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+;;; git-undo.el Foundation, Inc.
+
+;; Author: John Wiegley <johnw@newartisans.com>
+;; Created: 20 Nov 2017
+;; Version: 0.1
+
+;; Keywords: git diff history log undo
+;; X-URL: https://github.com/jwiegley/git-undo
+
+;; This program 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.
+
+;; This program 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, Inc., 59 Temple Place - Suite 330,
+;; Boston, MA 02111-1307, USA.
+
+;;; Commentary:
+
+;; Select a region and then use M-x git-undo to revert changes in that region
+;; to the most recent Git historical version. Use C-x z to repeatdly walk back
+;; through the history. M-x git-undo-browse will let you see the history of
+;; changes in a separate buffer.
+
+;;; Code:
+
+(require 'cl)
+
+(defgroup git-undo nil
+  "Successively undo a buffer region using Git history"
+  :group 'emacs)
+
+(defvar git-undo--region-start)
+(defvar git-undo--region-end)
+(defvar git-undo--history)
+
+(defun git-undo--apply-diff (hunk)
+  (with-temp-buffer
+    (insert hunk)
+    (goto-char (point-min))
+    (while (not (eobp))
+      (pcase (char-after)
+        (?\  (delete-char 1) (forward-line))
+        (?\+ (delete-char 1) (forward-line))
+        (?\- (delete-region (point) (and (forward-line) (point))))
+        (t (delete-region (point) (point-max)))))
+    (buffer-string)))
+
+(defun git-undo--replace-region ()
+  (goto-char git-undo--region-start)
+  (delete-region git-undo--region-start git-undo--region-end)
+  (if (null git-undo--history)
+      (error "There is no more Git history to undo")
+    (insert (git-undo--apply-diff (car git-undo--history)))
+    (setq git-undo--history (cdr git-undo--history)))
+  (goto-char git-undo--region-end))
+
+(defun git-undo--compute-offsets (start end)
+  "Taking uncommitted changes into account, find the location in
+Git history for a given line."
+  (let ((file-name (buffer-file-name))
+        (buffer-lines (line-number-at-pos (point-max))))
+    (with-temp-buffer
+      (shell-command
+       (format "git --no-pager diff -U%d HEAD -- %s"
+               buffer-lines (file-name-nondirectory file-name))
+       (current-buffer))
+      (goto-char (point-min))
+      (re-search-forward "^@@")
+      (forward-line)
+      (let ((adjustment 0)
+            (line 1)
+            adjusted-start
+            adjusted-end)
+        (while (not (eobp))
+          (pcase (char-after)
+            (?\+ (setq adjustment (1- adjustment)))
+            (?\- (setq adjustment (1+ adjustment)))
+            (t (setq line (1+ line))))
+          (when (= (- start adjustment) line)
+            (setq adjusted-start (+ start adjustment)))
+          (when (= (- end adjustment) line)
+            (setq adjusted-end (+ end adjustment))
+            (goto-char (point-max)))
+          (forward-line))
+        (cons (1+ adjusted-start) adjusted-end)))))
+
+(defun git-undo--build-history (start end)
+  (let ((file-name (buffer-file-name)))
+    (destructuring-bind (start-line . end-line)
+        (git-undo--compute-offsets (line-number-at-pos start)
+                                   (1- (line-number-at-pos end)))
+      (with-temp-buffer
+        (message "Retrieving Git history for lines %d to %d..."
+                 start-line end-line)
+        (shell-command
+         (format "git --no-pager log --no-expand-tabs -p -L%d,%d:%s"
+                 start-line end-line
+                 (file-name-nondirectory file-name))
+         (current-buffer))
+        (message "")
+        (goto-char (point-min))
+        (let ((commit t) history)
+          (while (and commit
+                      (re-search-forward "^@@" nil t)
+                      (forward-line))
+            (delete-region (point-min) (point))
+            (setq commit (and (re-search-forward "^commit " nil t)
+                              (match-beginning 0)))
+            (setq history (cons (buffer-substring-no-properties
+                                 (point-min) (or commit (point-max)))
+                                history)))
+          (nreverse history))))))
+
+;;;###autoload
+(defun git-undo (&optional start end)
+  "Undo Git-historical changes in the region from START to END."
+  (interactive "r")
+  (if (eq last-command 'git-undo)
+      (git-undo--replace-region)
+    (set (make-local-variable 'git-undo--region-start)
+         (copy-marker start nil))
+    (set (make-local-variable 'git-undo--region-end)
+         (copy-marker end t))
+    (set (make-local-variable 'git-undo--history)
+         (git-undo--build-history start end))
+    (git-undo--replace-region)))
+
+;;;###autoload
+(defun git-undo-browse (&optional start end)
+  "Undo Git-historical changes in the region from START to END."
+  (interactive "r")
+  (let ((history (git-undo--build-history start end)))
+    (display-buffer
+     (with-current-buffer
+         (get-buffer-create "*Git Region History*")
+       (delete-region (point-min) (point-max))
+       (dolist (entry history)
+         (insert (git-undo--apply-diff entry)
+                 #("-----\n" 0 5 (face bold))))
+       (delete-region (- (point) 6) (point))
+       (goto-char (point-min))
+       (current-buffer)))))
+
+;;; git-undo.el ends here