el
+++ b/evil-pkg.el
@@ -3,6 +3,5 @@
   "1.14.0"
   "Extensible Vi layer for Emacs."
   '((emacs "24.1")
-    (undo-tree "0.7.4")
     (goto-chg "1.6")
     (cl-lib "0.5")))
diff --git a/evil-test-helpers.el b/evil-test-helpers.el
index 397b5a6..696d899 100644
--- a/evil-test-helpers.el
+++ b/evil-test-helpers.el
@@ -140,7 +140,6 @@ raised.  Remaining forms are evaluated as-is.
                ;; necessary for keyboard macros to work
                (switch-to-buffer-other-window (current-buffer))
                (buffer-enable-undo)
-               (undo-tree-mode 1)
                ;; parse remaining forms
                ,@(mapcar
                   #'(lambda (form)
diff --git a/evil-vars.el b/evil-vars.el
index 07f478b..1fdf763 100644
--- a/evil-vars.el
+++ b/evil-vars.el
@@ -1971,6 +1971,46 @@ This variable must be set before evil is loaded."
                (add-hook 'minibuffer-setup-hook 'evil-initialize)
              (remove-hook 'minibuffer-setup-hook 'evil-initialize))))
 
+(defun evil--redo-placeholder (_count)
+  (user-error "Customize `evil-undo-system' for redo functionality."))
+
+(defvar evil-undo-function 'undo
+  "Function to be used by `evil-undo'.
+Customized via `evil-undo-system'.")
+
+(defvar evil-redo-function 'evil--redo-placeholder
+  "Function to be used by 'evil-redo'.
+Customized via `evil-undo-system'.")
+
+(defun evil-set-undo-system (system)
+  "Set `evil-undo-function' and `evil-redo-function` by SYSTEM."
+  (cond
+   ((not system)
+    (setq evil-undo-function 'undo
+          evil-redo-function 'evil--redo-placeholder))
+   ((eq system 'undo-redo)
+    (setq evil-undo-function 'undo-only
+          evil-redo-function 'undo-redo))
+   ((eq system 'undo-tree)
+    (setq evil-undo-function 'undo-tree-undo
+          evil-redo-function 'undo-tree-redo))
+   ((eq system 'undo-fu)
+    (setq evil-undo-function 'undo-fu-only-undo
+          evil-redo-function 'undo-fu-only-redo))
+   (t
+    (error "Unknown undo system %s" system))))
+
+(defcustom evil-undo-system nil
+  "Undo system Evil should use."
+  :type '(choice (const :tag "Vanilla undo" nil)
+                 (const undo-redo)
+                 (const undo-tree)
+                 (const undo-foo))
+  :group 'evil
+  :set #'(lambda (sym value)
+           (set-default sym value)
+           (evil-set-undo-system value)))
+
 (defun evil-version ()
   (interactive)
   (message "Evil version %s" evil-version))
diff --git a/evil.el b/evil.el
index 14d2b81..e584b31 100644
--- a/evil.el
+++ b/evil.el
@@ -107,11 +107,8 @@
 ;;      (require 'evil)
 ;;      (evil-mode 1)
 ;;
-;; Evil requires undo-tree.el for linear undo and undo branches:
-;;
-;;      http://www.emacswiki.org/emacs/UndoTree
-;;
-;; Otherwise, Evil uses regular Emacs undo.
+;; Evil requires undo-redo (Emacs 28), undo-fu or undo-tree for redo
+;; functionality.  Otherwise, Evil uses regular Emacs undo.
 ;;
 ;; Evil requires `goto-last-change' and `goto-last-change-reverse'
 ;; function for the corresponding motions g; g, as well as the